Meiosis is the process by which most eukaryotic organisms, those with cells having an organized nucleus, produces sex cells, the male and female gametes. Cells normally have two copies of each chromosome, one donated from each parent. This is the diploid chromosome condition and the paired chromosomes are called homologous. Meiosis transforms a diploid cell into four haploid granddaughter cells, each having a single copy of each chromosome. The process helps increase the genetic diversity of a species.
Meiosis occurs over two generations of cells. During normal cell division, or mitosis, each chromosome is copied, resulting in chromosomes containing twin sister chromatids. These joined chromatids later separate in mitosis to form the next generation of identical chromosomes. In meiosis I, the chromatids do not separate, which means each daughter cell receives only one copy of each chromosome, the haploid number, and each copy contains two chromatids. In meiosis II, the chromatids separate and are distributed to each resulting gamete.
During prophase of meiosis I, the double-chromatid homologous pairs of chromosomes cross over with each other and often exchange chromosome segments. This recombination creates genetic diversity by allowing genes from each parent to intermix, resulting in chromosomes with a different genetic complement. The exchange occurs between non-sister chromatids. Because genes often interact with each other, the new combination of genes on a chromosome can lead to new traits in offspring.
Reduction to Haploid
Because the duplicated chromatids remain joined during meiosis I, each daughter cell receives only one chromosome of each homologous pair. This reduces the diploid number to haploid, and the distribution of each chromosome is random. This means that it is equally likely for a given chromosome to be distributed to either of the two daughter cells. By shuffling the genetic deck in this way, the gametes resulting from meiosis II have new combinations of maternal and paternal chromosomes, increasing genetic diversity.
Random Chromatid Assortment
A third source of genetic diversity occurs during meiosis II, in which the sister chromatids separate and are randomly distributed to the daughter cells, the gametes. Crossing over in meiosis I leads to non-identical chromatids in meiosis II chromosomes. During anaphase of meiosis II, the centromere joining each chromatid pair dissolves, creating two chromosomes of each type. The outcome of which chromosome will go to which gamete is random, so that each gamete has a potentially unique combination of genetic material.
Fertilization creates genetic diversity by allowing each parent to randomly contribute a unique set of genes to a zygote. While fertilization is not part of meiosis, it depends on meiosis creating haploid gametes. The fertilized cell restores the diploid number. Without meiosis, the number of chromosomes per cell would double in each generation of offspring, leading to unstable conditions that could threaten the viability of a species.
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