Discussion
(I) The 3' to 5' exonuclease activity of eukaryotic DNA polymerase operates in reverse direction of DNA synthesis and participate in fidelity of replication.
(II) The ability of DNA polymerase to extend a primer only in 5' to 3' direction appears to make replication a complicated process, it is necessary for proofreading of duplicated DNA.
Choose the correct answer :
Answer: C
Both statements are correct.
(I) The 3' to 5' exonuclease of replicative DNA polymerases works in the direction opposite to synthesis. When a wrong base is added, the polymerase backs up, removes it and tries again. This proofreading improves fidelity by about a hundredfold.
(II) Synthesis only in the 5' to 3' direction makes replication complicated, requiring Okazaki fragments on the lagging strand. But it is what allows proofreading: the energy for adding each nucleotide comes from the incoming triphosphate, so after a mismatch is removed the 3' end can still be extended. In 3' to 5' synthesis, removing a mismatch would remove the activated end. Since both statements hold, the options calling either one wrong are incorrect.
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