a) Name the scientists who demonstrated that DNA replication take place in a sem
ID: 212648 • Letter: A
Question
a) Name the scientists who demonstrated that DNA replication take place in a semi-conservative manner in E. coli.
b) Explain how they did this.
c) Phosphorus is required to synthesize the dNTPs (nucleotides) used in DNA replication. A geneticist grows some E. coli in a medium containing non-radioactive phosphorus, and then transfers a sample to a medium containing radioactive phosphorus (32P). Sample of the bacteria are collected at the point of transfer (immediately), and after one and two rounds of DNA replication. Assume that all newly synthesized DNA contains 32P and all original DNA contains non-radioactive phosphorus. In each sample, will radioactivity be detected in neither strand, one strand or both? Give an answer for each sample.
Explanation / Answer
a] The semi-conservative mode of DNA replication was first proposed by Watson and Crick in 1953.
The experimental proof for this model of DNA replication was given by Matthew Meselson and Franklin Stahl in 1958. They performed experiments on E. coli to prove this .
b] The experiment that they performed included the following steps :
* They grew E. coli in a medium containing ^15NH4Cl [ in which ^15N is the heavy isotope of nitrogen] for many generations.
* As a result , ^15N got incorporated into newly synthesised DNA.
* This heavy DNA can be differentiated from normal DNA by centrifugation in caesium chloride [ CsCl] density gradient.
* Then they transferred the cells into a medium with normal with normal ^14NH4Cl and took the samples at various definite time intervals, as the cells multiplied.
* The extracted DNAs were centrifuged and measured to get their densities.
* The DNA extracted from the culture after one generation of transfer from ^15N medium to ^14N medium [ i.e after about 20 minutes ] showed an intermediate hybrid density .
* The DNA extracted from culture after rwo generations [ i.e. after 40 minutes] showed equal amount of light DNA and hybrid DNA .
Thus , it was experimentally proved that DNA replication is semi- conservative.
Similar experiments were performed by Taylor and colleagues in 1958, on Vicia faba to prove that DNA in chromosome also replicate semi-conservatively.
c] The sample of bacteria collected at the time of transfer will not possess any radioactive strand as they haven't got time to replicate.
The sample of bacteria collected after one generation will show a hybrid of radioactive and nonradioactive strands of DNA as the radioactive phosphorus will get incorporated into it.
The sample of bacteria collected after the second round of replication will possess one radioactive strand of DNA and one non radioactive strand as one of the DNA strands will be parental and one will be newly synthesised.
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