Meiosis is a reductioist devision of a diploid cell into four haploid gametes. It occurs in sex cells only and is responsible for genetic variation. Meiosis occurs in two steps, Meiosis I and Meiosis II, where the latter resembles mitosis.
Im meiosis one, Chromosomes double to produce an identical copy of themselves in interphase. They then supercoil and become visible under a light microscope. Two sister chromatids join together at the centromere, forming a homolougous chromosome. These chromatids in the homolougous chromosomes have the same genes in the same sequence, yet not neccisarily the same alleles. In prophase I, a pair of homologous chromosomes join together to produce bivalents. Crossing over occurs between the pairs of homolougous chromosomes, where base sequences are cut from the chromatids, and are swapped with a sequence from another chromatid. The point at which they cross over, chiasmata are formed. This results in genetic variation, and chromatids are no longer sister chromatids as the genetic information has been shuffeled. Also, spindle poles move to opposite sides of the cell and neuclear membrane begins to break down. In Metaphase I, bivalents line up on the equator of the cell, and orientate randomly. This random orientation also shuffles genes and causes genetic variation. In addition, spindle microtubes begin to form from the spindle poles. In Anaphase I, these spindle microtubes attach at the centromere of the homolougous chromosomes and pulls the bivalents apart. At this stage, the chromatids are not separated, one homologous chromosome from each bivalent pair is pulled to one side, the other one to the opposite. In telopahse I, the neuclear membrane reforms. The chromosome number has now been halfed in a reductionist devison.
Meiosis II occurs in both of the produced haploid cell, to produce two haploid cells out of each of them, making four in total. In Prophase II the previously reformed neuclear membrane dissapears and the spindle poles move to opposite sides again. In Mataphase II the chromosomes line up at the equator, again random orientation causes genetic variation. Then in Anaphase II, the spindle fibers attach to the centromere, pulling apart the two chromatids which make up the homolougous chromosome. One chromatid moves to one side, the other to the opposide. These chromatids are now chromosomes. Random seregation (seperation) in Anaphase also causes genetic variety. Then, cytokinesis occurs to produce 4 haploid neuclei.