|MoBio||Mutation Mechanisms||Chapter 7|
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).
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.