experiment meselson stahl

Meselson and Stahl Experiment

Meselson and Stahl experiment gave the experimental evidence of DNA replication to be semi-conservative type . It was introduced by the Matthew Meselson and Franklin Stahl in the year 1958 . Matthew Meselson and Franklin Stahl have used E.coli as the “ Model organism ” to explain the semiconservative mode of replication.

There are three modes of replication introduced during the 1950s like conservative, semi-conservative and dispersive. The researchers were confused between these three that what could be the actual pattern of DNA replication. In 1958, Matthew Meselson and Franklin Stahl presented their research, where they concluded that the replication of DNA is semiconservative type .

Matthew Meselson and Franklin Stahl have conducted several experiments after the discovery of DNA structure (by the two scientists Watson and Crick ). Watson and Crick’s model is widely accepted to demonstrate the replicative model of DNA. We will discuss the definition, steps and observation of the Meselson and Stahl experiment along with the semi-conservative model of DNA.

Content: Meselson and Stahl Experiment

Semi conservation model of dna, meselson and stahl experiment steps, observation, definition of meselson and stahl experiment.

Meselson and Stahl Experiment gave us the theory of semi-conservative replication of DNA. They have taken E.coli as the model organism and two different isotopes, N-15 and N-14 . The N-15 is the heavier isotope, whereas N-14 is the lighter or common isotope of nitrogen. Meselson and Stahl performed their experiment by first growing the E.coli in the medium containing 15 NH 4 Cl for several generations. They observed that the heavy isotope has incorporated in the genome of E.coli and the cells become more substantial due to 15 N heavy isotope.

Meselson and Stahl then transferred the E.coli cells incorporated with 15 N isotope to the medium containing 14 NH 4 Cl for several generations. After every twenty minutes, the E.coli cells multiply. For the processing of DNA, the cells were centrifuged by the addition of Caesium chloride, resulting in the formation of the concentration gradient. As a result, light, intermediate and heavy DNA strands will get separated.

After completing their experiment, Meselson and Stahl concluded that after each cell division, half of the DNA would be conserved for every next generation. Therefore, this experiment proves that the DNA replication obeys the semi-conservative mode of replication in which 50% of the DNA conserve for every next generation in a way like 100%, 50%, 25%, and 12.5% and so on.

It is the type of DNA replication. The term semi means “ Half ” and conservative means “ To store ”. The semi-conservative DNA replication results in the two daughter DNAs after the parent DNA replication.

In the two daughter DNA’s, each strand will contain a mixture of the parent DNA’s template strand, and the other with a newly synthesized strand (in F-1 gen ). When the parental DNA replicates, half of the 100%, i.e. 50% of the DNA is conserved by having parent strand and the remaining 50% will produce newly synthesized strands.

After the F-1 gen, the multiplication of the cell will get double, which will produce four DNA strands (in F-2 gen ). In F-2 gen half of 50%, i.e. only 25% of DNA is conserved by having parental strands, and the remaining 75% will produce newly synthesized strands.

Growth of E.coli : First, the E.coli were grown in the medium containing 15 NH 4 Cl for several generations. NH 4 provides the nitrogen as well as a protein source for the growth of the E.coli. Here, the 15 N is the heavy isotope of nitrogen.
Incorporation of 15 N : After several generations of E.coli, Meselson and Stahl observed that the 15 N heavy isotope has incorporated between the DNA nucleotides in E.coli.
Transfer of E.coli cells : The DNA of E.coli labelled with 15 N isotope were transferred to the medium containing 14 NH 4 Cl . Here, the 14 N is the light isotope of nitrogen. The E.coli cells were again allowed to multiply for several generations. The E.coli cells will multiply every 20 minutes for several generations.
Processing of DNA : For the processing or separation of DNA, the E.coli cells were transferred to the Eppendorf tubes. After that, caesium chloride is added, having a density of 1.71 g/cm 3 (the same of DNA). Finally, the tubes were subjected to high-speed centrifugation 140,000 X g for 20 hours.

The result, after two generations of E.coli, the following results were obtained:

predictions for f1 gen made by meselson and stahl

In the F-1 generation : According to the actual observations, two DNA strands (with a mixture of both 15 N and 14 N isotopes) will produce in F-1 gen. The above diagram shows that the semiconservative and dispersive model obeys the pattern of growth explained by Meselson and Stahl.

Thus, it is clear that the DNA does not replicate via “Conservative mode”. According to the conservative model, the DNA replicates to produce one newly synthesized DNA and one parental DNA. Therefore, the conservative model was disapproved, as it does not produce hybrid DNA in the F-1 generation.

predictions for f2 gen made by meselson and stahl

In the F-2 generation : According to the actual observation, four DNA strands ( two with hybrid and the remaining two with light DNA ) will produce in the F-2 generation. The hybrid DNA includes a mixture of 15 N and 14 N. The light DNA strands contain a pure 14 N. The diagram shows that only semi-conservative type of replication gave similar results conducted by Meselson and Stahl. Thus, both the conservative and dispersive modes of replication were disapproved.

Therefore, we can conclude that the type of replication in DNA is “ Semi conservative ”. The offsprings have a hybrid DNA containing a mixture of both template and newly synthesized DNA in the semi-conservative model. After each multiplication, the number of offspring will double, and half of the parental DNA will be conserved for the next generation.

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The Meselson-Stahl Experiment (1957–1958), by Matthew Meselson and Franklin Stahl

Illustration of the Meselson-Stahl Experiment

In an experiment later named for them, Matthew Stanley Meselson and Franklin William Stahl in the US demonstrated during the 1950s the semi-conservative replication of DNA, such that each daughter DNA molecule contains one new daughter subunit and one subunit conserved from the parental DNA molecule. The researchers conducted the experiment at California Institute of Technology (Caltech) in Pasadena, California, from October 1957 to January 1958. The experiment verified James Watson and Francis Crick´s model for the structure of DNA, which represented DNA as two helical strands wound together in a double helix that replicated semi-conservatively. The Watson-Crick Model for DNA later became the universally accepted DNA model. The Meselson-Stahl experiment enabled researchers to explain how DNA replicates, thereby providing a physical basis for the genetic phenomena of heredity and diseases.

The Meselson-Stahl experiment stemmed from a debate in the 1950s among scientists about how DNA replicated, or copied, itself. The debate began when James Watson and Francis Crick at the University of Cambridge in Cambridge, England, published a paper on the genetic implications of their proposed structure of DNA in May 1953. The Watson-Crick model represented DNA as two helical strands, each its own molecule, wound tightly together in a double helix. The scientists claimed that the two strands were complementary, which meant certain components of one strand matched with certain components of the other strand in the double helix.

With that model of DNA, scientists aimed to explain how organisms preserved and transferred the genetic information of DNA to their offspring. Watson and Crick suggested a method of self-replication for the movement of genetic information, later termed semi-conservative replication, in which DNA strands unwound and separated, so that each strand could serve as a template for a newly replicated strand. According to Watson and Crick, after DNA replicated itself, each new double helix contained one parent strand and one new daughter strand of DNA, thereby conserving one strand of the original double helix. While Watson and Crick proposed the semi-conservative model in 1953, the Meselson-Stahl experiment confirmed the model in 1957.

In 1954, Max Delbrück at Caltech published a paper that challenged the Watson-Crick Model for DNA replication. In his paper, Delbrück argued that the replication process suggested by Watson and Crick was unlikely because of the difficulty associated with unwinding the tightly-wound DNA structure. As an alternative, Delbrück proposed that instead of the entire structure breaking apart or unwinding, small segments of DNA broke from the parent helix. New DNA, Delbrück claimed, formed using the small segments as templates, and the segments then rejoined to form a new hybrid double helix, with parent and daughter segments interspersed throughout the structure.

After the release of Delbrück´s paper, many scientists sought to determine experimentally the mechanism of DNA replication, which yielded a variety of theories on the subject by 1956. Delbrück and Gunther Stent, a professor at the University of California, Berkeley, in Berkeley, California, presented a paper in June 1956 at a symposium at Johns Hopkins University in Baltimore, Maryland, which named and summarized the three prevailing theories regarding DNA replication at the time: semi-conservative, dispersive, and conservative. Delbrück and Stent defined conservative replication as a replication mechanism in which a completely new double helix replicated from the parent helix, with no part of the parent double helix incorporated into the daughter double helix. They described the semi-conservative process as Watson and Crick suggested, with half of the parental DNA molecule conserved in the daughter molecule. Lastly, Delbrück and Stent summarized Delbrück´s dispersive model, in which parental DNA segments distribute throughout the daughter DNA molecule. Delbrück and Stent´s paper provided the background for the Meselson-Stahl experiment.

In 1954, prior to publication of Delbrück´s initial challenge of the Watson-Crick model, Matthew Meselson and Franklin Stahl had joined the DNA replication discussion. During the spring of 1954, Meselson, a graduate student studying chemistry at Caltech, visited Delbrück´s office to discuss DNA replication. According to historian of science Frederic Holmes, during that meeting Meselson began brainstorming ways to determine how DNA replicated. In the summer of 1954, Meselson met Stahl at the Marine Biological Laboratory in Woods Hole, Massachusetts. Stahl, a graduate student studying biology at the University of Rochester in Rochester, New York, agreed to study DNA replication with Meselson the following year at Caltech.

