Chromosomes

Chromosomes are only visible as distinct, condensed, multicoiled bodies at the time the nucleus divides; for most of the cell's life the same genetic material exists as long, thin, uncoiled threads that stain to give the nucleus a granular look, and this granular material is called chromatin. Four features define a chromosome. Its shape is specific and constant for that chromosome: a long, thin structure interrupted by a short constricted region, the centromere, which always sits at the same position along a given chromosome. The number of chromosomes is fixed for each species - for example the mouse has 40, the onion has 16 and humans have 46 - and this number is a reliable identifying feature of the species. Chromosomes occur in homologous pairs, one member of each pair inherited from each parent; in humans, 23 of the 46 chromosomes come from the mother and 23 from the father through sexual reproduction. Finally, chromosomes are copied between nuclear divisions: while they are in their uncoiled, invisibl …