This chapter explains why cell division occurs and why there are two types— mitosis and meiosis. It describes the cell cycle and the role of mitosis in the cell cycle. The purpose of mitosis is to increase cell numbers in a conservative fashion. The function of mitosis is to distribute the replicated DNA to the two daughter cells that will result from cell division. The chapter describes the five phases of mitosis, and also explains what happens to the chromosomes and the role of the spindle. These phases are: prophase, prometaphase, metaphase, anaphase, and telophase. The chapter discusses the significant events in meiosis: reduction of chromosome number, random segregation of chromosomes, and crossing over, and the role of genes and how meiosis affects natural selection. Chromosomes exist throughout the cell cycle, but they are so dispersed during interphase that they are not visible through the ordinary light microscope.