Chromosome

A visible carrier of the genetic information. The 3 billion bp (base pairs) in the human genome are organized into 24 distinct, physically separate microscopic units called chromosomes. All genes are arranged linearly along the chromosomes. The nucleus of most human cells contains two sets of chromosomes, one set given by each parent. Each set has 23 single chromosomes—22 autosomes and an X or Y sex chromosome. (A normal female will have a pair of X chromosomes; a male will have an X and Y pair.) Chromosomes contain roughly equal parts of protein and DNA; chromosomal DNA contains an average of 150 million bases. DNA molecules are among the largest molecules now known. Chromosomes can be seen under a light microscope and, when stained with certain dyes, reveal a pattern of light and dark bands reflecting regional variations in the amounts of A and T vs G and C. Differences in size and banding pattern allow the 24 chromosomes to be distinguished from each other, an analysis called a karyotype. A few types of major chromosomal abnormalities, including missing or extra copies or gross breaks and rejoinings (translocations), can be detected by microscopic examination; Down's syndrome, in which an individual's cells contain a third copy of chromosome 21, is diagnosed by karyotype analysis. Most changes in DNA, however, are too subtle to be detected by this technique and require molecular analysis. These subtle DNA abnormalities (mutations) are responsible for many inherited diseases such as cystic fibrosis and sickle cell anemia or may predispose an individual to cancer, major psychiatric illnesses, and other complex diseases. The foregoing definition and discussion pertain to the chromosomes in the nucleus of the cell. The chromosome of the mitochondrion, which is in the cytoplasm of the cell, is a somewhat different story.

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One of the bodies (normally 46 in somatic cells in humans) in the cell nucleus that is the bearer of genes, has the form of a delicate chromatin filament during interphase, contracts to form a compact cylinder segmented into two arms by the centromere during metaphase and anaphase stages of cell divison, and is capable of reproducing its physical and chemical structure through successive cell divisons. In bacteria and other prokaryotes, the c. is not enclosed within a nuclear membrane and not subject to a mitotic mechanism. Prokaryotes may have more than one c.. [chromo- + G. soma, body]

- accessory c. a supernumerary c. that is not an exact replica of any of the chromosomes in the normal cellular complement. SYN: monosome (1), odd c., unpaired allosome, unpaired c..

- acentric c. a fragment of a c. lacking a centromere and unable to attach to the mitotic spindle, therefore unable to take part in the division of a nucleus and randomly distributed in daughter cells. SYN: acentric fragment.

- acrocentric c. a c. with the centromere placed very close to one end so that the short arm is very small, often with a satellite.

- bivalent c. a pair of chromosomes temporarily united.

- Christchurch c. an obsolete term describing an abnormal small acrocentric c. (no. 21 or 22) with complete or almost complete deletion of the short arm; found in cultured leukocytes in some cases of chronic lymphocytic leukemia, also in some normal relatives of patients.

- derivative c. an anomalous c. generated by translocation. SYN: translocation c..

- dicentric c. a c. with two centromeres that may result from reciprocal translocation.

- double minute chromosomes paired, extrachromosomal elements lacking centromeres, often associated with a drug resistance gene.

- fragile X c. an X c. with a fragile site near the end of the long arm, resulting in the appearance of an almost detached fragment; demonstrated only under special culture conditions; frequently associated with X-linked mental retardation. See Renpenning syndrome.

- giant c. 1. SYN: polytene c.. 2. SYN: lampbrush c..

- heterotypical c. c. that pairs with an unequal partner, e.g., the X and Y chromosomes.

- homologous chromosomes members of a single pair of chromosomes.

- lampbrush c., lamp-brush c. 1. a large c. found in oocytes of certain animals characterized by many fine lateral projections giving the appearance of a test tube brush or lampbrush. 2. multiply looped chromosomal area of the chromatin of some species. SYN: giant c. (2).

- late replicating c. a c. (often anomalous) that is shown, e.g., by incorporation of a labeled nucleotide, to undergo delayed duplication preliminary to mitosis; formerly used as a means of distinguishing members of a group of chromosomes.

- marker c. a c. with cytologically distinctive characteristics.

- metacentric c. a c. with a centrally placed centromere that divides the c. into two arms of approximately equal length.

- mitochondrial c. the DNA component of mitochondria, the chief function of which is synthesis of adenosine triphosphate and the management of cellular energy; the c. contains some 16,000 base pairs arranged in a circle. The inheritance is matrilineal, and the mutation rate is unusually high; since each cell contains thousands of copies,s a mutant form may assume an almost continuous gradation as in a galtonian process. Most of the mutations known have their impact on the respiratory chain.

