| nucleotide pair | <molecular biology> Two nitrogenous bases (adenine and thymine or guanine and cytosine) held together by weak bonds. Two strands of DNA are held together in the shape of a double helix by the bonds between base pairs. (09 Oct 1997) |
|---|---|
| accessory chromosome | A chromosome existing without its normal homologous chromosome; at the reduction division of gametogenesis an accessory chromosome 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. Synonym: monosome, odd chromosome, unpaired allosome, unpaired chromosome. (05 Mar 2000) |
| acentric chromosome | A fragment of a chromosome 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. Synonym: acentric fragment. (05 Mar 2000) |
| acrocentric chromosome | A chromosome with the centromere placed very close to one end so that the short arm is very small, often with a satellite. (05 Mar 2000) |
| balanced chromosome | <genetics> A chromosome which is unable to pair with its homologue and participate in homologus recombination during meiosis because it contains several inversion mutations (that is, has segments which have become flip-flopped). (09 Oct 1997) |
| B chromosome | <genetics> Small acentric chromosome, part of the normal genome of some races and species of plants. (18 Nov 1997) |
| bivalent chromosome | A pair of chromosome's temporarily united. (05 Mar 2000) |
| male chromosome complement | The large majority of males have a 46, xy chromosome complement (46 chromosomes including an x and a y chromosome). A minority of males have other chromosome constitutions such as 47,xxy (47 chromosomes including two x chromosomes and a y chromosome) and 47,xyy (47 chromosomes including an x and two y chromosomes). (12 Dec 1998) |
| marker chromosome | An abnormal chromosome that is distinctive in appearance but not fully identified. For example, the fragile x chromosome was once called the marker x. (12 Dec 1998) |
| p arm of a chromosome | The short arm of a chromosome (from the french petit meaning small). All human chromosomes have 2 arms: the p and q arms. (12 Dec 1998) |
| giant chromosome | <cell biology> Giant chromosomes produced by the successive replication of homologous pairs of chromosomes, joined together (synapsed) without chromosome separation or nuclear division. They thus consist of many up to 1000) identical chromosomes (strictly chromatids) running parallel and in strict register. The chromosomes remain visible during interphase and are found in some ciliates, ovule cells in angiosperms and in larval Dipteran tissue. The best known polytene chromosomes are those of the salivary gland of the larvae of Drosophila melanogaster which appear as a series of dense bands interspersed by light interbands, in a pattern characteristic for each chromosome. The bands, of which there are about 5,000 in Drosophila melanogaster, contain most of the DNA (ca 95%) of the chromosomes and each band roughly represents one gene. The banding pattern of polytene chromosomes provides a visible map to compare with the linkage map determined by genetic studies. Some segments of polytene chromosome show chromosome puffs, areas of high transcription. (18 Nov 1997) |
| Giemsa chromosome banding stain | <technique> A unique chromosome staining technique, used in human cytogenetics to identify individual chromosomes, which produces characteristic bands. It utilises acetic acid fixation, air drying, denaturing chromosomes mildly with proteolytic enzymes, salts, heat, detergents, or urea, and finally Giemsa stain; chromosome bands appear similar to those fluorochromed by Q-banding stain. Synonym: Giemsa chromosome banding stain. (05 Mar 2000) |
| metacentric chromosome | A chromosome with a centrally placed centromere that divides the chromosome into two arms of approximately equal length. (05 Mar 2000) |
| ring chromosome | A structurally abnormal chromosome in which the end of each chromosome arm has been lost and the broken arms have been reunited in ring formation. A ring chromosome is denoted by the symbol r. (12 Dec 1998) |
| Christchurch chromosome | An abnormal small acrocentric chromosome (no. 21 or 22) with complete or almost complete deletion of the short arm; found in cultured leukocytes in some cases of chronic lymphocytic leukaemia, also in some normal relatives of patients. (05 Mar 2000) |