||P.479 bottom paragraph to p.480 top paragraph: "The structure of the DNA double helix, with its bases facing inward, offers a measure of protection from all types of chemical attack by shielding its potentially reactive chemical groups, notably the amine side chains of the bases, from various mutagenic agents. In spite of this clever design, DNA molecules are subject to chemical alterations and physical damage. Some of this damage appears to occur through the actions of hydrogen and hydroxyl ions that are present at low concentration (~10^-7 M)at neutral pH. Often cited in this context is the process of depurination, in which the chemical bond linking a purine base (adenine or guanine) to deoxyribose breaks spontaneously (Figure 12.11A).” By some estimates, as many as 10,000 purine bases are lost by depurination each day in a mammalian cell. (This amounts to more than 10^17 chemically altered nucleotides generated each day in the human body!) Depyrimidination occurs at a 20- to 100-fold lower rate, but still results in as many as 500 cytosine and thymine bases lost per cell per day. Estimates of the steady-state level of base-free nucleotides present in a single human genome range from 4000 to 50,000."