Chromosome 21

Chromosome 21 (human)
Pair of human chromosome 21 (after G-banding). One is from mother, one is from father.
Chromosome 21 pair in human male karyogram.
Features
Length (bp)	46,709,983 bp
No. of genes	477 635
Type	Autosome
Centromere position	Acrocentric
Identifiers
RefSeq	NC_000021
GenBank	CM000683

Chromosome 21 is one of the 23 pairs of chromosomes in humans. Chromosome 21 is the smallest human autosome, with 48 million nucleotides (the building material of DNA) representing about 1.5 percent of the total DNA in cells. People without Down's syndrome have two copies of chromosome 21, while those with three copies of chromosome 21 have Down syndrome, also called "trisomy 21".

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. Because researchers use different approaches to genome annotation their predictions of the number of genes on each chromosome varies. In January 2017, two estimates differed by 33%, with one estimate giving 477 genes, and the other estimate giving 635 genes.

The following are some of the genes located on chromosome 21:

The following diseases are some of those related to genes on chromosome 21:

The following conditions are caused by changes in the structure or number of copies of chromosome 21: