SAT Subject Biology Practice Question 33: Answer and Explanation

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Question: 33

2. A plasmid containing a normal human beta-globin allele is digested with restriction enzymes. The results are shown in lane 2 of the electrophoresis gel. A second plasmid, contain the beta-globin allele that results in sickle-cell disease, is digested in the same way and placed in lane 3. The standard in lane 1 contains DNA fragments in lengths of 100 base pairs (bp), 200 bp, and so on.

Which of these describes the results shown on the gel?

A. Both lanes 1 and 2 both contain 100-bp fragments.
B. Only lane 2 contains a 175-bp fragment.
C. Only lane 3 contains a 375-bp fragment.
D. Both lanes 2 and 3 contain two equal-sized fragments.
E. Only lane 3 contains a 300-bp fragment.

Correct Answer: B


Lane 2 contains three DNA fragments: a very large fragment, a fragment of about 200 bp, and a fragment of about 175 bp. In contrast, lane 3 contains only the large fragment and a fragment of about 375 bp.

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