DNA ligase
Summary
DNA ligase is the enzyme that seals a nick in the sugar-phosphate backbone by forming a phosphodiester bond between a 3′ OH and an adjacent 5′ phosphate. It joins Okazaki fragments on the lagging strand, and it also seals nicks in DNA repair and recombinant DNA work.
Detail
Meaning: DNA ligase catalyzes a phosphodiester bond between the 3′ hydroxyl of one nucleotide and the 5′ phosphate of the next, closing a nick in a strand that is already base-paired to a template. The reaction is driven by an energy carrier, ATP in many organisms and NAD⁺ in many bacteria. Ligase does not add new nucleotides and does not need a template to choose bases.
On the MCAT: In replication, primase makes an RNA primer, DNA polymerase III extends it, DNA polymerase I removes the RNA and fills the gap with DNA, and ligase seals the final nick. Because the lagging strand is made in Okazaki fragments, it needs ligase repeatedly. A passage may describe a ligase-deficient mutant that accumulates short lagging-strand fragments. Ligase also finishes excision repair and recombinant DNA cloning: a restriction enzyme cuts a plasmid and a gene to leave complementary sticky ends, they anneal by hydrogen bonding, and ligase makes the bonds covalent.
Common confusion: Students mix up ligase and polymerase. Polymerase extends a 3′ OH by adding nucleotides complementary to a template. Ligase only joins two existing adjacent ends. Sticky ends held by hydrogen bonds alone are not a continuous backbone until ligase acts.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
- Molecular Biology of the Cell
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.