McMaster Ancient DNA Center: "Once an organism dies, presumably the single most important factor in the long-term preservation of its DNA is the rate at which specific cellular enzymes, called nucleases, can be stopped. These endonucleases are efficient and can rapidly cleave DNA into small fragments. However as these are energy requiring functions, and a cell without oxygen will deplete its energy sources quickly, nuclease degradation may cease relatively soon post mortem. The organism must then face the bacterial, fungal and insect onslaught which can be quite effective but often incomplete. Once bacterial onslaught has slowed, the DNA molecule is still subject to chemical degradation via hydrolysis and oxidation. To understand the processes which degrade the DNA molecule in the fossil record and under what conditions these reactions are minimized, one needs to look briefly at the molecule itself and its susceptible sites."