Figure 1.

Fragmentation and deamination. (A) A likely cause of fragmentation in ancient DNA is depurination, in which the N-glycosyl bond between a sugar and an adenine or guanine residue is cleaved, resulting in an abasic site. The DNA strand is then fragmented through β elimination, leaving 3′-aldehydic and 5′-phosphate ends. (B) Deamination of cytosine to uracil is the major mechanism leading to miscoding lesions in ancient DNA. DNA polymerases will incorporate an A across from the U, and in turn a T across from the A, causing apparent G to A and C to T substitutions.