Meiosis: Encyclopedia II - Meiosis - Chromosome segregation in meiosis

Meiosis - Chromosome segregation in meiosis

As a diploid cell is formed from the genetic recombination of two haploid cells, within a diploid cell there are normally two sets of chromosomes that code for the same information, one from the mother's haploid cell and the other from the father's. These chromosomes are called homologous chromosomes. Homologous chromosomes need not be genetically identical. For example, one particular locus (location) on one of the father's chromosomes may code for green eyes, while the same locus on the mother's chromosome may code for brown eyes. This genetic variety produced by sexual reproduction is the key to its power.

Before division, the genome is replicated. Each chromosome now contains two identical sister chromatids joined together by a region of DNA called the centromere. Meiosis I, the first round of division, separates homologous chromosomes. Meiosis II, the second round of division, separates sister chromatids. There are four haploid cells produced at the conclusion of meiosis.

This description suggests that two out of four gametes will contain the maternal set of chromosomes, while the other two will contain the paternal set. In reality, however, the gametes are genetically varied, containing a mix of both paternal and maternal genetic information. This is accomplished in two processes. During meiosis I, the concept of independent assortment is used to redistribute information. Homologous chromosomes will eventually part ways into separate cells. However, homologous chromosomes are oriented independently of their companions. That means that each daughter cell has a fifty-fifty chance of receiving the maternal chromosome or the paternal chromosome. At the same time during meiosis I, when the chromosomes are pairing up together for a short time before being split apart during synapsis, chromosomal crossover occurs. During this time, nonsister chromatids of homologous chromosomes may exchange segments at random locations called chiasmata. The chromosome that is subjected to crossing over is then called a recombinant chromosome.

The diagram shown above summarizes the segregation of the meiotic chromosomes. Chromosomes which are the same size (one light blue and one red to show parentage) are homologous to each other. They are replicated before meiosis so that each chromosome contains two genetically identical sister chromatids (the vertical bars of the H-like structure). Crossing over occurs between nonsister chromatids of the two homologous chromosomes. Homologous chromosomes are separated in meiosis I. In this case, each daughter cell receives one recombinant mother chromosome and recombinant father chromosome. Meiosis II splits up the sister chromatids. At conclusion, four genetically varied gametes are produced.

Adapted from the Wikipedia article "Chromosome segregation in meiosis", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki