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 loci. 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), that forms a compact and small complex called a chromatid. The complexes containing the duplicated DNA molecules, the sister chromatids, are 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. The sister chromatids will be distributed to each daughter cell at the end of the cell division.

The p region is represented in the shorter arm of the chromosome (p is for petit, French for small) while the q region is in the larger arm (chosen as next letter in alphabet after p).

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.