Question

Explain semi-conservation mode of replication of DNA with the help of one example.

Answer 1

Semi-conservative replication produces two daughter DNA molecules, each containing one parental strand and one newly synthesised strand.

Process

1. The two parental DNA strands unwind and separate as the hydrogen bonds between complementary bases break.
2. Each parental strand acts as a template. Free nucleotides pair with it according to the base-pairing rule: A pairs with T, and C pairs with G.
3. New strands are formed alongside the templates, producing two DNA molecules. Each contains one old and one new strand.

Evidence: Meselson–Stahl experiment

Meselson and Stahl grew E. coli for several generations in a medium containing heavy nitrogen ¹⁵N, then transferred the bacteria to a medium containing normal nitrogen ¹⁴N. After one round of replication, the DNA had intermediate density: each molecule contained one heavy parental strand and one light new strand. After a second round, both intermediate-density DNA and light DNA were present. This pattern supports semi-conservative replication.

Hence, each original strand is conserved in one daughter DNA molecule and guides the synthesis of its complementary strand.