Question

Describe the semiconservative method of DNA replication with diagram.

Hint:

Semiconservative replication = one parental strand + one new strand. Remember: Meselson–Stahl experiment is key proof.

Answer 1

Step 1: Definition.
The semiconservative method of DNA replication was proposed by Watson and Crick and experimentally proved by Meselson and Stahl (1958). According to this model, during replication each DNA molecule produces two daughter DNA molecules, each having: \[\begin{array}{rl} \bullet & \text{One original (parental) strand.} \\ \bullet & \text{One newly synthesized strand.} \\ \end{array}\] Thus, half (semi) of the parental DNA is conserved in each daughter molecule.

Step 2: Process of replication. \begin{enumerate} \item Initiation: The double helix unwinds at the origin with the help of helicase enzyme. Hydrogen bonds between bases break, forming replication forks. \item Elongation: Each parental strand acts as a template. DNA polymerase adds complementary nucleotides (A with T, G with C). \[\begin{array}{rl} \bullet & \text{Leading strand is synthesized continuously.} \\ \bullet & \text{Lagging strand is synthesized discontinuously as Okazaki fragments.} \\ \end{array}\] \item Termination: Okazaki fragments are joined by DNA ligase, resulting in two identical DNA molecules. \end{enumerate}

Step 4: Proof by Meselson and Stahl.
They used \textsuperscript{15}N heavy isotope in E. coli DNA and showed that after one generation, the DNA molecules had one parental (old) strand and one new strand, confirming the semiconservative nature.