Another more traditional way to distinguish one chromosome from another is to stain them with dyes that reveal a striking and reproducible pattern of bands along each mitotic chromosome (Figure 4–11). These banding patterns presumably reflect variations in chromatin structure, but their basis is not well understood. Nevertheless, the pattern of bands on each type of chromosome is unique, and it provided the initial means to identify and number each human chromosome reliably.
Figure 4–10 The complete set of human chromosomes. These chromosomes, from a female, were isolated from a cell undergoing nuclear division (mitosis) and are therefore highly compacted. Each chromosome has been “painted” a different color to permit its unambiguous identification under the fluorescence microscope, using a technique called “spectral karyotyping.” Chromosome painting can be performed by exposing the chromosomes to a large collection of DNA molecules whose sequence matches known DNA sequences from the human genome. The set of sequences matching each chromosome is coupled to a different combination of fluorescent dyes. DNA molecules derived from chromosome 1 are labeled with one specific dye combination, those from chromosome 2 with another, and so on. because the labeled DNA can form base pairs, or hybridize, only to the chromosome from which it was derived, each chromosome becomes labeled with a different combination of dyes. For such experiments, the chromosomes are subjected to treatments that separate the two strands of double-helical DNA in a way that permits base-pairing with the single-stranded labeled DNA, but keeps the overall chromosome structure relatively intact. (A) The chromosomes visualized as they originally spilled from the lysed cell.
The same chromosomes artificially lined up in their numerical order. This arrangement of the full chromosome set is called a karyotype. (Adapted from
N.
mcNeil and T. Ried, Expert Rev. Mol. Med. 2:1–14, 2000. with permission from Cambridge University Press.)
Figure 4–11 The banding patterns of human chromosomes. Chromosomes 1–22 are numbered in approximate order of size. A typical human cell contains two of each of these chromosomes, plus two sex chromosomes—two X chromosomes in a female, one X and one Y chromosome in a male. The chromosomes used to make these maps were stained at an early stage in mitosis, when the chromosomes are incompletely compacted. The horizontal red line represents the position of the centromere (see Figure 4–19), which appears as a constriction on mitotic chromosomes. The red knobs on chromosomes 13, 14, 15, 21, and 22 indicate the positions of genes that code for the large ribosomal RNAs (discussed in Chapter 6). These banding patterns are obtained by staining chromosomes with giemsa stain, and they can be observed under the light microscope. (Adapted from
U. Francke, Cytogenet. Cell Genet. 31:24– 32, 1981. with permission from the author.)
Figure 4–12 Aberrant human chromosomes. (A) Two normal human chromosomes, 4 and 6. (b) In an individual carrying a balanced chromosomal translocation, the DNA double helix in one chromosome has crossed over with the DNA double helix in the other chromosome due to an abnormal recombination event. The chromosome painting technique used on the chromosomes in each of the sets allows the identification of even short pieces of chromosomes that have become translocated, a frequent event in cancer cells. (Courtesy of Zhenya Tang and the NIgmS Human genetic Cell Repository at the Coriell Institute for medical Research: gm21880.)