Question

Explain the Semi-conservative mode of DNA replication with the Meselson and Stahl experiment.

Hint:

The Meselson and Stahl experiment is known as the {“most beautiful experiment in biology”} because it clearly demonstrated the {semi-conservative nature of DNA replication}.

Answer 1

Concept: DNA replication is the process by which a DNA molecule produces two identical copies of itself. According to the semi-conservative model, each daughter DNA molecule contains one original (parental) strand and one newly synthesized strand.
Step 1:Semi-conservative Mode of Replication During DNA replication:

• The double helix of DNA unwinds and separates into two strands.
• Each original strand acts as a template for the formation of a new complementary strand.
• As a result, each daughter DNA molecule contains one old strand and one new strand.

Thus, the replication is called semi-conservative because half of the parental DNA is conserved in each new molecule.
Step 2:Meselson and Stahl Experiment In 1958, Meselson and Stahl conducted an experiment using the bacterium {E. coli to prove the semi-conservative mode of DNA replication.

• Bacteria were first grown in a medium containing heavy nitrogen \((^{15N)\), which was incorporated into their DNA.
• These bacteria were then transferred to a medium containing normal nitrogen \((^{14N)\).
• DNA samples were taken after successive generations and analyzed using density gradient centrifugation.

Step 3:Results of the Experiment

• After the first generation, DNA showed an intermediate density (hybrid DNA containing one heavy and one light strand).
• After the second generation, two bands appeared: one intermediate and one light.

These results confirmed that each DNA molecule contained one old strand and one newly synthesized strand, supporting the semi-conservative model of DNA replication.