FIGURE 3.2 • Packaging of chromatin into the chromosomal structure. a. Sequential steps in the packaging of nuclear chromatin are shown in this diagram, beginning with the DNA double helix and ending with the highly condensed form found in chromosomes. b. Structure of human metaphase chromosome 2 as visible in atomic force microscopic image. 20,000. (Courtesy of Dr. Tatsuo Ushiki.) same shape and size). Twenty-two pairs have identical chromosomes (i.e., each chromosome of the pair contains the same portion of the genome) and are called autosomes. The twenty-third pair of chromosomes are the sex chromosomes, designated X and Y. Females contain two X chromosomes; males contain one X and one Y chromosome. The chromosomal number, 46, is found in most of the somatic cells of the body and is called the diploid (2n) number. To simplify the description of chromosomal number and DNA changes during mitosis and meiosis, we use the lower case letter (n) for chromosome number and lower case letter (d) for DNA content. Diploid chromosomes have the (2d) amount of DNA immediately after cell division but have twice that amount—that is, the (4d) amount of DNA—after the S phase (see page 89).
As a result of meiosis, eggs and sperm have only 23 chromosomes, the haploid (1n) number, as well as the haploid (1d) amount of DNA. The somatic chromosome number (2n) and the diploid (2d) amount of DNA are reestablished at fertilization by the fusion of the sperm nucleus with the egg nucleus.
In a karyotype, chromosome pairs are sorted according to their size, shape, and emitted fluorescent color.
A preparation of chromosomes derived from mechanically ruptured, dividing cells that are then fixed, plated on a microscope slide, and stained is called a metaphase spread. In the past, chromosomes were routinely stained with Giemsa stain; however, with the recent development of in situ hybridization techniques, the fluorescent in situ hybridization (FISH) procedure is now more often used to visualize a chromosomal spread. These spreads are observed with fluorescence microscopes, and computer-controlled cameras are then used to capture images of the chromosome pairs. Image-processing software is used to sort the chromosome pairs according to their morphology to form a karyotype (see Fig. F3.1.1a). A variety of molecular probes that are now commercially available are used in cytogenetic testing to diagnose disorders caused by chromosomal abnormalities such as nondisjunctions, transpositions (see Fig. F3.1.1a), deletions (see Fig. F3.1.1b), and duplications of specifc gene sites. Karyotypes are also used for prenatal determination of sex in fetuses and for prenatal screening for certain genetic diseases (see Fig. 1.7).
The Barr body can be used to identify the sex of a fetus.
Some chromosomes are repressed in the interphase nucleus and exist only in the tightly packed heterochromatic form. One X chromosome of the female is an example of such a chromosome. This fact can be used to identify the sex of a fetus. This chromosome was discovered in 1949 by Barr and Bartram in nerve cells of female cats, where it appears as a well-stained round body, now called the Barr body, adjacent to the nucleolus.