Bivalent (meiosis)

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A bivalent or a tetrad is a structure of chromosomes that is formed by the germ cells of the eukaryotes during maturation ( meiosis ) . In this structure, which occurs in the pachytan of meiosis, the chromosomes are already doubled, so that each chromosome consists of two chromatids . In addition, the homologous chromosomes, i.e. the maternal and paternal copies of a chromosome, are closely paired. At this stage, the recombination occurs through crossing-over .

The term tetrad refers to the four distinguishable chromatids, bivalent to the two chromosomes involved. Unpaired chromosomes that appear before or after mating are sometimes referred to as univalents . The bivalents a pairing, a distinction depending on whether none, one or two occasions a crossing-over occurred, non-crossover tetrads (NCT) of single- or double-crossover tetrads .

The four germ cells that arise during meiosis can also be described as a tetrad.

Multivalent

In the meiosis of polyploid organisms, the term multivalent is the term used for pairing associations consisting of more than one homologous chromosome (chromosome pairing). These are held together by chiasmata (chiasma). Associations consisting of two, three, four, five, ... homologous chromosomes are called bivalent , trivalent , quadrivalent , quinquevalent , ...

swell

  • Friedrich W. Stöcker, Gerhard Dietrich (Hrsg.): Brockhaus-ABC Biologie. Volume 1: A - Me. 6th, revised and expanded edition. FA Brockhaus Verlag, Leipzig 1986, ISBN 3-325-00073-8 .
  • CE M'Clung : The Spermatocyte Divisions of the Acrididæ. In: Kansas University Quarterly. Series A: Science and Mathematics. Jg. 9, 1900, ISSN  0885-4068 , pp. 73-100, here p. 78, The term "chromosome" being [...] restricted to the units of the division figures, there remains no name for the parts composing these when they are compound, as in tetrads and diads [...] I should like, therefore, to propose the term "chromatid" for each of these [...]. Digitized .
  • J. Howard McGregor: The Spermatogenesis of Amphiuma. In: Journal of Morphology. Vol. 15, Supplement, 1899, ISSN  0898-9249 , pp. 57-104, here p. 81, [...] and the only possibility of a true reduction [...] is by conceiving the chromosomes to be " bivalent "[...] and to assume [...] that the halves of these bivalent chromosomes form new connections before the final mitosis. Digitized .

Individual evidence

  1. ↑ Wilfried Janning, Elisabeth Knust: Genetics: General Genetics - Molecular Genetics - Developmental Genetics . 2nd Edition. Georg Thieme, Stuttgart 2008, ISBN 978-3-13-151422-6 , p. 41 f .