SAT Subject Biology Practice Question 265: Answer and Explanation

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Question: 265

15. 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.

At which time point(s) is the heaviest DNA extracted from the cells?

A. 0 minutes
B. 20 minutes
C. 40 minutes
D. 60 minutes
E. 40 and 60 minutes

Correct Answer: A

Explanation:

The heaviest DNA, signified by the bottom band, is obtained at the 0-minute extraction.

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