Differentiate sticky ends from blunt ends produced by restriction enzymes and explain why sticky ends facilitate ligation.

• A. Sticky ends have single-stranded overhangs that can base-pair with complementary fragments; blunt ends have no overhangs.
• B. Sticky ends are double-stranded cuts with no overhangs.
• C. Sticky ends are formed only by DNA ligase activity.
• D. Sticky ends do not affect ligation efficiency.

Answer: (A)

Restriction enzymes can cut DNA in two main ways: staggered cuts that leave short single-stranded overhangs, or straight cuts that produce flush ends with no overhangs. Those single-stranded overhangs, or sticky ends, can base-pair with complementary overhangs on another fragment, which helps the fragments align and stay together long enough for DNA ligase to seal the backbone. Blunt ends have no overhangs, so there’s no built-in base pairing to guide joining; they rely entirely on random collision and ligase action, which makes ligation less efficient. The base-pairing of sticky ends also enables more control over orientation when the overhangs are designed to be unique, reducing unwanted self-ligation. That’s why sticky ends facilitate ligation: they provide a natural way for fragments to find and pair with each other before sealing the nicks.