Question

Name and briefly describe the ways in which mistakes in DNA replication are kept to 1 mistake per 109 nucleotides copied.

Answer 1

The ways in which mistakes in DNA replication are kept to 1 mistake per 109 nucleotides copied are as follows:

1. Proofreading while the strand is growing : This step is carried out by the DNA polymerase, and it occurs just before a new nucleotide is added to the growing chain.The correct nucleotide has a higher affinity for the moving polymerase than does the incorrect nucleotide, because only the correct nucleotide can correctly base-pair with the template.

2. Exonucleolytic proofreading: This step takes place immediately after those rare instances in which an incorrect nucleotide is covalently added to the growing chain. Those DNA molecules with a mismatched nucleotide at the 3¢-OH end of the primer strand are not effective as templates because the polymerase cannot extend such a strand. DNA polymerase molecules deal with such a mismatched primer strand by means of a separate catalytic site.This 3¢-to-5¢ proofreading exonuclease clips off any unpaired residues at the primer terminus, continuing until enough nucleotides have been removed to regenerate a base-paired 3¢-OH terminus that can prime DNA synthesis. In this way, DNA polymerase functions as a “self-correcting” enzyme that removes its own polymerization errors as it moves along the DNA.