Chromosome 21 is one of the 23 pairs of humanchromosomes. It has 48 millionbase pairs (the building material ofDNA). This is about 1.5 percent of the total DNA in cells. Chromosome 21 is the smallest humanautosome (non-sex chromosome).
There are two copies of chromosome 21 in most people. Anyone with three copies of chromosome 21 hasDown syndrome, also known astrisomy 21.
In May 2000, researchers working on theHuman Genome Project announced that the sequence of base pairs which make up this chromosome had been identified. Chromosome 21 was the second human chromosome to be completely sequenced.Chromosome 22 was the first.