SAT Subject Biology Practice Question 266: Answer and Explanation

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Question: 266

16. Scientists grew E. colibacteria in a nutrient solution containing a heavy, stable isotope of nitrogen, 15N. They then transferred them to a medium with the more common light isotope of nitrogen. Periodically, DNA from the cells was extracted and subjected to density centrifugation. The strain of E. coliused divides once every 20 minutes. The results are illustrated below.

After two cell divisions, what types of chains make up the DNA of the cells?

I. Two light-nitrogen chains

II. Two heavy-nitrogen chains

III. One light-nitrogen chain and one heavy-nitrogen chain

A. I only
B. II only
C. III only
D. I and II
E. I and III

Correct Answer: E

Explanation:

After one cell division, the heavy chains have separated and formed double strands by pairing with light-isotope bases. After two cell divisions, the light chains have also paired with light-isotope chains.

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