Meiosis

Meiosis is a type of cell division that produces four haploid daughter cells from one diploid parent cell, each genetically different. It is called a reduction division because the chromosome number is halved.Diploid cells contain pairs of homologous chromosomes: one chromosome of each pair came from the mother and one from the father. Homologous chromosomes are the same size, have the centromere in the same position and carry the same genes at the same loci, although they may carry different alleles.Before meiosis, DNA replicates during interphase, so each chromosome consists of two sister chromatids. There are then two divisions:
• Meiosis I: homologous chromosomes pair up and are then separated into two cells. Each cell now has one chromosome from each pair, so it is haploid.
• Meiosis II: the sister chromatids of each chromosome are separated, as in mitosis, giving four haploid cells.Meiosis produces variation in two ways: crossing over between homologous chromosomes in prophase I, and independent assortment of homologous pairs in metaphase I.Comparison with mitosis: mitosis has one division, produces two diploid, genetically identical cells, and homologous chromosomes do not pair. Meiosis has two divisions and produces four haploid, genetically different cells.Worked example: a cell with 2n = 8 contains 8 chromosomes (16 chromatids after replication). After meiosis I each of the two cells has 4 chromosomes; after meiosis II each of the four cells has 4 chromosomes, each a single chromatid.