Meselson and Stahl began their collaboration in late 1956. By that time, Stahl had completed his PhD and Meselson had completed the experiments for his PhD, which he received in 1957. They worked on a variety of projects, including DNA replication. All of their projects, however, involved a method first devised by Meselson in 1954, called density-gradient centrifugation. Density-gradient centrifugation separates molecules based on their densities, which depend on the molecular weights of the molecules.

Meselson and Stahl used density-gradient centrifugation to separate different molecules in a solution, a method they later used to separate DNA molecules in a solution. In density gradient centrifugation, a solution is placed in an ultracentrifuge, a machine that spins the samples very fast on the order of 140,000 times the force of gravity or 44,770 revolutions per minute (rpm). As the samples spin, denser substances are pushed toward the bottom, while less dense substances distribute according to their weight in the centrifuge tube. By the end of centrifugation, the molecules reach a position called equilibrium, in which the molecules stop moving and remain in a gradient. The position of the molecules at equilibrium is dependent on the density of the molecule. Meselson and Stahl measured the areas in which DNA was at the highest concentration. Higher concentrations were represented by darker bands of DNA in the centrifuged sample. Stahl represented those bands on a graph, so that the peaks represented locations in the gradient where there was the highest concentration of molecules. Multiple peaks meant that molecules of different densities separated out of the solution.

To describe how DNA replicated, Meselson and Stahl needed to distinguish between parental and daughter DNA. They achieved that by modifying the molecules so each kind had a different density. Then Meselson and Stahl could separate the molecules using density-gradient centrifugation and analyze how much parental DNA was in the new daughter helices after every replication cycle. First they tried to alter the density of parental DNA by substituting a one nucleotide base, thymidine, with a heaver but similar DNA nucleotide base, 5-bromouriacil (5-BU). However, Meselson and Stahl struggled to substitute enough units of 5-BU into the DNA molecules to make the parental DNA significantly denser than normal DNA.

By July 1957, Meselson and Stahl successfully incorporated the heavy substitution in parental DNA, but the type of DNA they used still caused problems. Meselson and Stahl first used DNA from a specific type of virus that infects bacteria, called a bacteriophage. However, bacteriophage DNA not only broke apart in solution during centrifugation, but also replicated too quickly for the distribution of DNA to be adequately measured after each cycle. Consequently, Meselson and Stahl struggled to see clear locations within the density gradient with the highest concentration of bacterial DNA. Therefore, in September 1957, Meselson and Stahl switched to using the DNA from the bacteria Escherichia coli (E. coli) . E. coli DNA formed clearer concentration peaks during density gradient centrifugation.

At around the same time, in addition to changing the source DNA, Meselson and Stahl also changed the type of density label they used, from substitution labels to isotope labels. An isotope of an element is an atom with the same number of positive charged nuclear particles or protons, and a different number of uncharged particles, called neutrons. A difference in neutrons, for the most part, does not affect the chemical properties of the atom, but it alters the weight of the atom, thereby altering the density. Meselson and Stahl incorporated non-radioactive isotopes of nitrogen with different weights into the DNA of E. coli . As DNA contains a large amount of nitrogen, so long as the bacteria grew in a medium containing nitrogen of a specified isotope, the bacteria would use that nitrogen to build DNA. Therefore, depending on the medium in which E. coli grew, daughter strands of newly replicated DNA would vary by weight, and could be separated by density-gradient centrifugation.

Starting in October 1957, Meselson and Stahl conducted what later researches called the Meselson-Stahl experiment. They grew E. coli in a medium containing only the heavy isotope of nitrogen ( 15 N) to give the parental DNA a higher than normal density. As bacteria grow, they duplicate, thereby replicating their DNA in the process. The researchers then added an excess of light isotopes of nitrogen ( 14 N) to the heavy nitrogen environment.

Meselson and Stahl grew E. coli in the 14 N isotope environment for all subsequent bacterial generations, so that any new DNA strands produced were of a lower density than the original parent DNA. Before adding 14 N nitrogen, and for intervals of several bacterial generations after adding light nitrogen, Meselson and Stahl pulled samples of E. coli out of the growth medium for testing. They centrifuged each sample for initial separation, and then they added salt to the bacteria so that the bacteria released its DNA contents, allowing Meselson and Stahl to analyze the samples.

Next, Meselson and Stahl conducted density gradient centrifugation for each DNA sample to see how the parental and daughter DNA distributed according to their densities over multiple replications. They added a small amount of each sample of bacterial DNA to a cesium chloride solution, which when centrifuged had densities within the range of the bacterial DNA densities so that the DNA separated by density. The researchers centrifuged the DNA in an ultracentrifuge for twenth hours until the DNA reached equilibrium. Using ultraviolet light (UV), the researchers photographed the resulting DNA bands, which represented peaks of DNA concentrations at different densities. The density of the DNA depended on the amount of 15 N or 14 N nitrogen present. The more 15 N nitrogen atoms present, the denser the DNA.

For the bacterial DNA collected before Meselson and Stahl added 14 N nitrogen, the UV photographs showed only one band for DNA with 15 N nitrogen isotopes. That result occurred because the DNA from the first sample grew in an environment with only 15 N nitrogen isotopes. For samples pulled during the first replication cycle, the UV photographs showed fainter the 15 N DNA bands, and a new DNA band formed, which represented half 15 N DNA nitrogen isotopes and half 14 N DNA nitrogen isotopes. By the end of the first replication cycle, the heavy DNA band disappeared, and only a dark half 15 N and half 14 N DNA band remained. The half 15 N half 14 N DNA contained one subunit of 15 N nitrogen DNA and one subunit of 14 N nitrogen DNA. The data from the first replication cycle indicated some distribution of parental DNA, therefore ruled out conservative replication, because only parental DNA contained 15 N nitrogen isotopes and only parental DNA could represent the 15 N nitrogen isotopes in daughter DNA.

The same trends continued in future DNA replication cycles. As the bacteria continued to replicate and the bacterial DNA replicated, UV photographs showed that the band representing half 15 N half 14 N DNA depleted. A new band, representing DNA containing only 14 N nitrogen isotopes or light DNA, became the prevalent DNA band in the sample. The depletion of the half 15 N half 14 N band occurred because Meselson and Stahl never re-introduced 15 N nitrogen, so the relative amount of 15 N nitrogen DNA decreased. Meselson and Stahl then mixed the samples pulled from different replication cycles and centrifuged them together. The UV photograph from that run showed three bands of DNA with the half 15 N half 14 N DNA band at the midpoint between the 15 N DNA band and 14 N DNA band, making it an intermediate band. The result indicated that the half 15 N half 14 N DNA band had a density exactly between the 15 N and 14 N nitrogen DNA, showing that the DNA in the central band contained half of the 15 N nitrogen and half of the 14 N nitrogen isotopes, just as predicted by the Watson and Crick model. The exact split between heavy and light nitrogen characterized semi-conservative DNA replication.

Meselson and Stahl made three conclusions based on their results. First, they concluded that the nitrogen in each DNA molecule divided evenly between the two subunits of DNA, and that the subunits stayed intact throughout the observed replication cycles. Meselson and Stahl made that conclusion because the intermediate band had a density halfway between the heavy and light DNA bands. That conclusion made by Meselson and Stahl challenged the dispersive mechanism suggested by Delbrück, which involved breaking the DNA subunits into smaller pieces.

Meselson´s and Stahl´s second conclusion stated that each new DNA double helix contained one parental subunit, which supported semi-conservative replication. Assuming that DNA consists of two subunits, if a parent passes on one subunit of DNA to its offspring, then half of the parental DNA is conserved in the offspring DNA, and half of the parental DNA is not. The researchers made that conclusion because if parental DNA did not replicate in that way, then after the first replication, some DNA double helices would have contained only parental heavy nitrogen subunits or only daughter light nitrogen subunits. That type of replication would have indicated that that some parental DNA subunits did not separate in the semi-conservative fashion, and instead would have supported conservative replication. The presence of one parental subunit for each daughter DNA double helix supported semi-conservative replication.

The third conclusion made by Meselson and Stahl stated that for every parental DNA molecule, two new molecules were made. Therefore, the amount of DNA after each replication increased by a factor of two. Meselson and Stahl related their findings to the structure of DNA and replication mechanism proposed by Watson and Crick.

Before Meselson and Stahl published their findings, word of the Meselson-Stahl results spread throughout Caltech and the scientific community. According to Holmes, Delbrück, who had strongly opposed the semi-conservative method of DNA replication, immediately accepted DNA replication as semi-conservative after seeing the results from the Meselson-Stahl experiment. Some experiments earlier that year had pointed towards semi-conservative replication, and the Meselson-Stahl experiment served to further support semi-conservative replication.

