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Errors in DNA Replication
The low overall rate of mutation during DNA replication (1 base pair change in one billion base pairs per replication cycle) does not reflect the true number of errors that take place during the replication process. The number is kept so low by a proof-reading system that checks newly synthesized DNA for errors and corrects them when they are found. Errors in DNA replication can take different forms but usually revolve around the addition of a nucleotide with the incorrect base, meaning the pairing between the parent and daughter strand bases is not complementary. DNA retains its high level of accuracy with its proof-reading function.
The 3' to 5' Proof-Reading Exonuclease
The 3' to 5' proof-reading exonuclease works by scanning along directly behind as the DNA polymerase adds new nucleotides to the growing strand. If the last nucleotide added is mismatched, then the entire replication complex backs up, removes the last incorrect base, and attempts to add the correct base again. The enzyme is 3' to 5' because it scans in the opposite direction of DNA replication, which we learned must always be 5' to 3'. The mechanism of the proof-reading system offers an explanation as to why DNA replication must occur in this direction.
