In humans, the autosome that spans about 48 million base pairs, likely to contain about 200 to 300 genes, and with an acrocentric chromosome
In humans, the chromosomes occur in pairs and a typical somatic cell would contain 23 pairs of chromosomes in the nucleus. Each of these pairs has a characteristic length and banding pattern. There are two types of chromosomes: autosomes and allosomes. There are also two sets: the first set comes from the mother and the second set, from the father. Using idiograms, each chromosome may be identified one from the other based on the relative size and the banding pattern.
Chromosome 21 is an acrocentric autosome. It spans almost 48 million base pairs. It is likely to contain about 200 to 300 genes. Some of the genes found in chromosome 21 are CBS (encoding for cystathionine-beta-synthase), KCNE1 (for potassium voltage-gated channel, Isk-related family, member 1, KCNE2 (for potassium voltage-gated channel, Isk-related family, member 2), PDXK (for pyridoxal kinase), PSMG1 (for proteasome assembly chaperone 1), S100B (for calcium binding protein), and TMPRSS3 (for transmembrane protease, serine 3). Chromosme 21 is considered as the smallest human autosome.
Some of the diseases and disorders related to the genes on chromosome 21 include Down syndrome, amyotrophic lateral sclerosis, Erondu–Cymet syndrome, and homocystinuria