Despite the positive reception of the Meselson-Stahl experiment, years passed before scientists fully accepted the Watson-Crick Model for DNA based on the findings from the Meselson-Stahl experiment. The Meselson-Stahl experiment did not clearly identify the exact subunits that replicated in DNA. In the Watson and Crick model, DNA consisted of two one-stranded DNA subunits, but the Meselson-Stahl experiment also supported models of DNA as having more than two strands. In 1959, Liebe Cavalieri, a scientist at the Sloan-Kettering Institute for Cancer research in New York City, New York, and his research team had produced evidence supporting the theory that DNA consisted of two two-stranded subunits, making DNA a quadruple helix. Cavalieri´s proposal did not contradict the Meselson-Stahl experiment, because the Meselson-Stahl experiment did not define DNA subunits. However, later experiments performed by Meselson on bacteriophage DNA from 1959 to 1961, and experiments performed by John Cairns on E. coli DNA in 1962, settled the debate and showed that each subunit of DNA was a single strand.

As described by Holmes, many scientists highly regarded the Meselson-Stahl experiment. Scientists including John Cairns, Gunther Stent, and James Watson all described the experiment as beautiful in both its performance and simplicity. Holmes also described the academic paper published by Meselson and Stahl on their experiment as beautiful because of its concise descriptions, diagrams, and conclusions. The Meselson-Stahl experiment appeared in textbooks decades after Meselson and Stahl performed the experiment. In 2001, Holmes published Meselson, Stahl, and the Replication of DNA: A History of "The Most Beautiful Experiment in Biology," which told the history of the experiment.

The Meselson-Stahl experiment gave a physical explanation for the genetic observations made before it. According to Holmes, for scientists who already believed that DNA replicated semi-conservatively, the Meselson-Stahl experiment provided concrete evidence for that theory. Holmes stated that, for scientists who contested semi-conservative replication as proposed by Watson and Crick, the Meselson-Stahl experiment eventually changed their opinions. Either way, the experiment helped scientists´ explain inheritance by showing how DNA conserves genetic information throughout successive DNA replication cycles as a cell grows, develops, and reproduces.

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Dna Replication Experiment

DNA Replication and Meselson And Stahl's Experiment

Literally, replication means the process of duplication. In molecular biology, DNA replication is the primary stage of inheritance. Central dogma explains how the DNA makes its own copies through DNA replication, which then codes for the RNA in transcription and further, RNA codes for the proteins by the translation.

Let’s go through Meselson and Stahl Experiment and DNA replication.

Meselson and Stahl Experiment

Meselson and Stahl Experiment was an experimental proof for semiconservative DNA replication. In 1958, Matthew Meselson and Franklin Stahl conducted an experiment on E.coli which divides in 20 minutes, to study the replication of DNA.

Semi conservative DNA Replication through Meselson and Stahl’s Experiment

15 N (heavy) and 14 N (normal) are two isotopes of nitrogen, which can be distinguished based on their densities by centrifugation in Ca,esium chloride (CsCl). Meselson and Stahl cultured E.coli in a medium constituting 15 NH 4 Cl over many generations. As a result, 15 N was integrated into the bacterial DNA. Later, they revised the 15 NH 4 Cl medium to normal 14 NH 4 Cl. At a regular interval of time, they took the sample and checked for the density of DNA.

Observation

Sample no. 1 (after 20 minutes): The sample had bacterial DNA with an intermediate density. Sample no. 2 (after 40 minutes): The sample contained DNA with both intermediate and light densities in the same proportion.

Based on observations and experimental results, Meselson and Stahl concluded that DNA molecules can replicate semi-conservatively. Investigation of semi-conservative nature of replication of DNA or the copying of the cells , DNA didn’t end there. Followed by Meselson and Stahl experiment, Taylor and colleagues conducted another experiment on Vicia faba (fava beans) which again proved that replication of DNA is semi-conservative.

Frequently Asked Questions

Which mode of replication did the messelson and stahl’s experiment support.

Messelson and Stahl’s experiment supported the semi-conservative mode of replication. The DNA was first replicated in 14N medium which produced a band of 14N and 15N hybrid DNA. This eliminated the conservative mode of replication.

What are the different modes of replication of DNA?

The different modes of replication of DNA are:

Semiconservative
Conservative

How are semi-conservative and conservative modes of replication different?

Semi-conservative mode of replication produces two copies, each containing one original strand and one new strand. On the contrary, conservative replication produces two new strands and would leave two original template DNA strands in a double helix.

What is the result of DNA replication?

The result of DNA replication is one original strand and one new strand of nucleotides.

What happens if DNA replication goes wrong?

If DNA replication goes wrong, a mutation occurs. However, if any mismatch happens, it can be corrected during proofreading by DNA Polymerase.

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The Experimental Proof Of DNA Replication

The process by which cells duplicate their genetic material during cell division—the replication of DNA—was still largely a mystery. This sparked a race to understand how DNA replication happens among several well-known experts. The experimental evidence of DNA replication, which showed that DNA replication is a semi-conservative process, was one of the most important advances in this science.

Meselson and Stahl Experiment

Before we dive into the details of the Meselson and Stahl experiment, let’s first understand why it was such an important experiment in the field of molecular biology. At the time of the experiment, there was a lot of debate about how DNA replication occurred. There were three main hypotheses: conservative, semi-conservative, and dispersive . The conservative model suggested that the original DNA molecule remained intact during replication, with the newly synthesized DNA molecules consisting entirely of new nucleotides. The dispersive model suggested that the original DNA molecule was broken down and its nucleotides were randomly distributed between the newly synthesized DNA molecules. The semi-conservative model, on the other hand, proposed that each newly synthesized DNA molecule consisted of one original strand and one newly synthesized strand. The Meselson and Stahl experiment provided evidence in support of the semi-conservative model and helped to establish the basic mechanism of DNA replication

In 1958, Matthew Meselson and Franklin Stahl conducted a groundbreaking experiment that provided evidence for the semi-conservative nature of DNA replication. Their experiment involved growing E. coli bacteria in a medium containing a heavy isotope of nitrogen, N-15. This heavy isotope is incorporated into the DNA nucleotides as the bacteria grow and divide, resulting in a DNA molecule with a higher density than normal DNA.

Here’s a step-by-step breakdown of Meselson’s experiment:

E. coli bacteria were grown in a medium containing N-15 as the sole nitrogen source, allowing the bacteria to incorporate the heavy isotope into their DNA molecules.
The bacteria were then transferred to a medium containing a lighter isotope of nitrogen, N-14. This allowed the bacteria to begin replicating their DNA using the lighter nitrogen isotope.
After one round of replication, the DNA was extracted from the bacteria and subjected to a process called density gradient centrifugation . This technique separates molecules based on their density by subjecting them to a centrifugal force.
The DNA was loaded onto a tube containing a gradient of a heavy substance called cesium chloride (CsCl). The CsCl gradient allowed the DNA molecules to settle at the point in the tube where their density matched that of the surrounding CsCl.
The DNA was then spun at a high speed in the centrifuge, causing the DNA molecules to move through the CsCl gradient until they settled at their equilibrium density.
Meselson and Stahl observed that the DNA formed a single band in the tube, indicating that all the DNA molecules had the same density.
This result was unexpected, as it was predicted that if DNA replication was conservative, then all the DNA molecules would have either the heavy isotope or the light isotope, resulting in two distinct bands in the CsCl gradient.

The result of Meselson’s experiment led him to conclude that DNA replication is not conservative, but rather semi-conservative. This means that each strand of the original DNA molecule serves as a template for the synthesis of a new complementary strand, resulting in two DNA molecules, each with one original and one newly synthesized strand.

DNA Replication

DNA replication is the process by which cells make an exact copy of their genetic material before cell division. This process ensures that each daughter cell receives a complete set of genetic information. The replication of DNA is a complex process that involves several enzymes and other molecules.

Semi-Conservative Model

The mechanism of DNA replication was first proposed by Watson and Crick in 1953. They proposed that DNA replication is a semi-conservative process, meaning that each daughter DNA molecule contains one original strand and one newly synthesized strand. This model was supported by experiments conducted by Matthew Meselson and Franklin Stahl in 1958.

DNA Replication Fork

The site of DNA replication is called the replication fork. The replication fork is a Y-shaped structure that forms when the two strands of DNA are separated. At the replication fork, DNA synthesis occurs in both directions, creating two replication bubbles.

FAQs on Meselson and Stahl Experiment

Q1: how was the experimental proof of dna replication obtained.

The experimental proof of DNA replication was obtained through a series of experiments by researchers such as Meselson and Stahl, in which they used isotopes of nitrogen to trace the replication process.

Q2: What is the significance of DNA replication?

DNA replication is essential for the transmission of genetic information from one generation to the next. It ensures that each daughter cell receives an identical copy of the genetic material and maintains the genetic stability of the organism.

Q3: What is the role of enzymes in DNA replication?

Enzymes play a crucial role in DNA replication. They are responsible for unwinding the DNA double helix, separating the parent strands, synthesizing new strands of DNA, and proofreading the newly synthesized strands to ensure that they are accurate copies of the parent strands.

