What best defines a sticky end on a DNA fragment produced by restriction enzymes?

• A. A single-stranded overhang that can base-pair with a complementary end
• B. A blunt end with no overhang
• C. A double-stranded end with a 5' overhang
• D. A DNA fragment with a phosphate group

Answer: (C)

Sticky ends come from restriction enzymes that cut DNA in a staggered way, leaving short single-stranded overhangs at the fragment ends. Those overhangs can base-pair with a complementary sequence on another fragment, guiding correct pairing and enabling ligation. The defining feature is this single-stranded overhang capable of annealing; blunt ends have no overhang and thus don’t pair automatically. While a 5' overhang is one possible form of a sticky end, sticky ends can also be 3' overhangs, so the concept isn’t limited to a 5' overhang. A phosphate group at the end relates to ligation chemistry but doesn’t define a sticky end.