DNA Technology Practice Test

Why can fragments of human DNA cut with the same restriction enzyme join to plasmid DNA cut with the same enzyme?

Ends are blunt and require no base pairing Sticky ends created by staggered cuts allow complementary base pairing Complementary base pairing is irrelevant to ligation DNA ligase can join ends without base pairing

Correct Answer:

Sticky ends created by staggered cuts allow complementary base pairing

Explanation:

The concept here is sticky ends. Some restriction enzymes cut DNA in a staggered way, leaving short single-stranded overhangs called sticky ends. When the same enzyme cuts both the human DNA fragment and the plasmid, the overhangs they generate have complementary sequences. Those complementary overhangs can base-pair with each other, aligning the fragments precisely. Once aligned, DNA ligase seals the sugar-phosphate backbone to form a continuous, recombinant DNA molecule. This base-pairing guidance makes the joining efficient and specific. If the ends were blunt, there would be no such base-pairing guidance, making ligation much less efficient.

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