Figure 1. Types of human metaphase chromosomes. Metaphase chromosomes have two chromatids (also termed sister chromatids) which are held together at the centromere. In metaphase chromosomes the centromere is usually readily visible as a constriction. The centromere divides each of the chromatids into two chromosome arms. The short arm is referred to as the p arm, the long arm is referred to as the q arm. Chromosomes with centromeres close to the middle as shown on the left-hand side have arms of equal length and are known as metacentric chromosomes. If the centromere is not centrally located, resulting in arms which are unequal in length (as shown in the middle), the chromosome is termed submetacentric. A chromosome with a centromere very close to one end is called acrocentric. The p arm of acrocentric chromosomes is often referred to as satellite. The regions at both ends of the chromosome are the telomeres. Chromosomes can be identified by size, centromere location and banding pattern (see Figure 3).

Figure 2. Schematic view of the telomere structure and polymorphic subtelomere domains (for details see Flint et al. 1997). Human telomeres end with a simple repeat TTAGGG. This sequence is repeated hundreds of thousands times, resulting in a size ranging from 2 to 15 kb in length. Adjacent to the TTAGGG repeats are complex families of repetitive DNA. They may have a length of several hundred kilobases and they may be present on many other chromosomes. Interstitial degenerate (TTAGGG)_n repeats subdivide the subtelomere regions into distal and proximal subdomains with different patterns of homology to other chromosome ends. Adjacent proximally to the subtelomere sequences are the chromosome unique sequences.

Figure 3. GTG-banded normal male metaphase spread.

Figure 4. Different representations of chromosome 1. (a) GTG-banded chromosome 1, taken from Figure 3. (b) 850-band ideogram reflecting GTG bands. The relative widths of euchromatic bands are based on measurements and staining intensities (Francke, 1994). (c) In silicio bands for chromosome 1, which were derived computationally from the whole genome sequence (for details see Niimura and Gojobori 2002).

Figure 5. C-banded human male metaphase spread. C banding produces a selective staining of constitutive heterochromatin. C bands are located at the centromere of all chromosomes. They are best visible in chromosomes 1, 9 and 16 and at the long arm of the Y chromosome.

Figure 6. Ag-NOR banding of a normal female metaphase. This technique aims at the specific staining of the chromosomal regions that form and maintain the nucleoli in interphase nuclei, the so-called nucleolar organizing regions (NORs). The ‘Ag’ indicates that the staining was done by silver impregnation. Ag-NOR staining reflects the transcriptional activity of the NORs (DA Miller et al., 1976; OJ Miller et al., 1976). Thus, not all p arms of the acrocentric chromosomes show staining. Most individuals have four to seven active NORs per cell (Varley et al., 1981).

Figure 7. Folding and packing of chromosomal DNA. Scheme of different levels of packing of DNA from the double helix to a metaphase chromosome. (This is a composite of images taken from Passarge (2001) Color Atlas of Genetics, pp. 170–173 (stuttgart: Theme) and Horn and Peterson 2002 Science 297: 1824–1827.)