SAT Subject Biology Practice Question 267: Answer and Explanation

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Question: 267

17. 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.

If the scientists were to continue extracting and centrifuging DNA every 20 minutes, which result would describe the DNA bands in the test tubes?

A. The top band would move lower, while the middle band would disappear.
B. The top band would grow thinner, while the middle band would grow thicker.
C. The top and middle bands would both grow thicker and move lower in the tube.
D. The top and middle bands would both grow thicker but remain at the same level.
E. The top band would grow thicker, while the middle band would remain the same.

Correct Answer: E

Explanation:

As the bacteria continued to replicate, they would incorporate the light isotope of nitrogen into newly formed DNA. The top band would grow thicker as more DNA accrued. The heavy-isotope DNA strands would continue to separate and form new double strands of DNA, but no further heavy isotope would be incorporated. The middle band, formed by heavy and light chains, would remain the same.

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