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Base excision repair (BER) is a cellular mechanism that can repair damaged DNA during DNA replication. Repairing DNA sequence errors is necessary so that mutations are not induced during replication.
Single bases in DNA can be chemically mutated, for example by deamination or alkylation, resulting in incorrect base-pairing, and consequently, mutations in the DNA. Base excision repair involves flipping the mutated base out of the DNA helix and repairing the base alone. There are two main enzymes used, DNA glycosylases and AP endonucleases. The DNA glycosylase is used to break the β-N glycosidic bond to create an AP site. AP endonuclease recognizes this site and nicks the damaged DNA on the 5' side (upstream) of the AP site creating a free 3'-OH. DNA polymerase, Pol I, extends the DNA from the free 3'-OH using its exonuclease activity to replace the nucleotide of the damaged base, as well as a few downstream, followed by sealing of the new DNA strand by DNA ligase.
DNA glycosylases
There are specific DNA glycosylases that recognize different damaged bases. For example, there are two oxoG glycosylases – the first removes oxoG that is bound to cytosine, repairing the actual damaged base, whereas the second removes an adenine bound to oxoG, removing the incorrect base before the next replication. A Uracil DNA glycosylase recognizes uracil in DNA causing its removal. Yet another glycosylase recognizes a thymine to guanine pairing, a common occurrence due to the deamination of 5-methyl cytosine, and removes the thymine to allow for its replacement with cytosine. There is also an alkyladenine DNA glycosylase that recognizes the mutated 3-methyl adenine, 7-methyl guanine, and hypoxanthine.
External links
"Base excision repair"