What is 'semiconservative' DNA replictaion? How was it experimentally
proved and by whom?
Semiconservative DNA replication: The two strands separate as each acts as a template
for the synthesis of new complementary strands. After the completion of replication,
each DNA molecule has one parental and one newly synthesised strand. It is termed as 'semiconservative' DNA replication.
1. Messelson and Stahl proved the semiconservative nature of replication of DNA.
2. They grew E.coli bacteria in a medium containing the heavy nitrogen isotope
15N for many generations. This produced a population of bacterial cells that had
uniformly 15N labelled DNA, and this was heavier than the DNA obtained from E.coli
grown in 14N containing medium.
3. The bacterial cells with heavier DNA were then trasferred to a medium having the
ordinary 14N isotopes.
4. From the daughter cells of first generation, DNA was extracted, purified and centrifuged.
It was found that all the DNA molecules were hybrid 15N - 14N i.e. all were half heavy.
5. Daughter cells were allowed to divide again. The second generation cells were found
to have two types of DNA molecules, 50% half heavy with 15N - 14N hybrid density and
50% light with 14N - 14N density. This proved that DNA in chromosomes replicate
(Separation of DNA by centrifugation)
Messelson's and Stahl's experiment