As we discussed, mitotic cell division is important in the development of a multicellular organism and in the maintenance and repair of tissues and organs. Mitotic cell division is also the basis of asexual reproduction in unicellular eukaryotes. We now turn to the basis of sexual reproduction. In sexual reproduction, gametes fuse during fertilization to form a new organism. A new organism produced by sexual reproduction has the same number of chromosomes as its parents because the egg and sperm each contain half the number of chromosomes as each diploid parent. Gametes are produced by meiotic cell division, a form of cell division that includes two rounds of nuclear division. By producing haploid gametes, meiotic cell division makes sexual reproduction possible.

There are several major differences between meiotic cell division and mitotic cell division. First, meiotic cell division results in four daughter cells instead of two. Second, each of the four daughter cells contains half the number of chromosomes as the parent cell. (The word “meiosis” is from the Greek for “diminish” or “lessen.”) Third, the four daughter cells are each genetically unique. In other words, they are genetically different from each other and from the parental cell.

In multicellular animals, the cells produced by meiosis are the haploid eggs and sperm that fuse in sexual reproduction. In other organisms, such as fungi, the products are spores, and in some unicellular eukaryotes, the products are new organisms. In this section, we consider the steps by which meiosis occurs, its role in sexual reproduction, and how it likely evolved.