Hypermedia Glossary Of Genetic Terms
|Chromatid||The term which McClung (1900) proposed for each of the four threads making up a chromosome-pair at meiosis (Gk. chroma, colour; for the derivation of '-id', see diploid). The use of the term was subsequently extended to mitosis, and is now applied to the individual daughter-chromosomes (strand) into which each chromosome is divided in all nuclear divisions. The term chromatid is used so long as the daughter centromeres remain in contact with one another. As soon as they separate (anaphase of mitosis and anaphase 2 of meiosis), the expression daughter-chromosome is substituted for chromatid.|
|Meiosis||The term coined by Farmer and Moore (1905) for the process of two consecutive cell divisions in the diploid progenitors of sex cells. Meiosis results in four rather than two daughter cells (gametes), each with a haploid set of each chromosome pair. In meiosis I the prophase is more complex than that of mitosis. Five different stages can be differentiated: leptotene, zygotene, pachytene, diplotene and diakinesis. Prophase is followed by metaphase I, anaphase I, telophase I and interkinesis. Meiosis II could be described as a haploid mitosis resulting in four haploid gametes.
|Diploid||A full set of genetic material, consisting of paired chromosomes one chromosome from each parental set. Most animal cells except the gamete have a diploid set of chromosomes. The diploid human genome has 46 chromosomes. Compare haploid and polyploid.|
|Mitosis||The most frequent process of nuclear division (karyokinesis) in cells that produces daughter cells that are genetically identical to each other and to the parent cell. The mitosis is divided into four (or five) phases: prophase, prometaphase, metaphase, anaphase, telophase. Mitosis and interphase make the cell cylcle.|
|Centromere||The term introduced by Darlington (1936) for the specialized chromosome region which reacts to the spindle at nuclear division and to which spindle fibers attach during cell division. Appears as a distinct "waist" by microscopy. In a few organisms the centromeric properties are distributed over the entire length of the chromosome (diffuse centromere).|
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