Chromosome regions

Diagram of a duplicated and condensed (metaphase) eukaryotic chromosome. (1) Chromatid - one of the two identical parts of the chromosome after S phase. (2) Centromere - the point where the two chromatids touch, and where the microtubules attach. (3) Short arm (p). (4) Long arm (q).

Several chromosome regions have been defined by convenience in order to talk about gene locations. Most important is the distinction between chromosome region p and chromosome region q. These are virtual regions that exist in all chromosomes.

During cell division, the molecules that compose chromosomes (DNA and proteins) suffer a condensation process (called the chromatin condensation), and forms a compact and small complex. In diploid organisms, homologous chromosomes get attached to each other by the centromere. The centromere divides each chromosome into two regions: the smaller one, which is the p region, and the bigger one, the q region.

Usually, as a convention, the p region is represented in the upper part of an image while the q region is in the bottom part.

At either end of a chromosome is a telomere, a cap of DNA that protects the rest of the chromosome from damage. The areas of the p and q regions close to the telomeres are the subtelomeres, or subtelomeric regions. The areas closer to the centromere are the pericentronomic regions. Finally, the interstitial regions are the parts of the p and q regions that are close to neither the centromere nor the telomeres, but are roughly in the middle of p or q.