Of all the processes that permit the existence and perpetuation of life, perhaps the most critical is the ability of cells to accurately duplicate and then faithfully segregate their chromosomes at each cell division. During the cell cycle, a highly regulated process discussed in Chapter 19, cells duplicate their chromosomes precisely once during a period known as S phase (for DNA synthesis phase). Once the individual chromosomes have been duplicated, they are held together by proteins called cohesins. The cells then pass through a period called G2 (for gap 2) before entering mitosis, the process by which the duplicated chromosomes are segregated to the daughter cells. This process has to be very precise—