Bacteria are grown for many generations in a medium containing heavy nitrogen (), then moved to a medium containing normal nitrogen (). After two rounds of semi-conservative replication, what fraction of the DNA molecules contain some ?

Answer and explanation

Correct answer: B.

  1. Step 1: Start: every molecule has two strands. After round 1 every molecule is a hybrid with one old and one new strand.
  2. Step 2: In round 2 each hybrid gives one hybrid and one all- molecule, so there are molecules.
  3. Step 3: Only the hybrids contain , which is .
Answer
.
Why not the others: : halves the fraction in each round instead of counting the hybrid molecules. : would be correct after one round only, when every molecule is a hybrid. : assumes the heavy strands are lost, but the original strands survive in hybrid molecules.

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