Special chromosome staining procedures have revealed intricate sets of bands (transverse stripes) in many different organisms. The positions and sizes of the bands are highly chromosome-specific; therefore, they represent useful landmarks. There are Q bands (produced by quinacrine hydrochloride), G bands (produced by Giemsa stain), and R bands (produced by reversed Giemsa).
A rather specialized kind of banding occurs in a few organisms whose chromosomes can replicate their DNA many times without separating. This produces giant chromosomes, which are in essence magnified versions of the unreplicated forms. Consequently, the natural banding patterns of the chromosomes become readily visible and can serve as landmarks. These polytene chromosomes (polytene means “many-stranded”) are found in highly specialized cells of Malpighian tubules, rectum, gut, footpads, and salivary glands of the dipteran insects such as houseflies, mosquitoes, and fruit flies.
The fruit fly Drosophila melanogaster is a much-studied example. This insect (a diploid) has a 2n number of 8, and these eight chromosomes are present in most cells. However, in the cells of the special organs that contain the polytene chromosomes we see some interesting peculiarities. First, there are only four polytene chromosomes per cell (not eight) because during the specialized replication process the members of each homologous pair unexpectedly unite with each other. Second, all four polytene chromosomes become joined at a structure called the chromocenter, which is a coalescence of the heterochromatic areas around the centromeres of all four chromosomes. The chromocenter of Drosophila salivary gland chromosomes is shown in Figure, where L and R stand for arbitrarily assigned left and right arms.
Along the length of a polytene chromosome, there are transverse bands. Polytene bands are much more numerous than Q, G, or R bands, numbering in the hundreds on each chromosome. The bands differ in width and appearance, so that the banding pattern of each chromosome is unique and characteristic of that chromosome.
Molecular studies have shown that in any chromosomal region of Drosophila there are more genes than there are polytene bands, so the bands do not represent genes. Similarly, the significance of the Q, G, and R patterns in other eukaryotic cells is not clear. They probably reflect the degree of compactness of the DNA, but it is not known how this pattern is maintained.