E. coli grown for many generations in a N medium are transferred to a N medium and allowed to divide for generations. Counting single DNA strands, what fraction of all strands contains N?
Answer and explanation
Correct answer: A. 6.25%
- Step 1: DNA replication follows a semiconservative mechanism where template strands are conserved indefinitely in successive daughter molecules.
- Step 2: The original bacterial cell contains one double-stranded DNA molecule comprising two -labeled heavy strands; across subsequent divisions in medium, these original two strands persist intact.
- Step 3: After generations of binary fission, double-stranded DNA molecules are produced, containing a total of single DNA strands.
- Step 4: Of these individual strands, exactly strands contain . The fraction of -bearing strands is , or .
Answer
6.25%.
Why not the others: 12.5% is the fraction of double-stranded DNA molecules containing (), rather than individual single strands; 3.125% is the fraction after 5 generations (); 25% is the fraction after 3 generations ().
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