- nonhomologous chromosomes chromosomes that are not members of the same pair.

- nucleolar c. a c. regularly associated with a nucleolus.

- odd c. SYN: accessory c..

- Philadelphia c. (Ph1) an abnormally shortened c. 22, formed by translocation of a portion of the long arm of c. 22 to c. 9; found in cultured leukocytes of many patients with chronic granulocytic leukemia.

- polytene c. a stage of c. division that forms the giant c. found in the salivary gland of dipterous insects; the great width is the result of repeated divisions of the chromonema without subsequent lengthwise separation of the filaments. SYN: giant c. (1).

- c. puffs expansions of particular c. regions; sites of RNA syntheses.

- ring c. a c. with ends joined to form a circular structure. The ring form is abnormal in humans but the normal form of the c. in certain bacteria.

- sex chromosomes the pair of chromosomes responsible for sex determination. In humans and most animals, the sex chromosomes are designated X and Y; females have two X chromosomes, males have one X and one Y c. In certain birds, insects, and fishes the sex chromosomes are designated Z and W; males have two Z chromosomes, females may have one Z and one W c., or one Z and no W c.. SYN: gonosome.

- submetacentric c. a c. with the centromere so placed that it divides the c. into two arms of strikingly unequal length.

- telocentric c. a c. with a terminal centromere; such chromosomes in humans are unstable and arise by misdivision or breakage near the centromere and are usually eliminated within a few cell divisions.

- translocation c. SYN: derivative c..

- unpaired c. SYN: accessory c..

- W c., X c., Y c., Z c. sex chromosomes.

- c. walking sequential isolation of overlapping sequences of DNA ( i.e., clones); with this procedure large regions of the c. can be spanned. SYN: overlap hybridization.

- yeast artificial chromosomes (YAC) yeast DNA sequences that have incorporated into them very large foreign DNA fragments; the recombinant DNA is then introduced into the yeast by transformation; the use of yeast artificial chromosomes permits the cloning of large genes with their flanking regulatory sequences.

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chro·mo·some 'krō-mə-.sōm, -.zōm n any of the usu. linear bodies of the cell nucleus of eukaryotic organisms, the usu. circular bodies of prokaryotic organisms (as bacteria), or esp. in some schools of molecular biology the genomes of DNA viruses (as bacteriophages) that take up basophilic stains and contain most or all of the genes of the organism <both the \chromosomes of cells and those of viruses can duplicate only in the complex environment of a living cell (J. D. Watson)> <an episome, an element that may exist as a free circular plasmid, or that may become integrated into the bacterial \chromosome as a linear sequence (Benjamin Lewin)>

chro·mo·som·al .krō-mə-'sō-məl, -'zō- adj

chro·mo·som·al·ly -mə-lē adv

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n.

one of the threadlike structures in a cell nucleus that carry the genetic information in the form of gene. 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. 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.

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chro·mo·some (chr) (kroґmə-sōm) [chromo- + -some] 1. in eukaryotic cells, a structure in the nucleus consisting of chromatin (q.v.) and carrying the genetic information for the cell. Each organism of a species is normally characterized by the same number of chromosomes in its somatic cells; 46 is the usual number in humans, including the two (XX or XY) that determine the sex of the organism. During cell division, the chromatin is compactly coiled and supercoiled, making the chromosome visible with appropriate staining and permitting its movement in the cell with minimal entanglement. The hierarchical levels of compaction begin with the DNA double helix, which is coiled into nucleosomes; the nucleosomes supercoil into 30-nm diameter chromatin fibers, and the chromatin fibers are packed into 100 kb loops attached to a nonhistone protein scaffold to form the condensed chromosome.2. the analogous structure carrying the genetic material in prokaryotes, mitochondria, and chloroplasts; a closed circle of double-stranded DNA. chromosomal adj

Human male chromosomes with GTG banding, arranged as a karyotype.

Chromosome. (A), Long, threadlike interphase chromatin in nucleus. (B), Giemsa-stained, partially condensed chromatid in late prophase. (C), Giemsa-stained, fully condensed chromatid in metaphase; note that some of the sub-bands visible in late prophase have condensed into single bands. (In B and C, only one of a pair of sister chromatids is shown.)

Repeated symbols denote duplication of chromosome structure.

Symbols for rearrangements are placed before the chromosome number and the rearranged chromosomes are placed in parenthesis, e.g., t(14q21q), r(18).