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If a completely radioactive double stranded DNA molecule undergoes two rounds of replication in a non-radioactive medium, what will be the radioactive status of the four resulting molecules?

1. Half the number contain no radioactivity
2. All four still contain radioactivity
3. Three out of four contain radioactivity
4. Radioactivity is lost from all four

Correct answer: Half the number contain no radioactivity

Solution

DNA replicates semiconservatively, so each new double helix keeps one old strand and one new strand. Starting with fully labeled DNA, the first round makes two hybrid molecules; the second round produces four molecules, but only two still carry a labeled strand, so half are nonradioactive.

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