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Mutations may be caused by external factors (UV light,
chemical agents, etc.) or spontaneous cellular processes (replication errors, accidental deamination,
etc.). The following figure illustrates the deamination process. Conversion of the methylated cytosine (methylcytosine) to thymine may explain the observed CpG islands (more info).
Figure 7-F-1. Examples of
deamination which involves the removal of an amino group. Accidental deamination may change the cytosine to
uracil, or the methylated cytosine to thymine.
Cytosine is one of four bases in DNA
molecules. As shown in the above figure, it may be mutated to uracil by
deamination. Since uracil is not part of DNA, this mutation can easily
be detected and repaired by base excision.
Suppose DNA, like RNA, were made up of uracil, then the cytosine to uracil mutation
could be corrected only by mismatch repair which
is very inefficient. This may explain why DNA chooses
thymine, instead of uracil, even though the chemical
structure of uracil is simpler than thymine.
Review Article:
Oxidative
DNA damage: mechanisms, mutation, and disease - FASEB J., 2003.
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