sticky ends
Sticky ends are short single-stranded overhangs left dangling at the cut ends of a DNA fragment — little unpaired tails of a few bases. They are called sticky because those tails will base-pair with, and thus grab onto, any other DNA end carrying a complementary overhang, like two strips of hook-and-loop fastener clicking together.
They arise when certain restriction enzymes cut the two strands of DNA at staggered, offset positions rather than straight across. The offset leaves each fragment with a short single-stranded stretch. Crucially, every fragment cut by the same enzyme carries the same overhang, so any two such fragments — even from utterly different organisms — have ends that fit each other.
This matching is what makes sticky ends so useful for building recombinant DNA. When a gene and a vector are cut with the same enzyme, their complementary overhangs find each other and hold the pieces together by base-pairing, positioning them perfectly for DNA ligase to seal the join. Blunt ends, cut straight across with no overhang, can also be joined, but without the helpful pre-alignment that sticky ends provide.
EcoRI leaves a four-base AATT overhang on every fragment it cuts, so a human gene and a bacterial plasmid both cut with EcoRI snap together at their matching tails.
Identical overhangs from one enzyme let unrelated DNAs join.
Sticky ends from the same enzyme are interchangeable, which is convenient but also undirected — a fragment can insert in either orientation; researchers often use two different enzymes to force a fragment in a chosen direction.