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 which were 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.
Then 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 their experiments after the discovery of DNA structure by the two scientists Watson and Crick. This experiment is the most accepted model to demonstrate the replicative model of DNA.
Content: Meselson and Stahl Experiment
- Definition of Hershey and Chase Experiment
- Semi Conservation Model of DNA
- Meselson and Stahl Experiment Steps
Definition of Meselson and Stahl Experiment
Meselson and Stahl Experiment gave us the theory or evidence 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 15NH4Cl 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 15N heavy isotope. Meselson and Stahl then transferred the E.coli cells incorporated with 15N isotope to the medium containing 14NH4Cl for several generations.
After every twenty minutes, the E.coli cells will multiply. For the processing of DNA, the cells were centrifuged by the addition of Caesium chloride. Caesium chloride creates the concentration gradient, which will separate the light, intermediate and heavy DNA strands. 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 will conserve for every next generation in such a way like 100%, 50%, 25%, and 12.5% and so on.
Semi Conservation Model of DNA
It is the type of DNA replication. The term semi means “Half” and conservative means “To store”. Therefore the semi-conservative DNA replication can define as the process where the two daughters DNA produce as a result of replication of parent DNA.
In the two daughter DNA’s, each strand will contain a mixture of template strand of the parent DNA, 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 will conserve 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% will conserve by having parental strands, and the remaining 75% will produce newly synthesized strands.
Meselson and Stahl Experiment Steps
Meselson and Stahl performed a series of an experiment which includes the following steps:
Growth of E.coli: First, the E.coli were grown in the medium containing 15NH4Cl for several generations. NH4 provides the nitrogen as well as a protein source for the growth of the E.coli. Here the 15N is the heavy isotope of nitrogen.
Incorporation of 15N: After several generations of E.coli, Meselson and Stahl observed that the 15N heavy isotope has incorporated between the nucleotides of the DNA in E.coli.
Transfer of E.coli cells: The DNA of E.coli labelled with 15N isotope were transferred to the medium containing 14NH4Cl. Here the 14N 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, which is having a density of 1.71 g/cm3 the same as DNA. And, finally, the tubes were subjected to high-speed centrifugation 140,000 X g for 20hours.
After centrifugation, the DNA is separated based on mass or density. Different DNA bands like heavy, intermediate and light DNA forms as a result of the concentration gradient created by CsCl.
The light DNA will consist of a pure 14N isotope. An intermediate DNA band will indicate the combination or mixture of both 15N and 14N isotopes. The occurrence of heavy DNA bands will consist of a pure 15N isotope.
The result, after two generations of E.coli, the following results were obtained:
In the F-1 generation: According to the actual observations, in F-1 gen, two DNA strands have produced containing a mixture of both 15N and14N isotopes. In the above diagram, we can see that both the semiconservative and dispersive model follows 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 and produce one newly synthesized DNA and one parental DNA. Therefore, the conservative model disapproved as it does not produce hybrid DNA in the F-1 generation.
In the F-2 generation: According to the actual observation, the F-2 generation will produce four DNA strands where two contains hybrid and the remaining two with light DNA. The hybrid DNA includes a mixture of 15N and 14N. The light DNA strands contain a pure 14N. In the diagram, we can see 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”. In the semi-conservative model, the offspring produces a hybrid DNA containing a mixture of both template and newly synthesized DNA. After each multiplication, the number of offspring will get double, and half of the parental DNA will be conserved for the next generation.