Question

How Meselson and Stahl explained the concept of Semiconservative Replication of DNA experimentally?

Answer 1

The Meselson and Stahl experiment is as follows: 

1. They cultured E. coli bacteria in the medium containing ¹⁴N (light nitrogen) and obtained an equilibrium density gradient band by using 6M CsCl₂. The position of this band was recorded.
2. E. coli cells were then transferred to a ¹⁵N medium (heavy isotopic nitrogen) and allowed to replicate for several generations. At equilibrium point density gradient band was obtained, by using 6M CsCl₂. The position of this band was recorded.
3. The heavy DNA (¹⁵N) molecule can be distinguished from normal DNA by centrifugation in a 6M Caesium chloride (CsCl₂) density gradient. The density gradient value of 6M CsCl₂ and ¹⁵N DNA is almost the same. Therefore, at the equilibrium point, ¹⁵N DNA will form a band. In this, both strands of DNA are labelled with ¹⁵N.
4. Such E. coli cells were then transferred to another medium containing ¹⁴N i.e. normal (light) nitrogen. After the first generation, the density gradient band for ¹⁴N ¹⁵N was obtained and its position was recorded. After the second generation, two density gradient bands were obtained - one at ¹⁴N ¹⁵N position and the other at ¹⁴N position.
5. The position of bands after two generations clearly proved that DNA replication is semiconservative.