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meiosis

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agamic generation

agamic generation  

In the life cycle of some species (e.g. many members of the Cynipidae) a generation that comprises parthenogenetically reproducing females that are genetically diploid. The unfertilized eggs laid may ...
allele

allele  

Common shortening of the term ‘allelomorph’. One of two or more forms of a gene arising by mutation and occupying the same relative position (locus) on homologous chromosomes.
alternation of generations

alternation of generations  

Reproductive cycles in which a haploid phase alternates with a diploid phase. In mosses and vascular plants, the haploid phase is the gametophyte, the diploid the sporophyte.
ameiosis

ameiosis  

Meiosis in which the nucleus divides only once, so the number of chromosomes is not reduced.
amphidiploid

amphidiploid  

An allopolyploid in which the genetic behaviour of the constitutive genomes is diploid, such that bivalents between chromosomes originating from different genomes do not form during meiosis.
anaphase

anaphase  

One of several stages of cell division. In mitosis the chromatids of each chromosome move apart to opposite ends of the spindle. In the first anaphase of meiosis, the paired homologous chromosomes ...
apomixis

apomixis  

Reproduction in which meiosis and fertilization are altered, so that only one parent contributes genes to the offspring (e.g., agamospermy or thelytoky). Contrast with amixis, amphimixis.
apospory

apospory  

The development in plants of a diploid embryo sac by the somatic division of a nucellus or integument cell; a form of agamospermy (q.v.).
bivalent

bivalent  

n. (in genetics) a structure consisting of homologous chromosomes attached to each other by chiasmata during the first division of meiosis. —bivalent adj.
bouquet configuration

bouquet configuration  

A polarized arrangement of chromosome ends at the periphery while the remaining chromatin fills the volume of the nucleus. This is the result of telomeres (q.v.) moving along the inner surface of the ...
breakage and reunion

breakage and reunion  

The classical and generally accepted model of crossing over by physical breakage and crossways reunion of broken chromatids during meiosis. See Holliday model.
centromere

centromere  

The point at which the two halves of a chromosome, the chromatids, are joined. See kinetochore; spindle attachment.
chiasma

chiasma  

(pl. chiasmata)The point at which paired homologous chromosomes remain in contact as they begin to separate during the first prophase of meiosis, forming a cross shape. A number of chiasmata can ...
chiasma interference

chiasma interference  

The non-random frequency of more than one chiasma in a bivalent segment during meiosis. If the frequency of occurrence is higher than that expected from purely chance events then it is termed ...
chromatid

chromatid  

The two daughter strands of a duplicated chromosome that are joined by a single centromere. Upon the division of the centromere, the sister chromatids become separate chromosomes. See Chronology, ...
chromatid interference

chromatid interference  

The non-random participation of non-sister chromatids of a tetrad in successive crossings-over of meiosis. It results in a deviation from the expected 1:2:1 ratio for the frequencies of two-, three-, ...
chromomere

chromomere  

A small beadlike structure visible in a chromosome during prophase of meiosis and mitosis, when it is relatively uncoiled (in particular at the leptotene and zygotene stages of meiosis). Chromomeres ...
chromosome

chromosome  

1. in prokaryotes, the circular DNA molecule containing the entire set of genetic instructions essential for life of the cell. See genophore. 2. in the eukaryotic nucleus, one of the threadlike ...
cohesin

cohesin  

A multiprotein complex that binds sister chromatids together during the initial stages of nuclear division in cells. It includes members of the SMC (structural maintenance of chromosomes) family of ...
crossing over

crossing over  

The exchange of genetic material between homologous chromosomes by breakage and reunion. This occurs during pairing of chromosomes at meiosis, and in some organisms (e.g. certain fungi, and insects ...

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