Below is a double stranded DNA composed of one strand containing 15N atoms and the other strand containing 14N atoms. If the cell which contains this DNA is grown for one generation (replication) on 15N medium and then transferred to 14N media for a second generation, what would the resulting double stranded DNA molecules look like?

Question

Quizplus · Accepted Answer

After one generation on 15N medium, all the DNA in the cell should have one 15N strand and one 14N strand. When the cell is transferred to 14N medium for a second generation, two possible outcomes can occur:

1) Conservative replication: the original parental double-stranded DNA molecule remains intact and serves as a template for the synthesis of a completely new double-stranded DNA molecule. This would result in two molecules with one 15N strand and one 14N strand and two molecules with two 14N strands.

2) Semi-conservative replication: each parental DNA strand serves as a template for the synthesis of a new complementary strand. This would result in two molecules with one 15N and one 14N strand and two molecules with one 14N and one 15N strand.

Since we don't know which replication mechanism occurs, answer choices A and B are ruled out. Answer choice D is incorrect because it predicts the possibility of a double 15N strand which is not possible given the experimental setup. Answer choice E is incorrect because it predicts that all the molecules will have 14N strands, which is not possible since one parental strand contains 15N. Therefore, the correct answer is C, which accounts for both conservative and semi-conservative replication outcomes.

Below Is a Double Stranded DNA Composed of One Strand