Each functional eukaryotic chromosome has a centromere, to which spindle microtubules attach, and two telomeres, which stabilize the chromosome ends (see Figure 2.6). Chromosomes are classified into four basic types (see Figure 2.7):

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 (Figure 6.1). 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 routinely prepared by automated machines, which scan a slide using 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 help to distinguish among chromosomes of similar size and shape. For instance, special preparation and staining of chromosomes with a dye called Giemsa reveals G bands (Figure 6.2a), which distinguish areas of DNA that are rich in adenine–thymine (A–T) base pairs (see Chapter 8). Q bands (Figure 6.2b) are revealed by staining chromosomes with quinacrine mustard and viewing the chromosomes under ultraviolet light; variation in the brightness of Q bands results from differences in the relative amounts of cytosine–guanine (C–G) and adenine–thymine base pairs. Other techniques reveal C bands (Figure 6.2c), which are regions of DNA occupied by centromeric heterochromatin, and R bands (Figure 6.2d), which are rich in cytosine–guanine base pairs.

151