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Pollen Grains Pollen grains are minute structures of varying size and shape that contain the androecium, the male reproductive organ of a flower. Pollen grains are also called microgametophytes. The formation of pollen grains occurs through the process of microsporogenesis and consists of a protective outer laye 6 min read
The Structure and Functions of Pistil In flowering plants, sexual reproduction is a complex process that involves the mating of male and female gametes to create seeds for the following generation. The pistil, which is located in the centre of the flower, is the female reproductive structure in flowering plants. What is Pistil?A pistil 4 min read
Pollination Pollination is the biological process by which pollen from the male part of the flower transfers to the female part of the same or on different flowers. Pollination results in fertilization and the production of seeds. Pollination is important for the reproduction of plants. Pollination can occur in 6 min read
Double Fertilization: Process & Significance Double fertilization is a unique reproductive process that occurs in flowering plants (angiosperms). Unlike in most other organisms where a single sperm fertilizes an egg, in double fertilization, one male gamete fertilizes the egg cell to form the embryo, while another male gamete fuses with two po 8 min read
Post Fertilization Post-fertilization events are the processes that occur after the fusion of the male and female gametes during sexual reproduction. These post-fertilization events in flowering plants are crucial for the development of the zygote into a mature seed or fruit. Understanding post-fertilization events in 6 min read
Apomixis and Polyembryony: Differences, Types, Significance Apomixis and polyembryony are two different but related biological processes that result in the production of offspring without fertilization. Apomixis is a type of asexual reproduction where seeds are produced without gametic fusion. While polyembryony is a process in which multiple embryos are pro 5 min read

Chapter 2: Human Reproduction

NCERT Notes on Human Reproduction Class 12 Chapter 2 NCERT Notes of Class 12 Chapter 2 Human Reproduction: Human reproduction is the biological process by which a new individual offspring is produced from one or two parent organisms. The Human Reproduction process involves the fusion of gametes, which are specialized cells that carry genetic informati 15+ min read
Male Reproductive System - Structure, Organs, Functions All living creatures increase or duplicate and produce posterity of a comparable kind. Propagation is a fundamental interaction between the presence of animal groups and the continuation of life. Propagation is the cycle by which all life forms duplicate in number and increment their populace. Male 8 min read
Female Reproductive System The female reproductive system is made up of the internal and external sex organs which play an important part in reproduction. The female reproductive system parts and functions also play a crucial role in hormonal regulation and menstrual cycle regulation. The vagina, uterus, fallopian tubes, and 8 min read
Gametogenesis - Spermatogenesis and Oogenesis Gametogenesis is a process of producing male and female gametes, carried out by all sexually reproducing organisms. The process involves various multiple stages of division and differentiation and is highly regulated under hormonal control. GametogenesisGametogenesis produces male and female gametes 4 min read
Menstrual Cycle In a day-to-day existence cycle, a lady's body is powerless against different changes. The pattern of these progressions happens in ladies consistently, emphatically for pregnancy is known as the feminine cycle. At the point when an ovum is unfertilized, the uterus lining sheds and prompts a dischar 10 min read
Fertilizations And Implantation Fertilization and implantation are the 2 important events in human reproduction, which is the biological process of producing new individuals from a union of male and female gametes. This complex process involves the fusion of gametes, the development of a zygote, and the growth and differentiation 5 min read
Embryo Development - Development Process of Fetus Birth gives process to a child is known as reproduction. A species' survival depends on its ability to reproduce. There are two different ways to reproduce: Sexual reproduction is asexual reproduction. Asexual reproduction is a type of reproduction that occurs without the involvement of 2 parents. A 5 min read
Parturition And Lactation - Biology Notes Class 12 Parturition And Lactation: Several intricate physiological processes, such as fertilisation, implantation, gestation, and delivery, are involved in human reproduction. The act of giving birth, often referred to as parturition, is a significant occasion that signals the conclusion of pregnancy and th 4 min read

Chapter 3: Reproductive Health

Notes on NCERT for Class 12 Biology Chapter 3 Reproductive Health Notes on NCERT for Class 12 Biology Chapter 3 Reproductive Health: Reproductive health simply means people in a society living with physically and functionally normal reproductive organs and normal behavioral and emotional responses toward sex-related matters. According to WHO “reproductive health m 10 min read
Population Stabilization And Birth Control - Class 12 Population Stabilization And Birth Control: Reproductive Health means total well-being in all aspects of reproduction, i.e., physical, emotional, behavioral, and social. Counseling and raising awareness among people about reproductive organs, adolescence, and associated changes, safe and hygienic se 6 min read
Medical Termination of Pregnancy (MTP) Medical termination of Pregnancy (MTP) is an intentional or voluntary termination of pregnancy before its full term. Before the 1960s, surgical methods like vacuum aspiration or dilatation and curettage were common, but medication has since emerged as an alternative option. Medical Termination of Pr 5 min read
Sexually Transmitted Infections (STIS) of Reproductive Health Class 12 CBSE Class 12- Reproductive Health- Sexually Transmitted Infections (STIS): Sexually transmitted diseases (STDs) or sexually transmitted infections (STIs) are most commonly spread through sexual contact. Bacteria, viruses, and parasites that cause sexually transmitted illnesses can spread from perso 5 min read

Chapter 4: Principles Of Inheritance And Variation

Principles of Inheritance and Variation CBSE Notes for Chapter 4 Inheritance is the term given to the process by which characters are passed from parents to offspring which forms the basis of heredity. Heredity is the process of passing down genetic traits from parents to offspring. The degree of difference in characters between a parent and offspring is called v 15 min read
Mendel's Laws of Inheritance | Mendel's Experiments Mendel's law of inheritance states that offspring inherited from their parents that results in similar characteristics of parents and offspring. This law of inheritance depends upon three other laws including the law of dominance, the law of segregation, law of independent assortment. Gregor Mendel 8 min read
Inheritance of One Gene Notes We never wonder why Lion can give birth to Lions only, or why a bird can reproduce in the same species and no other species. Not everything is possible, Isn't it? Also, No human being look exactly identical, even with twins there are differences in every individual. Some siblings look similar while 6 min read
Chromosomal Theory of Inheritance The essential idea behind the chromosomal theory of inheritance is that genes are located on chromosomes and that the behavior of chromosomes during meiosis and fertilization provides the basis for inheritance patterns. In the early 1900s, pioneering geneticists Walter Sutton and Theodor Boveri form 6 min read
Linkage And Recombination - Principles Of Inheritance And Variation Class 12 NCERT CBSE Class 12- Principles Of Inheritance And Variation- Linkage And Recombination: Linkage and recombination are the phenomena that describe the inheritance of genes. Linkage and Recombination both are related to the genetic information inherited from parents to offspring. Linkage is the tendency of 6 min read
What is Polygenic Inheritance? Polygenic inheritance is a type of inheritance in which multiple genes control the phenotype of an organism. The phenotypes or traits can be height, skin color, the color of the eyes, etc. This type of inheritance is also known as quantitative inheritance or multifactorial inheritance. Such traits a 7 min read
Pleiotropy - An Overview and Related Disorders 'PLEIOTROPY' term refers to the phenomenon in which a single locus affects two or more apparently unrelated phenotypic traits and is frequently identified as a single mutation affecting two or more wild-type traits. Pleiotropy comes from the Greek words pleio, which means "many," and tropic, which m 5 min read
Sex Determination Genetic information is transferred to the offspring from their parents by the means of asexual or sexual reproduction. Transferring traits from parents to their offspring is called heredity and it is also known as biological inheritance or inheritance. Natural selection is the way in which the speci 7 min read
Mutation The human body might be visualized as a simple organism. But it is the combination of different complex processes. From the outside, a human body might resemble a very simple one. A body that has two arms, two legs & one head for monitoring purposes. But from the inside of the body, there are ma 15+ min read
Pedigree Analysis As a rule, including different plant and creature species, researchers utilize family investigation to break down the legacy of aggregates, or characteristics, utilizing mating tests called crosses. Mendel's analyses uncovered that the 'factors', what we know as qualities, are liable for the legacy 7 min read
Mendelian Disorder in Human Mendelian disorders are a form of genetic disorder that is caused by the inheritance of single or multiple mutant genes from one or both parents. The function of the mutant gene determines how severe a Mendelian disorder is. The condition is typically severe if the gene controls a crucial function. 6 min read
Chromosomal Disorders: Principles of Inheritance And Variation Class12 CBSE Class-12 Principles Of Inheritance And Variation - Chromosomal Disorders: The chromosomes are thread-like structures that are mainly present in the nucleus which carries the hereditary information of genes that are passed from the parents to the offspring. Due to some irregularities of cell div 5 min read

