DNA that has been  labeled with 15N is used as the template for replication. Replication is carried using 14N labeled dNTP's. After 3  rounds of replication, what percent of DNA molecules have one strand containing 15N and one strand containing 14N?   A. 50%   B. 75%   C. 25%   D. 100%

Biochemistry
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ISBN:9781319114671
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Chapter1: Biochemistry: An Evolving Science
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DNA that has been  labeled with 15N is used as the template for replication. Replication is carried using 14N labeled dNTP's. After 3  rounds of replication, what percent of DNA molecules have one strand containing 15N and one strand containing 14N?

  A.

50%

  B.

75%

  C.

25%

  D.

100%

Expert Solution
Step 1

Watson and Crick proposed that the double-strand DNA (dsDNA) replication is a semiconservative process. The dsDNA separates to form two single strands at the replication fork, and both the strands act as a template for new DNA strands synthesis. Meselson and Stahl proved semiconservative replication of DNA in prokaryote (E.coli)

Step 2

The 15N is the heavy isotope of nitrogen. The DNA containing 15N acts as a template, and normal 14N isotope-labeled deoxy-nucleoside triphosphates (dNTPs) are used for synthesizing new DNA strands. During each replication, the double strands separate, and a template strand and a new strand form a new dsDNA. Since there is only 14N in the medium, the new strands are formed from 14N dNTPs only. Thus, the number of 15N remains the same. 

Biochemistry homework question answer, step 2, image 1

From the given diagram

Biochemistry homework question answer, step 2, image 2

Therefore, option (c) is correct, and all other options are incorrect.

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