Why can blunt end ligation be harder than sticky end ligation?

Correct answer: Blunt ends lack complementary single-stranded overhangs.

Sticky ends have complementary single-stranded overhangs that base-pair and hold the two DNA fragments in the correct position temporarily. Blunt ends lack such overhangs, so the fragments associate less readily and ligation is usually less efficient. DNA ligase can join blunt ends; it just does so less efficiently in many cases. Exam tip: sticky end = complementary overhang; blunt end = no overhang.