Question

Meselson Stahl experiment is continued to 4th generation than ratio of DNA band in centrifugation tube which has density N$^{15}$N$^{15}$: N$^{14}$N$^{15}$: N$^{14}$ : N$^{14}$ and percentage of N$^{14}$ N$^{14}$ :-

• A. 0:1:1, 87.5%
• B. 1:0:1, 12.5%
• C. 0:1:1, 12.5%
• D. 0:2:14, 87.5%

Answer 1

Answer: (D)

0:2:14, 87.5%

Answer 2

The Meselson-Stahl experiment demonstrates the semi-conservative nature of DNA replication. We track the composition of DNA molecules through successive generations when bacteria initially grown in a heavy nitrogen (N$^{15}$) medium are transferred to a light nitrogen (N$^{14}$) medium.

1. Generation 0 (Parental): All DNA is N$^{15}$N$^{15}$ (heavy).

   • Number of DNA molecules: 1
   • Composition: 1 N$^{15}$N$^{15}$

2. Generation 1 (after one replication in N$^{14}$ medium): Each N$^{15}$N$^{15}$ molecule unwinds, and each N$^{15}$ strand serves as a template for a new N$^{14}$ strand.

   • Number of DNA molecules: 2 ($2^1$)
   • Composition: 2 N$^{14}$N$^{15}$ (hybrid)
   • Ratio N$^{15}$N$^{15}$ : N$^{14}$N$^{15}$ : N$^{14}$N$^{14}$ = 0 : 2 : 0

3. Generation 2 (after second replication in N$^{14}$ medium): Each of the two N$^{14}$N$^{15}$ molecules replicates.

   • Each N$^{14}$N$^{15}$ molecule produces one N$^{14}$N$^{15}$ and one N$^{14}$N$^{14}$.
   • Number of DNA molecules: 4 ($2^2$)
   • Composition: 2 N$^{14}$N$^{15}$ (hybrid) and 2 N$^{14}$N$^{14}$ (light)
   • Ratio N$^{15}$N$^{15}$ : N$^{14}$N$^{15}$ : N$^{14}$N$^{14}$ = 0 : 2 : 2

4. Generation 3 (after third replication in N$^{14}$ medium): The 4 DNA molecules replicate.

   • The 2 N$^{14}$N$^{15}$ molecules produce 2 N$^{14}$N$^{15}$ and 2 N$^{14}$N$^{14}$.
   • The 2 N$^{14}$N$^{14}$ molecules produce 4 N$^{14}$N$^{14}$.
   • Number of DNA molecules: 8 ($2^3$)
   • Composition: 2 N$^{14}$N$^{15}$ (hybrid) and (2+4) = 6 N$^{14}$N$^{14}$ (light)
   • Ratio N$^{15}$N$^{15}$ : N$^{14}$N$^{15}$ : N$^{14}$N$^{14}$ = 0 : 2 : 6

5. Generation 4 (after fourth replication in N$^{14}$ medium): The 8 DNA molecules replicate.

   • The 2 N$^{14}$N$^{15}$ molecules produce 2 N$^{14}$N$^{15}$ and 2 N$^{14}$N$^{14}$.
   • The 6 N$^{14}$N$^{14}$ molecules produce 12 N$^{14}$N$^{14}$.
   • Number of DNA molecules: 16 ($2^4$)
   • Composition: 2 N$^{14}$N$^{15}$ (hybrid) and (2+12) = 14 N$^{14}$N$^{14}$ (light)
   • Ratio N$^{15}$N$^{15}$ : N$^{14}$N$^{15}$ : N$^{14}$N$^{14}$ = 0 : 2 : 14

Now, calculate the percentage of N$^{14}$N$^{14}$ DNA:

Percentage N$^{14}$N$^{14}$ = (Number of N$^{14}$N$^{14}$ molecules / Total number of DNA molecules) $\times$ 100

Percentage N$^{14}$N$^{14}$ = (14 / 16) $\times$ 100 = (7 / 8) $\times$ 100 = 0.875 $\times$ 100 = 87.5%

Thus, the ratio of DNA bands N$^{15}$N$^{15}$: N$^{14}$N$^{15}$: N$^{14}$N$^{14}$ is 0:2:14, and the percentage of N$^{14}$N$^{14}$ is 87.5%.