Question

In Meselson--Stahl's experiment, if DNA of E.coli is extracted after 40 minutes, what will be the densities of DNA molecules formed (2nd generation)?

• A. 25% hybrid and 75% light
• B. 75% hybrid and 25% light
• C. 40% hybrid and 60% light
• D. 50% hybrid and 50% light

Hint:

20 minutes $\rightarrow$ 100% Hybrid 40 minutes $\rightarrow$ 50% Hybrid + 50% Light 60 minutes $\rightarrow$ 25% Hybrid + 75% Light

Answer 1

The Correct Option is D

Concept: Meselson and Stahl experimentally proved the semiconservative mode of DNA replication using isotopes of nitrogen: \[ ^{15}\text{N} \quad \text{and} \quad ^{14}\text{N} \] Generation time of E.coli is approximately: \[ 20 \text{ minutes} \]

Step 1: First generation (20 minutes).
After transfer from heavy nitrogen medium to light nitrogen medium: \[ 100% \text{ Hybrid DNA} \] is obtained.

Step 2: Second generation (40 minutes).
Each hybrid molecule produces: \[ 1 \text{ Hybrid} + 1 \text{ Light} \] Therefore: \[ 50% \text{ Hybrid} \] and \[ 50% \text{ Light} \] DNA molecules are obtained. Hence the correct answer is: \[ \boxed{50% \text{ Hybrid and } 50% \text{ Light}} \]