![]() An extra copy of an autosomal chromosome is known as 'Trisomy'. Many of these embryos die, but in the condition 'Down's syndrome', there is an extra copy of chromosome 21 (X-chromosome). As a result, embryos can form that either have extra copies, or lack copies, of one or more chromosomes. Sometimes, homologous chromosomes do not separate normally in meiosis 1, (nondisjunction). Because there is some swapping (or recombination) of genetic material between the pairs of homologous chromosomes, the actual number of genetically different gametes can be very much higher. ![]() In humans, which have 23 pairs of chromosomes, as a result of the random mixing, 2 23 = 8.4 x 10 6 genetically different gametes can be produced. Resulting cells are haploid, and have a single copy of each chromosomes, and either one X or one Y chromosome (female or male gametes respectively) The two copies are inherited as if they were a single chromosome.Ĭells then enter another round of cell division, without DNA duplication - meiotic division 2, to form a gamete. The paternal and maternal chromosomes are randomly sorted, such that the mix of chromosomes will be different from cell to cell. They may even be slightly different, if genetic recombination has occurred. either the maternal or the paternal chromosome. The resulting cells after meiotic division 1 are diploid - but unlike mitotic divisions:īoth chromosome copies are derived from only one of the original chromosomes in the parent cell - i.e. The chiasmata are also important in holding the maternal and paternal homologs together until they are separated at anaphase 1.Ĭross-overs do not occur between non-sister chromatids, as they are separate.Īt anaphase of meiotic division 1, the maternal and paternal chromatids are separated, such that one daughter cell inherits one mostly paternal homolog, and the other one mostly maternal homolog.Īt anaphase one of each pair of chromosomes goes to each daughter cell, such that each daughter cell inherits one copy of the paternal chromosome, and one copy of the maternal chromosome - as in the diagram on the right hand side. These random cross-over events scrambles the genetic material of each of the chromosomes in the gametes, helping to produce diverse offspring. This exhange of genetic material between maternal and paternal chromosomes is known as 'genetic recombination'Įach cross-over between two non-sister chromatids is called a chiasma (plural 'chiasmata') Each cross-over between two non-sister chromatids is called a chiasma (plural 'chiasmata') ![]() ![]() On average, 2-3 cross-overs per chromosome occur for humans. In the long prophase of meiotic division 1, genetic material can 'cross over' between maternal and paternal pairs of chromosomes (non-sister chromatids). Homologous chromosomes duplicate, but remain separate. However, in meiosis, the two pairs of chromosomes (paternal and maternal) stay together on the spindle of the equator to form a bivalent (contains 4 chromatids) The arms of sister chromatids are glued together by a protein called 'cohesin'. Homologous chromosomes duplicate to form sister chromatids, as in mitosis. The chromosomes in this pair are called homologs - there is one paternal and one maternal homolog.One of these chromosomes is derived from the male parent (parental chromosome) and one from the female (maternal chromosome).A diploid nucleus contains two pairs of each type of chromosome (autosomes) together with the sex chromosomes (X and X, or X and Y).Meiosis is from the greek work for 'diminuition'. That on the right shows mitotic (normal) cell division. The diagram on the left shows meiotic cell division.
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