Life can persist in one of two ways. First, living things can spurn death in the hope of living forever. This is usually not possible, because death by predation or other unintentional causes is virtually inevitable. Second, living things can make copies of themselves, or reproduce, as a means of "hedging their bets" against death. Then, all that is required for life to persist is for at least one of the copies, or offspring, to remain alive at any one time. Thus, all living things reproduce.

How does reproduction occur? Some organisms reproduce asexually. The single-celled prokaryotes undergo binary fission, meaning literally "to split in half." Binary fission occurs after the cell has doubled the amount of its cellular constituents. The single circular chromosome is replicated from a single initiation point, the replication fork moving bidirectionally around the chromosome. Finally, inward growth of the plasma membrane separates the...