It is a process by which DNA produces daughter DNA molecules, which are exact copies of the original DNA. In each new DNA molecule, one strand is old (original) while the other is newly formed. Hence, Watson and Crick described this method as semiconservative replication.
Meselson and Stahl conducted experiments on E.coli to prove that DNA replication is semi conservative.
1. They grew E.coli cells in a culture medium containing radioactive isotopes N15 (NH4cl). The 15N was incorporated into the newly synthesized DNA and other nitrogen containing compounds.
2. Bacteria labeled with 15N are transferred into a medium containing normal 14NH4cl.
3. The analysis was done after every generation at regular intervals.
4. The DNA samples were separated by centrifugation on CSCl2 gradient.
5. The hybrid DNA separated from fresh generations possesses one heavier strand contributed by parental with 15N isotope which is radioactive and lighter strand is newly synthesized with 14N isotope, is non-radioactive.
6. The DNA of the second generation showed that in one DNA molecule one strand was radioactive (15N) and other was non radioactive (14N), Whereas both the strands of other DNA molecule were non-radioactive (14N).
7. It is clear that the DNA of the first generation is intermediate it contains both 15N and 14N.
8. In the second generation there were two types of DNA one heavier DNA with 15N and 14N isotopes while the lighter DNA with 14N and 14N, the ratios of two types of DNA was 50:50.
9. In the third generation the ratio of intermediate (15N and 14N) was reduced from 50% to 25% while the lighter fraction (14N and 14N) increased from (50% – 75%).
10. In the 4th generation the percentage of intermediate still reduced to 12.5% while that of lighter fraction increased from 75% to 87.5%.
11. These experiments prove that DNA shows semi conservative replication.
