DNA Structure and Replication

ID #1138

I know it has something to do with the number of phosphate groups, but why doesn't DNA polymerase I have enough energy to seal the nicks in the sugar-phosphate backbone?

You're right, it boils down to the phosphate groups. The "built-in" energy supply in a nucleotide comes from its triphosphate (or to a lesser extent, its diphosphate) group. Since the bases on either side of the "nick" have already had their triphosphate groups hydrolyzed, there's no readily available energy source to tap into, so Ligase has to tap into the cell's energy supply, and that's ATP.

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