Why DNA replication is called semi-conservative replication?(b) Name the scientist who experimentally proved the same.

Answer: (a) Each daughter DNA molecule keeps one old (parental) strand and one newly made strand. (b) Meselson and Stahl (1958).

(a) During replication the two strands of the double helix separate and each acts as a template for a new complementary strand. Hence each of the two daughter DNA molecules has one parental (conserved) strand and one newly synthesised strand. Since only half of the parent molecule is conserved in each daughter, replication is called semi-conservative.

(b) Matthew Meselson and Franklin Stahl proved it in 1958 by growing E. coli in a medium with heavy nitrogen (¹⁵N), transferring them to ¹⁴N medium and separating the DNA by density-gradient (CsCl) centrifugation. After one generation all DNA was of hybrid (intermediate) density; after two generations it was half hybrid and half light.