Chapter 5: Molecular Basis Of Inheritance

Evolution Notes for Class 12 Chapter 6 Evolutionary biology is the study of the evolutionary processes that produced the diversity of life on Earth. Earth came into existence sometime between 4 and 5 billion years ago. Life evolved on planet Earth about 3.5 billion years ago. Since then, approximately 15 million different species of orga 11 min read
Molecular Basis of Inheritance Notes Class 12 CBSE Class 12 Molecular Basis of Inheritance: Inheritance is transmitted by certain molecules that Mendel termed as ‘factors’, but their nature was discovered later with the development of various scientific techniques. The molecules which govern the inheritance are called genes and it is of two typ 15+ min read
DNA: Structure, Types, and Functions DNA structure is made of nucleotide base pairs (other than RNA). DNA is the hereditary material that is possessed by all the organisms found on the Earth except certain virus species. DNA functions involve the transfer of genetic information from generation to generation. The full form of DNA is Deo 11 min read
Packaging of DNA Helix: Histones & Importance DNA packaging refers to the process through which DNA molecules are tightly compacted into a smaller volume so that they can fit into the nucleus of a cell. DNA packaging is important because the length of DNA molecules is much greater than the size of the cell nucleus, and therefore, if the DNA wer 5 min read
Search For Genetic Material The search for genetic material has been important in understanding inheritance and evolution. Scientists have explored various models and experiments to identify the substance responsible for transmitting hereditary traits. From Griffith's transformation experiments to Avery, MacLeod, and McCarty's 5 min read
Difference Between DNA and RNA The difference Between DNA and RNA lies in their structure, function, and location within cells, with DNA typically double-stranded, storing genetic information in the nucleus, while RNA is generally single-stranded, involved in protein synthesis, and present in various cellular compartments. DNA (D 6 min read
RNA - Definition, Structure, Types and Functions RNA is a ribonucleic acid that helps in the synthesis of proteins in our body. This nucleic acid is responsible for the production of new cells in the human body. It is usually obtained from the DNA molecule. RNA resembles the same that of DNA, the only difference being that it has a single strand u 11 min read
DNA Replication DNA replication is a fundamental biological process by which a cell duplicates its entire DNA. DNA is a self-replicating structure and the replication is catalyzed by enzymes. Through DNA Replication, genetic information is passed on from one generation of cells to the next during cell division. It 8 min read
The Experimental Proof Of DNA Replication The process by which cells duplicate their genetic material during cell division—the replication of DNA—was still largely a mystery. This sparked a race to understand how DNA replication happens among several well-known experts. The experimental evidence of DNA replication, which showed that DNA rep 5 min read
Transcription of DNA Transcription of DNA is a cellular process where the genetic information encoded in DNA is converted into RNA. It initiates with RNA polymerase binding to the DNA at a specific promoter region. Then, the enzyme unwinds the DNA and synthesizes a complementary RNA strand by following the DNA template. 6 min read
Genetic Code - Molecular Basis of Inheritance CBSE Class12- Molecular Basis Of Inheritance- Genetic Code: The sequence of nucleotides in deoxyribonucleic acid and ribonucleic acid which determines the amino acids sequence of proteins is known as Genetic code. DNA consists of information for protein sequences. RNA consists of four nucleotides: a 5 min read
Genetic Code and Mutations Genetic code and mutations are important to understand and explain the central dogma of biology. The set of rules governing how DNA sequences are translated into proteins is the genetic code. The four nucleotide bases adenine (A), thymine (T), guanine (G), and cytosine (C), which are organized in pa 5 min read
tRNA - the Adapter Molecule tRNA is also known as transfer RNA is a subtype of RNA, tRNA help in the protein synthesis process. tRNA carries the amino acid to the ribosome, which is the molecular machine that assembles the protein, and ensures that the amino acid is incorporated into the growing protein chain in the correct or 5 min read
RNA Translation The Central Dogma, claims that once "information" has transferred into protein, it cannot be retrieved. In greater detail, information transmission from nucleic acid to the nucleic acid or nucleic acid to protein may be conceivable, but transfer from protein to protein or protein to nucleic acid is 15+ min read
Lac Operon Lac operon consists of the genes that are required for the metabolism of lactose in a bacterium E. coli and some other enteric bacteria. The name Lac operon actually stands for lactose operon. Lac operon works only when the nutrient source lacks glucose and has only lactose as it takes more steps to 7 min read
Human Genome Project Human Genome Project was the world’s largest collaborative biological project that gave us the ability to examine the full genetic manual for creating a human being in nature. HGP was international scientific research that mainly aims to determine the base pairs that make human DNA, as well as the i 9 min read
What is DNA Fingerprinting? DNA Fingerprinting is a technique used to identify individuals by analyzing their unique DNA patterns. Studying the DNA Fingerprinting steps and process helps in understanding genetic relationships, solving crimes, and identifying individuals based on their unique DNA profiles. In this article, we w 10 min read

Chapter 6: Evolution

Origin of Life The origin of life on earth is one of the mysteries to mankind. According to a common man, life is gifted by god whereas scientists believe that life has originated from non-living matter by natural means. This mystery of whether life originated from non-living matter was solved by scientists Pirie. 4 min read
Evolution Of Life Forms – A Theory Evolution is a process of gradual changes in the heritable characteristics of a biological population, over successive generations, over a long period. (Population: - It is a group of individuals of the same species who live in the same area and can interbreed) Theories of EvolutionTill now, several 5 min read
Understanding Adaptive Radiation: Evolutionary Diversification Explained Adaptive radiation is a phenomenon observed in evolutionary biology, that involves the rapid diversification of species into various forms to exploit new ecological niches. This process leads to the exposure of multiple species with distinct adaptations, enhancing their survival in diverse environme 4 min read
Hardy-Weinberg Principle A system of guidelines for genetic inheritance is known as mendelian inheritance. A monk by the name of Gregor Mendel made the initial discoveries of genetics in the 1850s, and his findings were first published in 1866. People have been aware of how qualities are passed on from parents to their offs 13 min read
Evolution Of Humans - History, Stages, Characteristics, FAQs Humans, or Homo sapiens, are a species of upright-walking beings known for their cultural diversity, inhabiting the Earth's surface. Believed to have originated in Africa around 315,000 years ago, human evolution is a complex process involving the development of traits such as bipedalism and languag 6 min read

Chapter 7: Human Health and Disease

NCERT Notes on Class 12 Biology Chapter 7 - Human Health and Disease NCERT Chapter 7 of Class 12 Notes on Human Health and Disease: According to the World Health Organisation, health can be defined as a state of complete physical, mental, and social well-being and not merely the absence of disease and infirmity. Good health has many benefits like it helps to keep us 15+ min read
Common Diseases In Humans Disease: - A disease is a physiological condition in which the human body fights against the external or internal causes of infection. On the basis of externally caused diseases, various examples are present, ranging from bacteria, viruses, protozoans, helminths, and many more. Pathogen: - The patho 5 min read
Immunity - Definition, Types and Vaccination Immunity is a defense mechanism of the body that is provided by the immune system and helps in fighting disease-causing organisms. There are two immunity types: innate and acquired immunity. Immunity-enhancing foods help boost the body's immune system Vaccination also enhances immunity by exposing t 11 min read
Innate And Acquired Immunity The immune system fights against germs and foreign substances on the skin, in the body's tissues, and in bodily fluids such as blood. The overall ability of the host to fight the disease-causing organisms conferred by the immune system is called Immunity. The immune system can be broadly categorized 5 min read
What are HIV and AIDS? AIDS (Acquired Immune Deficiency Syndrome) is a sexually transmitted disease caused by HIV (Human immunodeficiency virus). HIV HIV (human immunodeficiency infection) is an infection that assaults cells that assist the body with battling contamination, making an individual more powerless against diff 8 min read
Difference Between Vaccination And Immunization The difference between vaccination and immunization is that vaccination has to be given externally from outside the body. The vaccination process involves the introduction of a vaccine into the individual whereas immunization involves producing antibodies against vaccines containing weak pathogens. 5 min read
What is Cancer? Introduction, Types, Stages, Treatment Health is "a condition of total physical, mental, and social well-being and not only the absence of disease or disability," according to the World Health Organization. Over time, several definitions have been employed for various objectives. Healthy behaviors can be encouraged, such as regular exerc 14 min read
Alcohol and Drug Abuse Prevention Control As opposed to the normal thoughts pervasive in general society, substance use is very far-reaching. So is substance misuse. It's anything but a little issue, confined to the domain of the feeble and detestable. The utilization of medications rises above race, orientation, age, or financial status. T 10 min read
Adolescence and Drug Abuse Adolescence is a critical period of development during which individuals experience physical, emotional, and social changes. However, it is also a time when many young people may be exposed to drugs and alcohol, which can have serious and long-lasting effects on their health and development. In this 5 min read
Addiction And Dependence Human health refers to a person's overall well-being and physical and mental health. It includes various aspects such as physical, emotional, social, and mental well-being and can be influenced by factors such as genetics, lifestyle, environment, and access to medical care. This includes healthy beh 7 min read

Chapter 8: Microbes In Human Welfare

Microbes in Human Welfare Notes CBSE Class 12 Chapter 8 Microbes in Huaman Welfare: Microbes are the smallest living organisms that can only be seen under the microscope. Microbes are found everywhere. Examples- are air, water, soil, inside and outside the bodies of plants and animals, thermal vents (1000 degree Celsius), under th 6 min read
Microbes In Human Welfare Microbes are microscopic organisms, that can be classified under protozoa, bacteria, fungi, and microscopic plants viruses, viroid, and prions (proteinaceous infectious agents). They are present everywhere– in soil, water, and air, inside our bodies, animals, and plants. Not only in life forms, but 6 min read
Biofertilizers Biofertilizers are biologically active substances that help in enriching the soil's fertility. Biofertilizers are microbes or microbial products. It helps to reduce the use of chemical fertilizers. Reducing the use of chemical fertilizers from the environment biofertilizers helps to protect the ecos 8 min read

