sticky and blunt ends

When you cut a zipper, you can cut straight across both rows of teeth at the same spot, or you can cut one row a little ahead of the other. The first leaves a clean flush edge; the second leaves a short stretch of teeth on one side that can grab a matching stretch on another zipper. Restriction enzymes cut DNA in these two styles too, giving blunt ends or sticky ends.

DNA is two strands. If an enzyme cuts both strands at exactly the same position, the result is a blunt end: a flat double-stranded edge with no overhang. If instead the enzyme cuts the two strands a few bases apart (a staggered cut), each fragment ends with a short single-stranded overhang — a sticky end (also called a cohesive end). Because the enzyme makes the same staggered cut everywhere, every sticky end it produces has the same overhang sequence. Those overhangs are complementary, so they spontaneously pair up by base-pairing: a 5'-AATT overhang from one fragment finds and hybridizes with an AATT overhang from any other fragment cut by the same enzyme. That brief base-pairing holds the pieces together long enough for an enzyme to seal them.

Sticky ends are a gift to cloning because they make joining efficient and directional: two DNAs cut with the same enzyme will lock together at their complementary overhangs. Blunt ends are more universal — any blunt end can join any other blunt end regardless of sequence — but the joining is less efficient and gives no control over orientation. A common trick is to cut a plasmid with two different enzymes so its two ends are different sticky ends, forcing an insert to drop in one way only (directional cloning).