Karyotype

                 The complete set of chromosomes possessed by an organism is called its karyotype. An organism’s karyotype is usually presented as a picture of metaphase chromosomes lined up in descending order of their size. Karyotypes are prepared from actively dividing cells, such as white blood cells, bone marrow cells, or cells from meristematic tissues of plants. After treatment with a chemical (such as colchicine) that prevents them from entering anaphase, the cells are chemically preserved. They are then burst open to release the chromosomes onto a microscope slide, and the chromosomes are stained and photographed. The photograph is then enlarged, and the individual chromosomes are cut out and arranged in a karyotype. For human chromosomes, karyotypes are routinely prepared by automated machines, which scan a slide using a video camera attached to a microscope, looking for a chromosome spread (a group of chromosomes that are well separated). When a spread has been located, the camera takes a picture of the chromosomes, the image is digitized, and the chromosomes are sorted and arranged electronically by a computer.