DNA Structure and Replication

ID #2218

DNA Polymerase I has exonuclease activity, but it starts on the inside of a chain. How is this possible?

The exonuclease activity of DNA Pol I is for removing the short RNA primers created by Primase. In the lagging strand, there is a nick between the 5' end of the RNA primer of one Okazaki fragment and the 3' end of another DNA fragment adjacent to it (i.e, they are not covalently linked via the phosphodiester backbone). Even though this takes place within a nucleic acid chain, this is not technically endonuclease activity because of the existence of a free 5' phosphate group at the beginning of the primer. 

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