Chapter 9: Biotechnology _ Principles And Processes

NCERT Notes Biology Class 12 Chapter 9 Biotechnology: Principles and Processes NCERT CBSE Class 12th Science Notes Chapter 9 Biotechnology: Principles and Processes: Biotechnology Principles and Processes is an important part of Class 12 Science Notes for quick revision. They will benefit from having challenging study material to use in preparing for the exam. Students can get 15+ min read
Restriction Enzymes Restriction enzyme is a bacterial protein that cleaves DNA at particular locations, these sites are called restricted sites. The restriction enzymes guard against bacteriophages in living bacteria. They identify the bacteriophage and cleave it at its restriction sites, destroying its DNA. Important 8 min read
Competent Host in Recombinant DNA Competent Host - For Transformation With Recombinant DNA: Competent Host refers to a living organism, such as bacteria or yeast that has been modified or treated in such a way that it can uptake and express foreign DNA molecules. These competent hosts are commonly used in the process of genetic tran 3 min read
Recombinant DNA Technology DNA is a collection of molecules that is in charge of transporting and passing genetic information from parents to offspring. DNA is the genetic material of a cell that carries information from generation to generation. It is essential for the survival of the cell. For the betterment of an individua 10 min read

Chapter 10: Biotechnology and Its Application

CBSE Class 12 Biology Biotechnology And Its Application Revision Notes CBSE Class 12 Chapter 10 Biotechnology and Its Applications: Biotechnology refers to the production of biopharmaceuticals and biologicals on a large scale, which involves using genetically modified organisms such as microbes, fungi, plants, and animals. Biotechnology has various applications, includ 11 min read
Application of Biotechnology Biotechnology is an applied branch of science and Biotechnologists use a living organism and its systems, or its products, to improve the quality of life of people. It is a highly advanced branch in today’s world that utilizes genetic, molecular, and cellular processes to bring about signific 7 min read
Genetically Engineered Insulin Genetically Designed Insulin, commonly known as recombinant insulin, is a type of insulin created using genetic engineering techniques. Insulin is a hormone that regulates blood sugar levels in the body and is vital for diabetics who cannot make enough insulin on their own. Prior to the development 6 min read
Biotechnology And Its Application- Gene Therapy BiotechnologyBiotechnology is focused on the large-scale production of biopharmaceuticals, including microorganisms, fungi, plants, and animals that have undergone genetic modification. Its applications include the fields of medicine and diagnostics as well as food processing, GM crops for agricultu 10 min read
Molecular Diagnosis Biotechnology is the application of biological processes, organisms, cells, and molecular biology to technology, engineering, and medicine. It involves using biological systems and techniques to develop new products and processes that can improve human health, food production, and the environment. B 7 min read
Transgenic Animals Notes - Biotechnology And Its Application CBSE Class 12- Biotechnology And Its Application - Transgenic Animals: When a foreign gene inserts into the genome of the animals to alter its DNA or the animals with a modified genome are Known as Transgenic Animals. It is a method that helps to improve the genetic traits of targeted animals. Trans 4 min read
Ethical Issues Related to Genetically Modified Organisms Genetically Modified Organisms (GMOs) are used in laboratories for research to know the organism and its function in a better way. Biotechnology is the field of study that involves the application of biological systems, organisms, or cells to make products that benefit human beings. Genetically Modi 7 min read

Chapter 11: Organisms And Populations

Organism and Population Notes Class 12 Biology Chapter 11 CBSE Class 12 Organisms and Population: The study of organisms and populations is an important area of biology known as ecology. An organism is a single living individual that is capable of carrying out all basic life processes. Organisms can be unicellular, or multicellular. Whereas population refe 9 min read
Responses To Abiotic Factors - Organisms And Populations Responses to abiotic factors are the ways in which living organisms react and adapt to changes in the non-living components of their habitat. There are majorly four abiotic factors, namely, Temperature, Water, Light and Soil which affect living organisms. Living organisms show responses to these abi 6 min read
What is Adaptation? Adaptation refers to a change in an organism's structure and function as a result of a natural process that makes the organism more suited to endure and proliferate in a given environment. Adaptation occurs in plants and animals, allowing them to adjust well within a given environment. E.g. Dessert 9 min read
Population Attributes - Overview Notes- Class 12 Population Attributes are the characteristics used to define a population. Population attributes play an essential role in understanding the dynamics and characteristics of a population. These attributes are measured for a population, not for an organism. There are five important attributes of a pop 6 min read
Population Growth - CBSE Class 12 Population Growth: Population growth refers to the increase in the number of people in a given area in a particular period of time. It is the main cause of this world because the population of human beings is not a static factor. Population growth depends on various factors such as weather, food ava 6 min read
Population Interactions Population interaction in the ecosystem occurs between the populations which interact with one another living in a community. Population interactions are divided into several types. There are two kinds of factors- biotic and abiotic factors. Different kinds of population interaction affect a lot to 4 min read
Predators One species completely depends on the other in this connection for food and survival. The species that are fed upon is known as the prey, while the species that feeds on another species is known as the predator. Predation is the name given to the entire relationship. Because the predator is typicall 6 min read
What Is Parasitism? Definition, Types and Examples Parasitism is a kind of symbiosis, a close and continuous long-term biological relationship between two species, where one organism, the parasite, lives on or inside another organism, the host. The parasite may have a negative impact on the host's health and is adapted structurally to this way of li 7 min read
Commensalism A commensal symbiosis is a symbiotic relationship between two species in which one species benefits from association, while the other species neither benefits nor harms. What is Commensalism? In other words, an organism that benefits from the relationship has a positive effect on its survival or rep 7 min read
Mutualism Mutualism is a type of symbiotic relationship in which both species involved are benefited from interaction. In mutualism, each species provides something of value to the other. This type of relationship is critical to the survival of one or both species and plays a role in shaping ecosystems and in 5 min read

Chapter 12: Ecosystem

Ecosystem Notes Class 12 Biology Chapter 12 Class 12 CBSE Biology Chapter 12- Ecosystem: Living organisms interact with one another and their physical surroundings in a functioning ecosystem. The size of ecosystems can vary, from little ponds to enormous forests or seas. According to some ecologists, the entire biosphere is made up of all the 13 min read
What is Ecosystem? Definition, Structure, Types, and Functions The ecosystem term was first coined by an ecologist Arthur Tansley in 1935. The ecosystem is a balance or equilibrium between living and non-living factors of the ecosystem where they tend to interact with each other. All living things, including plants, animals, and microorganisms, depend on non-li 12 min read
Energy Flow of Ecosystem The energy flow of ecosystem means the pathway energy takes to move from one organism to another in an ecosystem. The energy flow of an ecosystem is a fundamental concept of ecological studies. The direction of flow of energy in an ecosystem is unidirectional and is typically in the form of food ene 8 min read
Ecological Pyramid - Definition, Types, Importance, Limitations An ecological pyramid is a graphical representation of the relationship that every living creature present at different levels of the ecosystem shares with each other. Ecological Pyramids represent the different forms of bio-productivity of an ecosystem i.e. how much biomass, energy, or number of in 8 min read
Ecological Succession - Definition, Types, Characteristics, Causes Ecological succession is the process by which the structure and composition of a biological community change over time. Each of the ecological succession stages is characterized by different species compositions and environmental conditions. Understanding ecological succession and its types helps in 7 min read
What is Nutrient Cycling? To survive, organisms need nutrients. The natural recycling process is called the nutrient cycle. From one organism to the next, an element travels in a circular pattern. Recycling is the ecological process that supports and makes additional contributions to human welfare. Nutrient CycleThe term "nu 7 min read
Carbon Cycle The Carbon cycle is a type of Biogeochemical Cycle. The carbon cycle definition states that it is a natural process of a continuous cycle of carbon on the planet. The carbon cycle steps maintain the balance of carbon within the environment. It is a complex web of interconnected processes that involv 8 min read
Phosphorus Cycle The phosphorus cycle is a natural phenomenon by which phosphorus cycles through the three components of the biosphere which are the hydrosphere, lithosphere, and atmosphere. The phosphorus cycle is a very gradual process. The phosphorus cycle steps include weathering, release of phosphates into soil 7 min read
Types of Ecosystem Services - CBSE Class 12- Ecosystem Types of Ecosystem Services: Ecosystem Services is an effort sponsored by the UN in order to study and analyze the impact of human actions on the ecosystem because humans directly interact with the ecosystem and derive a number of benefits from the ecosystem and these actions of humans also lead to 5 min read

