Question

Explain the experimental proof of DNA replication by Meselson and Stahl.

Hint:

Remember: Meselson-Stahl → Hybrid DNA → Semi-conservative replication.

Answer 1

Concept: Meselson and Stahl (1958) experimentally proved that DNA replication is semi-conservative.
Step 1: Objective.
To determine whether DNA replication is conservative, semi-conservative, or dispersive.
Step 2: Experimental design.

• Bacteria (E. coli) were grown in a medium containing heavy nitrogen ($^{15}$N).
• DNA became labelled with heavy nitrogen.
• These bacteria were then transferred to a medium containing light nitrogen ($^{14}$N).

Step 3: Observations.

• After one generation: DNA showed intermediate density (hybrid DNA: $^{15}$N + $^{14}$N).
• After two generations: Two bands appeared
  • One light DNA ($^{14}$N-$^{14}$N)
  • One intermediate DNA ($^{15}$N-$^{14}$N)

Step 4: Interpretation.

• Each new DNA molecule contains one parental strand and one newly synthesized strand.

Step 5: Conclusion.
Thus, DNA replication is semi-conservative in nature.