chromosome

chromosome
—chromosomal, adj. —chromosomally, adv.
/kroh"meuh sohm'/, n. Genetics.
any of several threadlike bodies, consisting of chromatin, that carry the genes in a linear order: the human species has 23 pairs, designated 1 to 22 in order of decreasing size and X and Y for the female and male sex chromosomes respectively.
[1885-90; CHROMO- + -SOME3]

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Microscopic, threadlike part of a cell that carries hereditary information in the form of genes.

The structure and location of chromosomes differentiate prokaryotic cells from eukaryotic cells (see prokaryote, eukaryote). Every species has a characteristic number of chromosomes; humans have 23 pairs (22 pairs of autosomal, or nonsex, chromosomes and one pair of sex chromosomes). Human chromosomes consist primarily of DNA. During cell division (see meiosis, mitosis), chromosomes are distributed evenly among daughter cells. In sexually reproducing organisms, the number of chromosomes in somatic (nonsex) cells is diploid, while gametes or sex cells (egg and sperm) produced by meiosis are haploid (see ploidy). Fertilization restores the diploid set of chromosomes in the zygote.

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 the microscopic, threadlike part of the cell that carries hereditary information in the form of genes.

 The structure and location of chromosomes is one of the chief differences between the two basic types of cells: prokaryotic (prokaryote) cells and eukaryotic cells. Among organisms with prokaryotic cells (i.e., bacteria and blue-green algae), chromosomes consist entirely of deoxyribonucleic acid (DNA). The single chromosome of a prokaryotic cell is not enclosed within a nuclear membrane. Among all other organisms (i.e., the eukaryotes (eukaryote)), the chromosomes are contained in a membrane-bound cell nucleus. The chromosomes of a eukaryotic cell consist primarily of DNA attached to a protein core. They also contain ribonucleic acid (RNA). Among both prokaryotes and eukaryotes, the arrangement of components in the DNA molecules determines the genetic information. The remainder of this article pertains to eukaryotic chromosomes.

      Every species has a characteristic number of chromosomes (chromosome number). In species that reproduce asexually, the chromosome number is the same in all the cells of the organism. Among sexually reproducing organisms, the number of chromosomes in the body (somatic) cells is diploid (2n; a pair of each chromosome), twice the haploid (1n) number found in the sex cells, or gametes (gamete). The haploid number is produced during meiosis (q.v.). During fertilization, two gametes combine to produce a zygote, a single cell with a diploid set of chromosomes. See also polyploidy.

      Somatic cells reproduce by dividing, a process called mitosis (q.v.). Between cell divisions the genetic material (chromatin) is diffused throughout the nucleus in a tangled network of filaments called chromonemata. These long filaments are formed from the uncoiled chromosomes and probably provide a large surface area, thereby facilitating DNA synthesis. During this phase, DNA duplicates itself in preparation for cell division.

      At the onset of cell division, the chromonemata coil up and are surrounded by a protein sheath, forming a tiny rod, or chromosome. Each chromosome actually consists of a set of duplicate chromatids that are held together by the centromere. The centromere is the point of attachment to the spindle fibres (part of a structure that pulls the chromatids to opposite ends of the cell). During the middle stage in cell division, the centromere duplicates, and the chromatid pair separates; each chromatid becomes a separate chromosome at this point. The cell divides, and both of the daughter cells have a complete (diploid) set of chromosomes. The chromosomes uncoil in the new cells, again forming the diffuse network of filaments.

      Among many organisms that have separate sexes, there are two basic types of chromosomes: sex chromosomes and autosomes. Autosomes control the inheritance of all the characteristics except the sex-linked ones, which are controlled by the sex chromosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes (see sex chromosome). All act in the same way during cell division. For information on sex-linked characteristics, see linkage group.

      Chromosome breakage is the physical breakage of subunits of a chromosome. It is usually followed by reunion (frequently at a foreign site, resulting in a chromosome unlike the original). Breakage and reunion of homologous chromosomes during meiosis is the basis for the classical model of crossing over, which results in unexpected types of offspring of a mating.

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Universalium. 2010.

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Look at other dictionaries:

  • chromosome — [ kromozom ] n. m. • 1889; mot all. 1888, du gr. khrôma « couleur » et sôma « corps », « parce qu [ils] absorbent électivement certaines matières colorantes » (J. Rostand) ♦ Chacun des éléments essentiels du noyau cellulaire, de forme déterminée… …   Encyclopédie Universelle

  • Chromosome 7 — humain Le chromosome 7 est un des 24 chromosomes humains. C est l un des 22 autosomes. Sommaire 1 Caractéristiques du chromosome 7 2 Anomalies ch …   Wikipédia en Français

  • Chromosome W — ● Chromosome W celui des deux chromosomes sexuels d une femelle hétérogamétique (oiseau par exemple) qui diffère des chromosomes sexuels du mâle, notés Z. (C est le chromosome W qui oriente le développement sexuel de l embryon dans le sens… …   Encyclopédie Universelle

  • Chromosome X — ● Chromosome X chromosome sexuel commun aux deux sexes lorsque c est le mâle qui est hétérogamétique. (Dans ce cas, qui est celui de l espèce humaine, des mammifères, de certains insectes, la formule femelle est XX, la formule mâle XY.) …   Encyclopédie Universelle

  • Chromosome Y — ● Chromosome Y celui des deux chromosomes sexuels d un mâle hétérogamétique qui diffère des chromosomes sexuels de la femelle, notés X. (L existence du chromosome Y dans le noyau de l œuf fécondé est responsable de l orientation du développement… …   Encyclopédie Universelle

  • Chromosome Z — ● Chromosome Z chromosome sexuel commun aux deux sexes lorsque c est la femelle qui est hétérogamétique. (Dans ce cas, qui est celui des oiseaux et de certains insectes, la formule mâle est ZZ, la formule femelle WZ.) …   Encyclopédie Universelle

  • Chromosome — Chro mo*some , n. [Gr. ? color + ? the body.] (Biol.) One of the minute bodies into which the chromatin of the nucleus is resolved during mitotic cell division; the {idant} of Weismann. [1913 Webster] …   The Collaborative International Dictionary of English

  • chromosome — chromosome. См. хромосома. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) …   Молекулярная биология и генетика. Толковый словарь.

  • Chromosome — Chromosome, s. Pflanzenzelle und Zelle …   Meyers Großes Konversations-Lexikon

  • chromosome — 1889, from Ger. Chromosom, coined 1888 by German anatomist Wilhelm von Waldeyer Hartz (1836 1921), from Latinized form of Gk. khroma color (see CHROMA (Cf. chroma)) + soma body. So called because the structures contain a substance that stains… …   Etymology dictionary

  • chromosome — ► NOUN Biology ▪ a thread like structure found in the nuclei of most living cells, carrying genetic information in the form of genes. DERIVATIVES chromosomal adjective. ORIGIN from Greek khr ma colour + s ma body …   English terms dictionary

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