Chromosome 21 (human)
Chromosome 21 is one of the 23 pairs of chromosomes in humans. People without Down's syndrome have two copies of this chromosome. The trisomy of the 21st chromosome causes Down's syndrome. Chromosome 21 is the smallest human chromosome, with 48 million nucleotides (the building material of DNA) representing about 1.5 percent of the total DNA in cells.
In 2000, researchers working on the Human Genome Project announced that they had determined the sequence of base pairs that make up this chromosome. Chromosome 21 was the second human chromosome to be fully sequenced, after chromosome 22.
Identifying genes on each chromosome is an active area of genetic research. However, because researchers use different approaches to predict the number of genes on each chromosome, the estimated number of genes varies. Depending on the genome annotation used, chromosome 21 contains 477 or 635 genes.
Genes
The following are some of the genes located on chromosome 21:
APP: amyloid beta (A4) precursor protein (peptidase nexin-II, Alzheimer disease)
