I 9.3 A human karyotype consists of 46 chromosomes. A karyotype for a male is shown here; a karyotype for a female would have two X chromosomes. (ISM/Phototake).

Figure 2.8). On human chromosomes, the short arm is designated by the letter p and the long arm by the letter q.

The complete set of chromosomes that an organism possesses is called its karyotype and is usually presented as a picture of metaphase chromosomes lined up in descending order of their size ( FIGURE 9.3). 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, spread on a microscope slide, 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 often routinely prepared by automated machines, which scan a slide with a video camera attached to a microscope, looking for chromosome spreads. 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.

Preparation and staining techniques have been developed to help distinguish among chromosomes of similar size and shape. For instance, chromosomes may be treated with enzymes that partly digest them; staining with a special dye called Giemsa reveals G bands, which distinguish areas of DNA that are rich in adenine - thymine base pairs ( FIGURE 9.4a). Q bands ( FIGURE 9.4b) are revealed by staining chromosomes with quinacrine mustard and viewing the chromosomes under UV light. Other techniques reveal C bands ( FIGURE 9.4c), which are regions of DNA occupied by centromeric heterochromatin, and R bands ( FIGURE 9.4d), which are rich in guanine - cytosine base pairs. Pictures of karyotypes, including specific chromosome abnormalities, the analysis of human karyotypes, and links to a number of Web sites on chromosomes

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