Answer:
Average mutation rates in RNA viruses are estimated to be about 100 times higher than those in DNA viruses for a couple of reasons.
Replication errors
The enzymes that occur in RNA viruses and participate in copying viral genomes is a key reason for this difference. These enzymes lack the built-in capabilities to recognize DNA damage that enzymes in most organisms have.
Retroviruses take their RNA-based genome, use it to make DNA within a host cell and use the new DNA to replicate more viral RNA. This process is error-prone and results in an unusually high mutation rate. HIV, for example, has a mutation rate of 3.4 x 10^-5 errors per base pair each time its genome goes through this process. Retroviruses have higher mutation rates than most other viruses, including other RNA viruses.
Another interesting difference between RNA and DNA mutations involves the bases thymine, cytosine and uracil, typically represented as T, C and U in the DNA code. DNA uses thymine, while RNA uses uracil instead. Cytosine can sometimes spontaneously change to uracil by deamination. In DNA, this error will be detected because DNA doesn't ordinarily contain uracil; the cell has enzymes that can recognize and fix the substitution. In RNA, however, this kind of error cannot be detected because RNA ordinarily contains both cytosine and uracil bases. So, some mutations are less likely to be recognized and repaired in RNA viruses, and the mutation rate increases.
