Question

Matthew Meselson and Franklin Stahl grew E. coli(doubling time is $20$min) in medium containing ${}^{15}N{H}_{4}Cl$ for many generations. Then the E. coli was transferred to medium containing ${}^{14}N{H}_{4}Cl$. After $40$ minutes, the cells were harvested and DNA was extracted and subjected to cesium chloride density gradient centrifugation. The proportion of light and hybrid DNA densities will be.

• A. $50\%$ light and $50\%$ hybrid DNA
• B. $100\%$ light DNA
• C. $100\%$ hybrid DNA
• D. $25\%$ light and $75\%$ hybrid DNA

Answer 1

Answer: (A)

$50\%$ light and $50\%$ hybrid DNA

• DNA shows a semi-conservative method of replication.
• If the culture of E. coli containing ${}^{15}$N is added in a medium containing ${}^{14}$NH${}_4$Cl, after 20 minutes (first generation), only hybrid DNA will be present in the medium. After 40 minutes (second generation), there will be 50% light and 50% hybrid DNA in the medium.
• Hence, upon density gradient centrifugation, two bands are observed, one of light DNA and one of hybrid DNA.
  

Thus, the correct answer is option A.