DNA REPAIR The structure of DNA must be preserved because of its importance to the genetic process. Mutations can occur when the wrong base is added in the replication process; in the same way, chemicals can result in DNA mutations, which need to be repaired. There are mechanisms in place that repair the damaged DNA. There are two mechanisms in place that can do this. There is the direct reversal of whatever reaction caused the DNA damage in the first place as well as removal of damaged bases and replacement with the correct base. If this fails, there are other mechanisms in place that help the cell deal with the results of the mutation. Spontaneous damage to DNA can happen with deamination of cytosine, guanine, and adenine. There can also be loss of purine bases—called depurination. Spontaneous damage can occur from radiation and certain mutagenic chemicals. UV light can form pyrimidine dimers. This is when two side-by-side pyrimidines form a connected structure. Alkylation can occur, in which methyl groups get added to the base pairs. Most of the DNA damage gets repaired by removing the damaged bases, making new DNA where the damaged bases were once located. Pyrimidine dimers, however, can be repaired directly as can alkylated guanine fragments. UV light mostly causes pyrimidine dimers but can cause other DNA changes. These dimers can block the transcription and replication processes. In some species, UV light can also repair pyrimidine dimers but this is not the case in humans. Another DNA damage that can be seen is the addition of methyl or ethyl group to guanine. Methylation of guanine will lead to a hydrogen bond with thymine rather than cytosine. There is an enzyme that can reverse this process. It is seen in many organisms, including humans. If direct repair cannot happen, excision repair can take place. This is the most important type of repair in most prokaryotic and eukaryotic cells. Damaged DNA is recognized by the cell, leaving behind a space where new bases are added based on the preexisting template. There is base-excision repair, mismatch repair, and nucleotide excision repair. Uracil is cut out and replaced in DNA using base-excision repair. There
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