Discussion
DNA ligase is used in genetic engineering to
Answer: C
Correct answer: join DNA fragments by forming phosphodiester bonds
DNA ligase seals a break in the sugar-phosphate backbone by forming a phosphodiester bond between an adjacent 3’ hydroxyl and 5’ phosphate. In cloning it is what joins an insert to a vector after both have been cut. Its natural role is in replication and repair: it joins the Okazaki fragments of the lagging strand, and seals the nick left after excision repair. The enzyme most used in the laboratory is T4 DNA ligase, from bacteriophage T4, which will join blunt ends as well as sticky ones - the E. coli enzyme requires a sticky-ended join. The distractors are the other enzymes of the toolkit. A restriction endonuclease cuts at a recognition sequence; a helicase unwinds the helix; a methyltransferase adds methyl groups, which is how a cell protects its own DNA from its restriction enzymes. Ligase and restriction enzyme together are often described as the scissors and glue of recombinant DNA work.
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