| meiotic | Pertaining to meiosis. (12 Dec 1998) |
|---|---|
| meiotic division | A specialised form of nuclear division in which there two successive nuclear divisions (meiosis I and II) without any chromosome replication between them. Each division can be divided into 4 phases similar to those of mitosis pro, meta, ana and telophase). Meiosis reduces the starting number of 4n chromosomes in the parent cell to n in each of the 4 daughter cells. Each cell receives only one of each homologous chromosome pair, with the maternal and paternal chromosomes being distributed randomly between the cells. This is vital for the segregation of genes. During the prophase of meiosis I (classically divided into stages: Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis), homologous chromosomes pair to form bivalents, thus allowing crossing over, the physical exchange of chromatid segments. This results in the recombination of genes. Meiosis occurs during the formation of gametes in animals, which are thus haploid and fertilization gives a diploid egg. In plants meiosis leads to the formation of the spore by the sporophyte generation. (18 Nov 1997) |
| meiotic drive | Differential fitness in males and females. (05 Mar 2000) |
| meiotic nondisjunction | Failure of two memberrs of a chromosome pair to separate (disjoin) during meiosis so that both go to one daughter cell and none to the other. This mechanism is responsible for the extra chromosome 21 in trisomy 21 (down syndrome) and for extra and missing chromosomes causing other birth defects and many spontaneous abortions (miscarriages). (12 Dec 1998) |
| meiotic phase | The stage of nuclear changes in the sexual cells during which reduction of the chromosomes takes place; it embraces the cell generations of the spermatocytes and oocytes. Synonym: reduction phase. (05 Mar 2000) |
| meiotic spindle | The meiotic equivalent of the mitotic spindle. (18 Nov 1997) |
| second meiotic division | The second of two consecutive divisions of the nucleus of an eukaryotic cell during the process of meiosis. It includes the following stages of meiosis: prophase II, metaphase II, anaphase II, and telophase II. (09 Oct 1997) |
| first meiotic division | The first of two consecutive divisions of the nucleus of an eukaryotic cell during the process of meiosis. It includes the following stages of meiosis: prophase I, metaphase I, anaphase I, and telophase I. (09 Oct 1997) |
| abscopal effect | A reaction produced following irradiation but occurring outside the zone of actual radiation absorption. (05 Mar 2000) |
| additive effect | <biochemistry, chemistry> An additive effect is the overall biological effect two chemicals acting together and which is the simple sum of the effects of the chemicals acting independently. Compare: antagonism. (15 Jan 1998) |
| adverse effect | This is an abnormal or harmful effect to an organism caused by exposure to a chemical. It is indicated by some result such as death, a change in food or water consumption, altered body and organ weights, altered enzyme levels, or visible illness. An effect may be classed as adverse if it causes functional or anatomical damage, causes irreversible change in the homeostasis of the organism, or increases the susceptibility of the organism to other chemical or biological stress. A non-adverse effect will usually be reversed when the organism is no longer being exposed to the chemical. (09 Oct 1997) |
| Anrep effect | A small transient positive inotropic effect of abrupt increases of systolic aortic and left ventricular pressures related to recovery from transient subendocardial ischemia (e.g., cold pressor test). (05 Mar 2000) |
| antagonistic effect | This is the consequence of one chemical (or group of chemicals) counteracting the effects of another chemical, the opposing chemicals cancel out each other's effects. (09 Oct 1997) |
| Arias-Stella effect | Focal, unusual, decidual changes in endometrial epithelium, consisting of intraluminal budding, and nuclear enlargement and hyperchromatism with cytoplasmic swelling and vacuolation; may be associated with ectopic or uterine pregnancy. Synonym: Arias-Stella effect, Arias-Stella reaction. (05 Mar 2000) |
| Auger effect | <physics> Transition of an electron in an atom from a discrete electronic level to an ionised continuous level with the same energy. Synonym: autoionisation. (13 Jan 1998) |