Chapter 13: Biodiversity and Its Conservation

Biodiversity and Conservation Notes Class 12 Chapter 13 CBSE Class 12 Science Notes Chapter 13 Biodiversity and Conservation are an important part of Class 12 Science Notes for quick revision. They will benefit from having challenging study material to use in preparing for the exam. Students can get CBSE Class 12th Science Notes Chapter 13 Biodiversity a 11 min read
How Many Species Are There On Earth And How Many In India? Biodiversity refers to the variety of life on earth, including the different species of plants and animals, the ecosystems they inhabit, and the genetic variation within each species. Biodiversity plays a critical role in maintaining the balance of our planet's ecosystem and the survival of life on 4 min read
Pattern of Biodiversity The word "biodiversity" refers to the variety of life on Earth at all its levels, from genes to ecosystems, and can cover the evolutionary, ecological, and cultural processes that support life. The term "biodiversity" refers to a wide range of living things, from people to microorganisms, fungi, and 6 min read
In-Situ and Ex-Situ Conservation of Biodiversity In-situ and ex-situ conservation of biodiversity are two approaches to the conservation of biodiversity. In-situ conservation mainly focuses on protecting the organism in its natural habitat whereas ex-situ conservation mainly focuses on protecting the organism by relocating it into an ideal protec 8 min read

NCERT Solutions

NCERT Solutions for CBSE Class 12 Biology CBSE Class 12th Biology NCERT Solutions 2023-24 is available here. These solutions will help students in their preparation for Class 12th CBSE Boards exams for the year 2023-2024. NCERT Solutions gives a detailed explanation of questions in the NCERT textbooks. By studying these solutions you will a 11 min read
Sexual Reproduction in Flowering Plants NCERT Solutions *As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 2, has now been renumbered as Chapter 1. Stay updated with the latest changes in the curriculum. Sexual Reproduction in Flowering Plants Class 12 NCERT Solutions is all about the process of Sexual Repr 12 min read
NCERT Solutions for Class 12 Chapter 2 Human Reproduction As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 3, has now been renumbered as Chapter 2. Stay updated with the latest changes in the curriculum. NCERT Solutions for Class 12 Chapter 2 Human Reproduction is all about the process of sexual reproduction 12 min read
NCERT Solutions for Class 12 Biology Chapter 3 Reproductive Health As per the revised curriculum of CBSE Syllabus 2023-24, the Reproductive health chapter, previously known as Chapter 4, has now been renumbered as Chapter 3. Stay updated with the latest changes in the curriculum. NCERT Solutions for Class 12 Biology Chapter 3 Reproductive Health is all about the im 15 min read
NCERT Solutions Class 12 Biology Chapter 4 Principles of Inheritance and Variation As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 5, has now been renumbered as Chapter 4. Stay updated with the latest changes in the curriculum. Principles of Inheritance and Variation Class 12 NCERT Solution is all about the process and principle of 15+ min read
NCERT Solutions of Class-12 Biology Chapter-5: Molecular Basis of Inheritance Molecular Basis of Inheritance Class 12 NCERT Solution is all about the process of inheritance at the molecular level. These NCERT Solutions are prepared by our Top Biology Experts in order to take care of all Important Topics that might be asked in the upcoming examination 2023. So, Students can al 9 min read
NCERT Solutions for Class 12 Biology Chapter 6 Evolution *As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 7, has now been renumbered as Chapter 6. Stay updated with the latest changes in the curriculum. Evolution Class 12 NCERT Solution is all about the process of Evolution, the benefits of evolution reaso 8 min read
NCERT Solutions for Class 12 Biology Chapter 7 Human Health and Disease As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 8, has now been renumbered as Chapter 7. Stay updated with the latest changes in the curriculum. Human Health and Disease Class 12 NCERT Solutions are all about human health and human-related disease. T 10 min read
NCERT Solutions for Class 12 Biology Chapter 8 As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 10, has now been renumbered as Chapter 8. Stay updated with the latest changes in the curriculum. Microbes in Human Welfare Class 12 NCERT Solution is all about microbes and their usage in daily life. T 8 min read
NCERT Solutions Class-12 Chapter 9 Biotechnology: Principles and Processes *As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 11, has now been renumbered as Chapter 9. Stay updated with the latest changes in the curriculum. Biotechnology: Principles and Processes Class 12 NCERT Solution is all about the process of Biotechnolo 8 min read
NCERT Solutions for Class 12 Biology Chapter 10 Biotechnology and Its Applications *As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 12, has now been renumbered as Chapter 10. Stay updated with the latest changes in the curriculum. Biotechnology and Its Applications Class 12 NCERT Solution is all about the process of Biotechnology a 7 min read
CBSE Solutions for Class 12 Biology Chapter 11 Organisms and Populations *As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 13, has now been renumbered as Chapter 11. Stay updated with the latest changes in the curriculum. Organism and Populations Class 12 NCERT Solution is all about the basics of organisms, populations, an 9 min read
CBSE Solutions for Class 12 Biology Chapter 13 Biodiversity and Conservation *As per the revised curriculum of CBSE Syllabus 2023-24, this chapter, previously known as Chapter 15, has now been renumbered as Chapter 13. Stay updated with the latest changes in the curriculum. Biodiversity and Conservation Class 12 NCERT Solution is all about biodiversity its importance and dif 7 min read
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Today we know that this is the pattern used by living cells, but the experimental evidence in support of semiconservative replication was not published until 1958 . In the 5 years between Watson and Crick's suggestion and the definitive experiment, semiconservative replication was controversial and other patterns were considered.

Three hypothesized patterns were proposed:

Semiconservative - The original double strand of DNA separates and each strand acts as a template for the synthesis of a complimentary strand.
Conservative replication - the original double strand of DNA remains intact and is used as a template to create a new double stranded molecule.
Dispersive replication - similar to conservative replication in that the original double strand is used as a template without being separated, but prior to cell division, the strands recombine such that each daughter cell gets a mix of new and old DNA. With each round of replication, the original DNA gets cut up and dispersed evenly between each copy.

The methods Meselson and Stahl developed allowed them to distinguish existing DNA from newly synthesized DNA and to track new and old DNA over several rounds of replication.

They accomplished this by labeling cells with different stable isotopes of nitrogen. First, a culture of bacterial cell were grown for several generations in a media containing only 15 N ( a stable, heavy isotope of Nitrogen). After this period * of growth, all of the DNA in the cells contained 15 N. These cells were then rinsed and put into a media containing only the more common, lighter isotope of nitrogen ( 14 N). As the cells grew and divided in this fresh media, all newly synthesized DNA would contain only the lighter nitrogen isotope, while DNA from the original cells would still contain 15 N. In this illustration above, 15 N labeled DNA is shown in orange and 14 N labeled in green.

The 15 N and 14 N labeled DNA was then tracked using high speed centrifugation and a density * gradient created with cesium chloride (CsCl).

During centrifugation in a CsCl gradient, DNA accumulates in bands along the gradient based on its density. Since 15 N is more dense than 14 N, 15 N enriched DNA accumulates lower down in the centrifuge tube than the 14 N DNA. DNA containing a mixture of 15 N and 14 N ends up in an intermediate position between the two extremes.

By spinning DNA extracted at different times during the experiment, Meselson and Stahl were able to see how new and old DNA interacted during each round of replication.

The beauty of this experiment was that it allowed them to distinguish between the three different hypothesized replication patterns. The key result occurs at the second generation when all three proposed replication patterns give different results in the CsCl gradient.

That Meselson and Stahl's experiment showed the pattern predicted by the semiconservative hypothesis provided the definitive experimental evidence in support of the process proposed by Watson and Crick.

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Book Review
Published: 01 March 2002

Meselson, Stahl and the Replication of DNA: A History of “The Most Beautiful Experiment in Biology”

Bruce Stillman 1

Nature Medicine volume 8 , page 211 ( 2002 ) Cite this article

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Meselson, Stahl and the Replication of DNA

Frederic Lawrence Holmes

Great experiments either prove a previous notion, or they reveal unexpected results that lead to new ideas. In science, ideas are propagated and the very best of them survive for many years, if not forever. Experiments are, by their very nature, often transitory and useful but for a moment in time. However, at least one notable experiment is an exception: the famous Meselson and Stahl experiment. In a recent labor of love, Frederic Lawrence Holmes delves into this experiment, telling us how it came about, how it was conceived, how it was executed and what it meant at the time. Along the way, one gets a glimpse of what it was like to do science in the earliest days of molecular biology and a sense of the social aspects of science in those heady times.

The second of the famous papers by Jim Watson and Francis Crick deals with the implications of the double-helix structure for inheritance and states that “each chain then acts as a template for the formation on to itself of a new companion chain so that eventually we shall have two pairs of chains, where we only had one before”. Therein, they proposed that the DNA unwound and each strand was a template for the synthesis of a complementary strand, begetting two identical helices. They suggested that DNA might replicate in a semi-conservative manner, rather than the alternative conservative mode whereby the parental double helix remained intact and the new double helix was identical to the parent, but composed of entirely new strands. The Meselson and Stahl experiment demonstrated that Watson and Crick were correct in their assumption.

It seems difficult these days to comprehend that there was ever any doubt about how DNA must replicate. But masterfully, and in great detail, Holmes takes us back to the discourse that emerged immediately after the double-helix revelation. Many were concerned about what the great Max Delbrück thought of the double helix, and although he enthusiastically spread the word about its structure, true to form, Max had a problem: the “untwiddling problem”. How could the two strands that were intertwined so many times separate during replication? He was not only concerned about the problem, as were Watson and Crick, but he proposed a complicated (and incorrect) solution in a Proceedings of the National Academy of Sciences paper in the Spring of 1954.

