Molecular Biology of the Gene

DNA Repair

Errors can occur during DNA replication.
Considering that there is an enormous amount of material to be replicated every time a cell (prokaryotic or eukaryotic) divides, errors may occur during replication. These errors could be caused by chemicals entering the system, radiation exposure, or many other environmental factors. It is essential that each new cell has the exact same DNA as its parent cell in order to function properly. There are three major mechanisms that cells can use to repair problems that may arise during replication: proofreading, mismatch repair, and excision repair.

The first way DNA can repair mistakes is by proofreading the new strands. This happens as the DNA polymerase assembles the new strands. Each time a new nucleotide is added, the DNA polymerase checks to make sure the nucleotide matches up correctly with its complement. If it does not, the DNA polymerase immediately removes the incorrect base and finds the correct one. This effort is very effective as a first line of defense. Proofreading significantly lowers the frequency of errors. However, even though proofreading is very effective, some replication mistakes still go undetected.

The second method used to repair mistakes is called mismatch repair, the method of DNA repair that involves the scanning of the new strands after replication and correction of errors. Several enzymes scan the DNA after the new strands are formed to see if any bases are incorrectly matched with their complements, such as G-T or A-C. If this happens, a set of repair proteins removes the incorrect match and replaces it with the correct one. The failure of this repair mechanism results in an overall change in the DNA sequence. This can then alter the proteins produced and the traits expressed in the organism.

The third method of repair is called excision repair, a mechanism used to repair a damaged section of DNA by removing the incorrect bases and replacing them with correct ones. Here, short strands of damaged DNA are removed and replaced. This is particularly useful when the bases have been damaged by chemicals or radiation from the environment and are no longer the normal purines or pyrimidines.

Modes of DNA Repair during Replication

Modes of DNA Repair
Mechanism Damage to DNA Type of Repair
Proofreading Errors in replication made by DNA polymerase Elimination of the incorrect nucleotide and replacement with the correct one
Mismatch repair Errors made when copying the bases Removal of the incorrect strand and replacement with correct bases
Excision repair Damage to a base due to chemicals or radiation Removal of the damaged fragment and replacement with the correct nucleotides

Because the process of DNA replication is not perfect, there are protocols in place that repair the errors in replication before they get passed on.