Question

E. coli with completely $N^{15}$ DNA was allowed to replicate in $N^{14}$ medium for two generations. Percentage of bacteria with heavy DNA will be

• A. $6.25\%$
• B. $50\%$
• C. $25\%$
• D. $100\%$

Answer 1

Answer: (B)

Meselson and Stahl found that the DNA of the first generation of bacteria was hybrid or intermediate (N₁₄ N₁₄). The second generation of bacteria, after 40 minutes, contained two types of DNA, 50% light (N₁₄ N₁₄) and 50% intermediate (N₁₄ N₁₅). The third generation of bacteria after 60 minutes contained two types of DNA, 25% intermediate (N₁₅ N₁₄) and 75% light (N₁₄ N₁₄) in a 1:3 ratio. The fourth-generation after 80 minutes contained 12.5% N₁₅ N₁₄ and 87.5% N₁₄ N₁₄ DNA in a 1:7 ratio.