Chromatid Health Dictionary

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Chromatid: From 1 Different Sources

n. one of the two threadlike strands formed by longitudinal division of a chromosome during *mitosis and *meiosis. They remain attached at the *centromere. Chromatids can be seen between early prophase and metaphase in mitosis and between diplotene and the second metaphase of meiosis, after which they divide at the centromere to form daughter chromosomes.
Health Source: Oxford | Concise Colour Medical Dictionary
Author: Jonathan Law, Elizabeth Martin

Meiosis

Meiosis, or reduction division, is the form of cell division that only occurs in the gonads (see GONAD) – that is, the testis (see TESTICLE) and the ovary (see OVARIES) – giving rise to the germ cells (gametes) of the sperms (see SPERMATOZOON) and the ova (see OVUM).

Two types of sperm cells are produced: one contains 22 autosomes and a Y sex chromosome (see SEX CHROMOSOMES); the other, 22 autosomes and an X sex chromosome. All the ova, however, produced by normal meiosis have 22 autosomes and an X sex chromosome.

Two divisions of the NUCLEUS occur (see also CELLS) and only one division of the chromosomes, so that the number of chromosomes in the ova and sperms is half that of the somatic cells. Each chromosome pair divides so that the gametes receive only one member of each pair. The number of chromosomes is restored to full complement at fertilisation so that the zygote has a complete set, each chromosome from the nucleus of the sperm pairing up with its corresponding partner from the ovum.

The ?rst stage of meiosis involves the pairing of homologous chromosomes which join together and synapse lengthwise. The chromosomes then become doubled by splitting along their length and the chromatids so formed are held together by centromeres. As the homologous chromosomes – one of which has come from the mother, and the other from the father – are lying together, genetic interchange can take place between the chromatids and in this way new combinations of GENES arise. All four chromatids are closely interwoven and recombination may take place between any maternal or any paternal chromatids. This process is known as crossing over or recombination. After this period of interchange, homologous chromosomes move apart, one to each pole of the nucleus. The cell then divides and the nucleus of each new cell now contains 23 and not 46 chromosomes. The second meiotic division then occurs, the centromeres divide and the chromatids move apart to opposite poles of the nucleus so there are still 23 chromosomes in each of the daughter nuclei so formed. The cell divides again so that there are four gametes, each containing a half number (haploid) set of chromosomes. However, owing to the recombination or crossing over, the genetic material is not identical with either parent or with other spermatozoa.... meiosis

Mitosis

The process of cell division for somatic cells and for the ovum after fertilisation. Each chromosome becomes doubled by splitting lengthwise and forming two chromatids which remain held together by the centromere. These chromatids are exact copies of the original chromosomes and contain duplicates of all the genes they bear. When cell division takes place, the pull of the spindle splits the centromere and each double chromatid separates, one passing to one pole of the nucleus and the other to the opposite pole. The nucleus and the cell itself then also divide, forming two new daughter cells containing precisely the same 23 pairs of chromosomes and carrying exactly the same complement of genes as did the mother cell. (See CHROMOSOMES; FERTILISATION; GENES; HEREDITY.)... mitosis

Anaphase

n. the third stage of *mitosis and of each division of *meiosis. In mitosis and anaphase II of meiosis the chromatids separate, becoming daughter chromosomes, and move apart along the spindle fibres towards opposite ends of the cell. In anaphase I of meiosis the pairs of homologous chromosomes separate from each other. See disjunction.... anaphase

Centromere

(kinetochore) n. the part of a chromosome that joins the two *chromatids to each other and becomes attached to the spindle during *mitosis and *meiosis. When chromosome division takes place the centromeres split longitudinally.... centromere

Chromosome

n. one of the threadlike structures in a cell nucleus that carry the genetic information in the form of *genes. It is composed of a long double filament of *DNA coiled into a helix together with associated proteins, with the genes arranged in a linear manner along its length. It stains deeply with basic dyes during cell division (see meiosis; mitosis). The nucleus of each human somatic cell contains 46 chromosomes, 23 of which are of maternal and 23 of paternal origin (see illustration). Each chromosome can duplicate an exact copy of itself between each cell division (see interphase) so that each new cell formed receives a full set of chromosomes. See also chromatid; centromere; sex chromosome. —chromosomal adj.... chromosome

Crossing Over

(in genetics) the exchange of sections of chromatids that occurs between pairs of homologous chromosomes, which results in the recombination of genetic material. It occurs during *meiosis at a *chiasma.... crossing over

Diplotene

n. the fourth stage in the first prophase of *meiosis, in which *crossing over occurs between the paired chromatids of homologous chromosomes, which then begin to separate.... diplotene

Disjunction

n. the separation of pairs of homologous chromosomes during meiosis or of the chromatids of a chromosome during *anaphase of mitosis or meiosis. Compare nondisjunction.... disjunction

Tetrad

n. (in genetics) 1. the four cells resulting from meiosis after the second telophase. 2. the four chromatids of a pair of homologous chromosomes (see bivalent) in the first stage of meiosis.... tetrad

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