Spontaneous Mutations Are Nucleotide Sequence Changes

Instructions for making all cellular components are contained in DNA molecules. (RNA serves the purpose with some viruses.) New copies of DNA are made by DNA polymerase joining nucleotides at a rate of about 800 per second. DNA polymerase occasionally makes errors. (Error rate is about 10^-5.)¹⁰⁶ Proofreading mechanisms correct some of those errors, but the proofreaders are also imperfect. (Proofreading reduces error rate to about 10^-6 to 10^-7.) Another set of enzymes corrects mismatches in the DNA strands, reducing the error rate further. However, the rate is still significant in large pathogen populations. The result is that a population of progeny pathogens has a few members that differ slightly from their parents. Those differences, which at the DNA level are called mutations, are often deleterious: They lower the chance that the mutant will survive. But some mutations are beneficial within a particular environment. For example, mutations occasionally arise in a gene encoding an antibiotic target and cause changes that block binding of the antibiotic. Other mutations cause over-production of an efflux pump or lower membrane permeability. Still other mutations increase the activity of an enzyme that breaks down an antibiotic.