Holmes' well-written book describes every detail from thenceforth. The chance meeting of Matt Meselson and Frank Stahl at Woods Hole, the seminar by Monod that induced Meselson to think about density transfer, the trials of experimentation and of course the “beautiful experiment” itself. Although dense, the story is worth reading to understand what science was like in the 1950s and how a great experiment came about. It also describes the environment at Caltech during that era, scientifically exciting, but socially bleak. I assume the social environment in Pasadena has improved, but clearly the science there remains as strong as it was. Students who do science, or those who study the process will learn much from this book on how great science can be accomplished.

What struck me while reading this treatise was the remarkably open exchange of ideas between the early phage investigators, via letters and discussions at meetings. Scientists traveled (and reveled) more than I would have thought, a common thread that has emerged in other books I have read about the early phage days. For example, Holmes reports that Meselson and Stahl wrote many times to Jim Watson and others about the design and progress of their experiments. Obviously Watson had a more than passing interest in the matter, but more interestingly, Meselson and Stahl wrote to and visited Gunther Stent at Berkeley to discuss their progress. They did this even though Stent was working on the replication problem and favored the Delbrück proposal that DNA replication was not semi-conservative. We should learn from history, because unfortunately, in modern molecular biology where scientists are not as technique-limited as they once were, the free exchange of ideas is in danger of being lost.

The measure of a great technique is what it reveals and whether it lasts. The Meselson and Stahl experiment is still in wide use today. It has been used to demonstrate the distributive nature of histone deposition during chromosome replication and most recently to study the mechanism and timing of replication of the entire genome of the yeast Saccharomyces cerevisiae . Very few experimental methods have survived as long as the density-transfer idea. Thus, I expect that Holmes' book will be read for many years to come, and justifiably so.

Also reviewed by Sydney Brenner

Salk Institute for Biological Studies La Jolla, California, USA

In these days of high throughput science, when advances in technology have literally given us the power to make atom-by-atom descriptions of all living matter, it is refreshing to look back at an earlier time, when advances in science required both a good idea and the means to show it was true. We were like Houdinis, strapped in chairs with our hands tied behind our backs trying to escape from locked rooms. This book is the history of the Meselson–Stahl experiment—the most beautiful experiment in biology—and reconstructs both the background and the event itself in a most meticulous and admirable way. Although we learn about the revolution in biology consequent upon the discovery of the double helix, it is not history in the large but rather history on the minute scale of what actually happened in the creation and execution of the experiment. The author has had access both to the notebooks and the memories of the scientists as well as to others and he has marshaled all of this detail into a narrative that is interesting and informative.

When the double helical structure of DNA was proposed, the intertwining of the strands created an objection in the minds of some who became concerned that the strands would have to be unwound in order for them to be replicated. Max Delbruck, in particular, was most troubled by it. It was fortunate that, at the time, people did not know that there were DNA molecules that were closed circles, because they would have declared the replication model proposed by Watson and Crick impossible. Somewhere the book says that there was a small band of enthusiastic supporters who were not troubled by this difficulty. I was one of them and took the view that if it were a problem, biological systems would have found a way to solve it. Indeed, I think it was Leslie Orgel who said that nature would have invented an enzyme to do it, a most perceptive insight.

The consequences of the replication model were clear: after one replication step two molecules would be present, each with one old and one new strand. How could one prove this? I met Matt Meselson outside Blackford Hall in Cold Spring Harbor in September 1954 when he had already conceived of the idea of doing the experiment with heavy isotopes using some sort of density centrifugation to separate the molecules. Frank Stahl knew how to work with phages and the partnership was formed. However, Meselson had to complete his PhD thesis research in crystallography, and while making the transition from physical chemistry to biology, he kept detailed notes about what he was reading in a workbook. The evolution of his thinking can be followed from these books.

After spending time trying to do the transfer experiment with 5-bromouracil-labeled bacteriophage T2, density-gradient ultracentrifugation became possible and they switched to using bacteria and 15 N labeling. They were able to show that the difference in density between light 14 N- and heavy 15 N-labeled DNA was sufficient to allow a molecule of intermediate density to be resolved, whereupon Meselson decided to do a double-transfer experiment from heavy to light and light to heavy medium against the advice of Stahl who had to go to an interview in Missouri. Meselson also added several controls and labeled the tubes from this large series of experiments with a complicated code before proceeding to analyze them in the ultracentrifuge. His memory was that the experiment had worked, but an examination of the original films showed that his recollection of the result was wrong. None of the films showed the expected three bands that Meselson thought he saw when he rushed over to announce the result at a party being held at his house. Of course, later experiments gave the expected result.

It could be said that if historians have the benefit of hindsight, scientists have the advantage of foresight. Meselson had sketched the expected result before doing the experiments and I think he superposed in his mind the individual results of his experiments to generate an answer compatible with it. All experimentalists know you have to do an experiment four times. The first one is a complete mess and shows only a hint that it might have worked. The second one is better but still messy. Then you do it the third time for the book. This is when you forget to add a reagent, or mix up the tubes or the centrifuge leaks. That is why there is always a fourth time.

I urge every young scientist to read this book. In 1957, when the experiment was performed, Meselson was 27 and barely with a PhD in chemistry. Frank Stahl was 28 and a postdoctoral fellow at the California Institute of Technology. Both were doing an experiment that had nothing to do with their official programs of research. They simply went ahead and did it. They filled out no forms, made no applications, had no reviews. They only had the judgments of their real scientific peers.

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Stillman, B. Meselson, Stahl and the Replication of DNA: A History of “The Most Beautiful Experiment in Biology”. Nat Med 8 , 211 (2002). https://doi.org/10.1038/nm0302-211

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Meselson-Stahl experiment
The Meselson-Stahl experiment is an experiment by Matthew Meselson and Franklin Stahl in 1958 which supported Watson and Crick's hypothesis that DNA replication was semiconservative.In semiconservative replication, when the double-stranded DNA helix is replicated, each of the two new double-stranded DNA helices consisted of one strand from the original helix and one newly synthesized.
Meselson and Stahl Experiment
Meselson and Stahl experiment gave the experimental evidence of DNA replication to be semi-conservative type.It was introduced by the Matthew Meselson and Franklin Stahl in the year 1958.Matthew Meselson and Franklin Stahl have used E.coli as the "Model organism" to explain the semiconservative mode of replication. There are three modes of replication introduced during the 1950s like ...
The Meselson-Stahl Experiment (1957-1958), by Matthew Meselson and
The Meselson-Stahl experiment stemmed from a debate in the 1950s among scientists about how DNA replicated, or copied, itself. The debate began when James Watson and Francis Crick at the University of Cambridge in Cambridge, England, published a paper on the genetic implications of their proposed structure of DNA in May 1953. ...
The Meselson And Stahl Experiment on DNA Replication
Experiment. 15 N (heavy) and 14 N (normal) are two isotopes of nitrogen, which can be distinguished based on their densities by centrifugation in Ca,esium chloride (CsCl). Meselson and Stahl cultured E.coli in a medium constituting 15 NH 4 Cl over many generations. As a result, 15 N was integrated into the bacterial DNA. Later, they revised the 15 NH 4 Cl medium to normal 14 NH 4 Cl.
PDF MeselsonandStahl:TheartofDNA replication I
placed Meselson and Stahl's experiment on the pages of many a syllabus. At the Massachusetts Institute of Technology (Cambridge, MA), Professor of Biology Tania Baker says the experiment is part of a course required of all molecular biology graduate students. ''It is a very nice test of a model of replication,'' she says.
Meselson and Stahl Experiment-DNA Replication
Meselson and Stahl Experiment. Before we dive into the details of the Meselson and Stahl experiment, let's first understand why it was such an important experiment in the field of molecular biology. At the time of the experiment, there was a lot of debate about how DNA replication occurred.
Semi-Conservative DNA Replication: Meselson and Stahl
Meselson and Stahl opted for nitrogen because it is an essential chemical component of DNA; therefore, every time a cell divides and its DNA replicates, it incorporates new N atoms into the DNA of ...
Meselson-Stahl Experiment
The methods Meselson and Stahl developed allowed them to distinguish existing DNA from newly synthesized DNA and to track new and old DNA over several rounds of replication. ... By spinning DNA extracted at different times during the experiment, Meselson and Stahl were able to see how new and old DNA interacted during each round of replication.
PDF The Most Beautiful Experiment in Biology The Replication of DNA in
Met Matthew Meselson in 1954. *To find more information on the various awards, see the Historical Timelines of Meselson and Stahl In 1957 he, along with Matthew Meselson, developed the technique of density gradient centrifugation, which led to the realization that DNA replication is semi-conservative through their experiment with E.coli DNA.
Meselson, Stahl and the Replication of DNA: A History of ...
The Meselson and Stahl experiment demonstrated that Watson and Crick were correct in their assumption. It seems difficult these days to comprehend that there was ever any doubt about how DNA must ...

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