- Also called:
- reduction division
- Key People:
- Cyril Dean Darlington
meiosis
What is meiosis?
How does meiosis differ from mitosis?
What is the outcome of meiosis?
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meiosis, specialized type of cell division in germ cells that consists of two successive nuclear divisions, producing four genetically distinct haploid cells (gametes, or sex cells), each possessing half the number of chromosomes of the original diploid cell.
A brief treatment of meiosis follows. For further discussion, see cell: Cell division and growth.
The process of meiosis is characteristic of sexually reproducing organisms, whose somatic cells contain a diploid (2n) set of chromosomes—two haploid sets, one inherited from each parent. These haploid sets are homologous, carrying the same genes at corresponding positions, though the genes may exist in different forms, known as alleles. In humans, for example, one homologous chromosome may carry the A allele for blood type while the corresponding chromosome carries the B allele, resulting in AB blood type.
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Prior to meiosis, each of the chromosomes in the diploid germ cell has replicated and thus consists of a joined pair of duplicate chromatids. Meiosis begins with prophase I, during which the chromosomes condense and homologous maternal and paternal chromosomes pair closely with one another in a process known as synapsis. Each pair of chromosomes—called a tetrad, or a bivalent—consists of four chromatids. At this point, the homologous chromosomes exchange genetic material by the process of crossing over (see linkage group). In metaphase I, the homologous pairs align at the cell’s equatorial plane. During anaphase I, the homologous chromosomes separate and are pulled to opposite poles of the cell. . In telophase I and cytokinesis then divide the elongated cell into two daughter cells. Each daughter cell is haploid, containing one chromosome from each homologous pair, although each chromosome still consists of two joined sister chromatids.
In the second meiotic division (meiosis II), each haploid daughter cell divides. There is no further reduction in chromosome number during this division, as it involves the separation of each sister chromatid pair. The chromatids are pulled to opposite poles of the cell, effectively becoming individual chromosomes. Each daughter cell then divides in half via cytokinesis, thereby producing a total of four different haploid gametes. When fertilization occurs, two gametes fuse, each contributing one haploid set of chromosomes, thereby restoring the diploid number in the resulting zygote. See also mitosis.