The enzyme employed by RNA viruses, on the other hand, is “error prone,” according to Eddie. “It’s just a really crappy polymerase,” which doesn’t proofread, doesn’t backtrack, doesn’t correct erroneous placement of those nucleotide bases, A, C, G, and U. Why not? Because the genomes of RNA viruses are tiny, ranging from about two thousand nucleotides to about thirty thousand, which is much less than what most DNA viruses carry.