Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells. They share some similarities, but also exhibit distinct differences that lead to very different outcomes (Figure 1 and Table 1). Mitosis is a single nuclear division that results in two nuclei that are usually partitioned into two new cells. The nuclei resulting from a mitotic division are genetically identical to the original nucleus. They have the same number of sets of chromosomes, one set in the case of haploid cells and two sets in the case of diploid cells. In most plants and all animal species, it is typically diploid cells that undergo mitosis to form new diploid cells. In contrast, meiosis consists of two nuclear divisions resulting in four nuclei that are usually partitioned into four new cells. The nuclei resulting from meiosis are not genetically identical and they contain one chromosome set only. This is half the number of chromosome sets in the original cell, which is diploid.
The main differences between mitosis and meiosis occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs become associated with each other, are bound together with the synaptonemal complex, develop chiasmata and undergo crossover between sister chromatids, and line up along the metaphase plate in tetrads with kinetochore fibers from opposite spindle poles attached to each kinetochore of a homolog in a tetrad. All of these events occur only in meiosis I.
|Table 1. Meiosis v. Mitosis|
|DNA Synthesis||Occurs in S phase of Interphase||Occurs in S phase of Interphase|
|Synapsis of homolgous chromosomes||During prophase I||Does not occur in mitosis|
|Crossover||During prophase I||Does not occur in mitosis|
|Homologous chromosomes line up at metaphase plate||During metaphase I||Does not occur in mitosis|
|Sister chromatids line up at metaphase plate||During metaphase II||During metaphase|
|Outcome: Number and genetic composition of daughter cells||Four haploid cells at the end of meiosis II||Two diploid cells at the end of mitosis|