Chromosome

A chromosome is a single long molecule of DNA packaged with proteins into a compact, thread-like structure that carries genes. In eukaryotes, chromosomes are linear and sit inside the nucleus, with the DNA wound around spool-like proteins called histones to form chromatin. Most bacteria instead have a single circular chromosome lying free in the cytoplasm. Chromosomes are normally too loosely packed to see, but they condense before a cell divides and then become visible under a light microscope.

A human body cell holds 46 chromosomes in 23 pairs: 22 pairs of autosomes, numbered roughly from largest to smallest, plus the sex chromosomes, either XX or XY. One member of each pair comes from each parent, and eggs and sperm carry a single set of 23. Each chromosome has a narrowed region, the centromere, that divides it into a short arm (p) and a long arm (q), which helps describe where genes lie. Chromosome number varies widely between species.

Doctors examine chromosomes in a karyotype, an ordered picture of a cell's full set, to spot changes such as the extra copy of chromosome 21 in Down syndrome. The name means 'colored body', because chromosomes take up stains strongly.