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Base excision repair is a mechanism in the cell cycle that allows for repairs to the DNA when base pairs are damaged. The damage creates a small lesion that may lead to problems in the future, and the body relies on base excision repair to address DNA errors before they multiply. If a lesion is larger, it requires a nucleotide excision repair; this occurs when the variation in the DNA is large enough to pull the helix out of shape.
In a base excision repair, an enzyme called DNA glycosylase cuts out the damaged base pair. DNA follows some very specific patterns, and the body can recognize a base that doesn't belong because it contains incorrect bases or bases out of order. After the excision, there is a small gap in the DNA that must be filled in through the actions of DNA ligase and DNA polymerase, two additional enzymes.
The enzymes involved can vary by species. In all cases, the cell is able to recognize the problem with the DNA and can dispatch appropriate enzymes to make a repair. The base excision repair could be compared to that of a knitter who notices a wrong stitch. Rather than throwing out his knitting and giving up, he can carefully take out the stitch and correct it to restore the project so he can continue working on it. As with knitting, there are several ways to repair the problem, depending on the species and the type of error.
DNA can be damaged in a number of ways. Oxygenation and alkylation are two common culprits. Organisms exposed to ionizing radiation may also develop damage in their DNA. The base excision repair allows the body to repair itself as necessary, in recognition of the fact that normal wear and tear tend to create small errors in DNA over time. When the cell fails to identify swapped or incorrect bases, the DNA can replicate and may compound the error, as seen when cells become cancerous because of scrambling that occurs in their DNA.
Researchers study base excision repair in the lab to learn more about how and why it happens, and what happens when it goes wrong or the body fails to identify a problem with a DNA strand. This information can be important for treatment of conditions with a genetic or DNA component. It can also be useful for the development of medical treatments that may involve direct manipulation of DNA. The enzymes involved in base excision repair can also be harnessed to insert or swap out DNA to remove harmful genes or recode a patient's DNA to address a health problem.