Meiosis
Say it: my-OH-sis
Meiosis is a two-stage cell division that turns one diploid cell into four genetically different haploid cells, the precursors of eggs and sperm.
Also: meioses, reduction division
Definition
What is meiosis?
Meiosis is the type of cell division that produces gametes in sexually reproducing organisms. After a single round of DNA replication, the cell divides twice, in meiosis I and meiosis II, giving four cells that each have half the original number of chromosomes. In humans, a diploid cell with 46 chromosomes yields haploid cells with 23, and fertilization restores the full set of 46.
Meiosis I is the reduction division. Homologous chromosomes, one inherited from each parent, pair up along their length, and non-sister chromatids swap matching segments in a process called crossing over. The paired chromosomes are then pulled to opposite poles. Meiosis II resembles mitosis: the sister chromatids separate, with no DNA copying in between.
Crossing over and the random orientation of each chromosome pair at metaphase I, known as independent assortment, make every gamete genetically unique; in humans, independent assortment alone allows more than eight million combinations. Mistakes in chromosome separation, called nondisjunction, can produce gametes with an extra or missing chromosome, the cause of conditions such as Down syndrome.
Word parts
Where does the word “meiosis” come from?
- meio-
- less, a lesseningGreek meiosis, from meion (less)
- -sis
- processGreek -sis
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Terms that go with it
- MitosisMitosis is the division of a cell's nucleus that gives each of two daughter cells an identical, complete set of chromosomes.
- GameteA gamete is a haploid reproductive cell, such as a sperm or an egg, that fuses with a gamete of the opposite type at fertilization to form a zygote.
- ChromosomeA chromosome is a thread-like structure of DNA and protein that carries an organism's genes; most human cells hold 46 of them in 23 pairs.
- FertilizationFertilization is the fusion of a sperm and an egg, the male and female gametes, into a single diploid cell, the zygote, that can develop into a new organism.
- AlleleAn allele is any of the alternative forms of a gene or DNA sequence that can occupy the same position, or locus, on a chromosome.
- Genetic linkageGenetic linkage is the tendency of genes that lie close together on the same chromosome to be inherited together rather than sorted independently.
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