Spillover: Animal Infections and the Next Human Pandemic

by David Quammen

The different attributes of DNA and RNA account for one of the most crucial differences among viruses: rate of mutation. DNA is a double-stranded molecule, the famed double helix, and because its two strands fit together by way of those very specific relationships between pairs of nucleotide bases (adenine linking only with thymine, cytosine only with guanine), it generally repairs mistakes in the placement of bases as it replicates itself. This repair work is performed by DNA polymerase, the enzyme that helps catalyze construction of new DNA from single strands. If an adenine is mistakenly set in place to become linked with a guanine (not its correct partner), the polymerase recognizes that mistake, backtracks by one pair, fixes the mismatch, and then moves on. So the rate of mutation in most DNA viruses is relatively low. RNA viruses, coded by a single-strand molecule with no such corrective arrangement, no such buddy-buddy system, no such proofreading polymerase, sustain rates of mutation that may be thousands of times higher.