Which enzyme seals nicks between Okazaki fragments?

• A. DNA polymerase I seals nicks between Okazaki fragments.
• B. DNA ligase seals nicks between Okazaki fragments.
• C. Helicase seals nicks.
• D. Primase seals nicks.

Answer: (B)

Joining the short fragments on the lagging strand is accomplished by DNA ligase. Okazaki fragments are made in pieces, each starting with an RNA primer. After the primer is removed and the fragments are filled in, a nick remains where two fragments meet. DNA ligase seals this nick by forming a new phosphodiester bond, stitching the fragments together into one continuous strand. The energy for this reaction comes from ATP in eukaryotes (or NAD+ in bacteria). The other enzymes set things up: helicase unwinds the DNA, primase lays down the RNA primer, and DNA polymerase fills in the gaps and replaces primers with DNA before ligation.