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

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

Answer: (B)

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.