Shuffles the genes between the two chromosomes in each pair (one received from each parent), producing recombinant chromosomes with unique genetic combinations in every gamete It is an essential process for oogenesis and spermatogenesis.Īlthough the process of meiosis is related to the more general cell division process of mitosis, it differs in two important respects: Meiosis occurs in all sexually-reproducing single-celled and multicellular organisms (which are all eukaryotes), including animals, plants and fungi. This same pattern, but not the same number of chromosomes, occurs in all organisms that utilize meiosis.
When two gametes (an egg and a sperm) fuse, the resulting zygote is once again diploid, with the mother and father each contributing 23 chromosomes. Meiosis produces haploid gametes (ova or sperm) that contain one set of 23 chromosomes. For example, diploid humanĬells contain 23 pairs of chromosomes including 1 pair of sex chromosomes (46 total), half of maternal origin and half of paternal origin. Thus, alternating cycles of meiosis and fertilization enable sexual reproduction, with successive generations maintaining the same number of chromosomes. fertilization) to form a diploid zygote that contains two copies of each chromosome, one from each parent. Because the number of chromosomes is halved during meiosis, gametes can fuse (i.e. In female animals, three of the four meiotic products are typically eliminated by extrusion into polar bodies, and only one cell develops to produce an ovum. In some cases, all four of the meiotic products form gametes such as sperm, spores or pollen. In most organisms, these links can help direct each pair of homologous chromosomes to segregate away from each other during Meiosis I, resulting in two haploid cells that have half the number of chromosomes as the parent cell.ĭuring meiosis II, the cohesion between sister chromatids is released and they segregate from one another, as during mitosis. A subset of recombination events results in crossovers, which create physical links known as chiasmata (singular: chiasma, for the Greek letter Chi (Χ)) between the homologous chromosomes. During this time, homologous chromosomes pair with each other and undergo genetic recombination, a programmed process in which DNA may be cut and then repaired, which allows them to exchange some of their genetic information. Immediately following DNA replication, meiotic cells enter a prolonged G 2-like stage known as meiotic prophase. This S-phase can be referred to as "premeiotic S-phase" or "meiotic S-phase". Before meiosis begins, during S phase of the cell cycle, the DNA of each chromosome is replicated so that it consists of two identical sister chromatids, which remain held together through sister chromatid cohesion. The two meiotic divisions are known as meiosis I and meiosis II. In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells, each with half the number of chromosomes as the original parent cell. Later on, during fertilisation, the haploid cells produced by meiosis from a male and female will fuse to create a cell with two copies of each chromosome again, the zygote.Įrrors in meiosis resulting in aneuploidy (an abnormal number of chromosomes) are the leading known cause of miscarriage and the most frequent genetic cause of developmental disabilities.
#Mitosis vs meiosis venn diagram word document code#
Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome.
It involves two rounds of division that ultimately result in four cells with only one copy of each chromosome ( haploid). Meiosis ( / m aɪ ˈ oʊ s ɪ s/ ( listen) from Ancient Greek μείωσις ( meíōsis) 'lessening', since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms used to produce the gametes, such as sperm or egg cells. A video of meiosis I in a crane fly spermatocyte, played back at 120× the recorded speed
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