Cytogenetics is the study of the structure and numeric characteristics of the cell’s chromosomes. Chromosome studies can be done on any tissue or cell that grows and divides in culture. Lymphocytes from venous blood are frequently used for this purpose. After the cells have been cultured, a drug called colchicine is used to arrest mitosis in metaphase. A chromosome spread is prepared by fixing and spreading the chromosomes on a slide. Subsequently, appropriate staining techniques show the chromosomal banding patterns so they can be identified. The chromosomes are photographed, and the photomicrographs of each of the chromosomes are cut out and arranged in pairs according to a standard classification system (see Fig. 6.6). The completed picture is called a karyotype, and the procedure for preparing the picture is called karyotyping. A uniform system of chromosome classification was originally formulated at the 1971 Paris Chromosome Conference and was later revised to describe the chromosomes as seen in more elongated prophase and pro-metaphase preparations.
In the metaphase spread, each chromosome takes the form of chromatids to form an “X” or “wishbone” pattern. Human chromosomes are divided into three types according to the position of the centromere. If the centromere is in the center and the arms are of approximately the same length, the chromosome is said to be metacentric; if it is not centered and the arms are of clearly different lengths, it is submetacentric; and if it is near one end, it is acrocentric. The short arm of the chromosome is designated as “p” for “petite,” and the long arm is designated as “q” for no other reason than it is the next letter of the alphabet. The arms of the chromosome are indicated by the chromosome number followed by the p or q designation (e.g., 15p). Chromosomes 13, 14, 15, 21, and 22 have small masses of chromatin called satellites attached to their short arms by narrow stalks. At the ends of each chromosome are special DNA sequences called telomeres. Telomeres allow the end of the DNA molecule to be replicated completely.
The banding patterns of a chromosome are used in describing the position of a gene on a chromosome. Each arm of a chromosome is divided into regions, which are numbered from the centromere outward (e.g., 1, 2). The regions are further divided into bands, which are also numbered (Fig. 6.9). These numbers are used in designating the position of a gene on a chromosome. For example, Xp22 refers to band 2, region 2 of the short arm (p) of the X chromosome.