| prophase | Classical term for the first phase of mitosis or of one of the divisions of meiosis. During this phase the chromosomes condense and become visible. (18 Nov 1997) |
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| prophase i | The first stage in the first meiotic division of meiosis. The individual chromosomes of the cell become clearly visible within the nucleus with a light microscope as they condense from long, thin, wispy structures to thick structures, and they appear as a tangled jumble of paired, identical chromatids. Prophase ends with the disappearance of the nuclear membrane. This is identical to the prophase stage of mitosis. (09 Oct 1997) |
| prophase II | The first stage in the second meiotic division of meiosis, following telophase I. In each of the two cells which were produced by the first meiotic division, the chromosomes migrate towards the cells centre. The stage ends when the chromosomes are aligned along a single plane in the centre of each cell. (09 Oct 1997) |
| 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) |