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chromosome

chromosome (kro´mo-som)

One of the bodies (normally 46 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 microbes, the c. is prokaryotic, not being enclosed within a nuclear membrane and not being subject to a mitotic mechanism. [chromo- + G. soma, body]
accessory c. a c. existing without its normal homologous c.; at the reduction division of gametogenesis an accessory c. is likely to be included in one daughter cell and not in the other, but may be lost completely by lagging behind on the equatorial plate.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.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 c.'s temporarily united.
Christchurch c. (Ch1) 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.translocation c;
dicentric c. a c. with two centromeres that may result from reciprocal translocation.
double minute c.'s 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's syndrome.
giant c. 1. polytene c 2. lampbrush c
heterotypical c. allosome
homologous c.'s members of a single pair of c.'s.
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.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 c.'s.
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 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 c.'s c.'s that are not members of the same pair.
nucleolar c. a c. regularly associated with a nucleolus.
odd c. accessory c
Philadelphia c. (Ph1) an abnormal minute c. formed by a rearrangement of c.'s 9 and 22; 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.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 c.'s the pair of c.'s responsible for sex determination. In humans and most animals, the sex c.'s are designated X and Y; females have two X c.'s, males have one X and one Y c. In certain birds, insects, and fishes the sex c.'s are designated Z and W; males have two Z c.'s, females may have one Z and one W c., or one Z and no W c.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 c.'s are unstable and arise by misdivision or breakage near the centromere and are usually eliminated within a few cell divisions.
translocation c. derivative c
unpaired c. accessory c
W c. , X c. , Y c. , Z c. See sex c.'s.
yeast artificial c.'s (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 c.'s permits the cloning of large genes with their flanking regulatory sequences.

 

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