CELL reproduction occurs in two different ways: MITOSIS and MEIOSIS. Each involves the creation of new cells, but with one critical difference: mitotic cell division produces cells with identical DNA, but meiotic does not—it involves the fusion of GAMETES, creating cells with new DNA profiles.
Mitosis is one part of the CELL CYCLE. During the mitotic phase of the cell cycle (which is further divisible into prophase, prometaphase, metaphase, anaphase and telophase) two essential steps take place: mitosis itself (the division of the cell NUCLEUS) and cytokinesis (the division of the CYTOPLASM). Preparation for these takes place in the second major component of the cell cycle, the interphase. The interphase (accounting for some 90% of the cycle) is divisible into three components: G1 (the first gap) (a period of cell growth), the S phase (during which CHROMOSOMES replicate, in preparation for mitosis each divided cell nucleus must have an exact copy of the GENOME) and finally the G2 phase (second gap, during which further cell growth takes place). Mitosis is regulated by chemical control systems, involving various GROWTH FACTORS external to the cell and a class of kinases known as cyclin-dependent kinases (Cdks) that are found within cells (see KINASE). Mitotic cell division occurs continuously in some cells. Fresh BLOOD cells are, for example, produced in bone marrow throughout an individual’s life. Other cells however assume a final form and divide no more: the term POSTMITOTIC indicates a cell that has completed all the mitotic divisions it will go through.
Meiosis involves gametes—sperm and egg cells (see SPERMATOZOA and OVUM). The normal cells of the human body (called SOMATIC CELLS) have 46 chromosomes, arranged in pairs: these are DIPLOID cells, in possession of two sex chromosomes (see X AND Y CHROMOSOMES) and 44 AUTOSOMES (the non-sex chromosomes).
The gametes however are HAPLOID, with only half the chromosome set: 22 autosomes plus a single sex chromosome (either an X or a Y chromosome). The combination of male and female gametes produces a diploid ZYGOTE (a fertilized egg) which undergoes mitosis to produce more cells. Like mitosis, meiosis is a complex process involving several stages: interphase I is followed by meiosis I (with prophase, metaphase, anaphase and telophase I) followed by meiosis II (with each subphase repeated). At the start of the first interphase, the diploid cell has homologous pairs of chromosomes. During interphase I these replicate within the cell, yielding a homologous pair of chromosomes, each composed of two sister CHROMATIDS. In meiosis I there is cell division to produce two cells, each with a set of homologous chromosomes: these cells are haploid, each with a pair of identical chromatids. In meiosis II there is a second division to produce four cells, each with only one sister chromatid. These are also haploid, in possession of one chromatid with half the genetic material of the organism. This is what each gamete possesses, and their combination of these into a new zygote during reproduction, one from each parent, is the engine of genetic development. One further process encourages genetic variation. A process known as crossing over occurs during meiosis I (in the prophase) when a complex process (mediated by the synaptonemal complex) allows homologous parts of two non-sister chromatids to change places. The DNA from two parents can, by this means, be combined on a single chromosome.
