MCAT Question A Day - 4/26/14 - Answer!

E. coli previously grown in medium with normal nitrogen were placed in cultures containing labeled heavy nitrogen and were allowed to replicate. Based on your knowledge of the semiconservative model of DNA replication, what proportion of DNA strands will contain labeled heavy nitrogen after two cycles of replication?

A. 25%

B. 50%

C. 75%

D. 100%

The correct answer is (C). After one cycle of replication, two out of the four DNA strands will contain labeled heavy nitrogen. After the second cycle of replication, six out of the eight DNA strands will contain labeled heavy nitrogen.

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