SAT Subject Biology Practice Question 34: Answer and Explanation

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Question: 34

3. 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.

Based on the results shown, which of these BEST describes the mutation that resulted in the sickle-cell allele?

A. It created a site that is recognized by the restriction enzyme.
B. It removed two restriction sites recognized by the enzyme.
C. It changed a restriction site cut by the restriction enzyme.
D. It inserted a second restriction site specific for the enzyme.
E. It inserted a third restriction site specific for the enzyme.

Correct Answer: C


The mutation in the sickle cell allele changed the DNA sequence at one of the recognition sites for the enzyme. This removed a restriction site, causing the 175-and 200-bp fragments seen in lane 2 (the normal allele) to remain joined.

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