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In cellular organisms (the prokaryotes and in most eukaryotic cell division) cells divide by mitosis (called binary fission in the prokaryotes; Figure 5.18). During mitosis, DNA is replicated, generating two cells with exactly the same genetic composition as the original cell. All prokaryotes divide in this way. It is also referred to as asexual reproduction. In prokaryotes, the rate of cell division can be sufficiently high that a cycle of DNA replication can be begun before the previous one is completed. Mitosis is used in multicellular eukaryotes for replicating cells such as skin cells.

Figure 5.18 The process of mitosis or “binary fission.”

In many eukaryotes, including animals, plants and fungi, an additional form of replication is achieved, referred to as meiosis (Figure 5.19). Meiosis, put simply, is the process of making sex cells that can come together to make new individuals – this defines sexual reproduction. It seems logical, then, that the central process of meiosis is to make cells with half the genetic complement, so that when the mother's and father's sex cells come together, they produce a full complement again.

Figure 5.19 The process of meiosis.

Meiosis can be explained by reference to typical animal cells involved in this process (Figure 5.19). Animal cells contain two sets of chromosomes. As a consequence, they are called diploid cells. One set of chromosomes has come from the mother and one set from the father. In the first stage of meiosis, these sets of chromosomes are replicated in a diploid cell (Figure 5.19a). This process occurs before mitosis or meiosis. In mitosis these chromosomes would just separate into daughter cells, creating identical cells to the parent cell. However, in doubling up the chromosomes in meiosis, the cell has entered into the stage called Meiosis I. In the next step, the sets of doubled up chromosomes line up (Figure 5.19b) and then exchange genetic information (Figure 5.19c), generating chromosomes that are no longer identical to one another. This event is unique to meiosis and is called cross-over. By crossing over, segments of the chromosomes are mixed, generating variation. These chromosomes can now be divided into two new cells (Figure 5.19d). In Meiosis II, the next stage of meiosis, these cells are again divided in an identical way to mitosis, generating four cells (Figure 5.19e). These are the sex cells. These sperm or eggs (gametes) contain half the genetic complement and are called haploid cells. They can join together in sexual reproduction in which chromosomes from the mother and father come together to generate new adult diploid cells, which begin the process again in the new individuals.