Mitosis and Meiosis Compared

Mitosis and meiosis are both forms of nuclear division that rely on the same essential machinery -- condensed chromosomes, a spindle apparatus built of microtubules, and the orderly, stage-by-stage movement of chromosomes toward opposite poles of the cell -- yet the two processes differ so fundamentally in their outcome and biological purpose that they are never interchangeable.

Mitosis occurs in somatic (body) cells and is a single division that produces two daughter cells from one parent cell, each carrying exactly the same, diploid chromosome number as the parent (2n to 2n), and each genetically identical to the parent cell and to one another. Meiosis, by contrast, occurs only in the specialised germ cells that give rise to gametes, and consists of two successive divisions, meiosis I and meiosis II, which together produce four daughter cells from one original parent cell, each carrying only half the parent's chromosome number (2n to n).

The two processes also differ sharply in what happens to homologous chromosomes. In mitosis, the two members of a homologous pair behave entirely independently of one another throughout the division; they never pair up, and no exchange of genetic material occurs between them. In meiosis, by contrast, homologous chromosomes actively pair with one another during zygotene of prophase I, a process called synapsis that forms structures called bivalents, and during pachytene the paired homologues undergo crossing over, exchanging segments of genetic material between non-sister chromatids -- an event that has no counterpart anywhere in mitosis. …