Chromosome 21 is one of the 23 pairs of chromosomes in the human karyotype. The population normally has two copies of this chromosome. Chromosome 21 is the smallest human autosome, containing about 47 million nucleotides, representing about 1.5% of total DNA in the cell.
In 2000, researchers working on the Human Genome Project announced that they had determined the sequence of the base pairs that make up this chromosome. Chromosome 21 was the second human chromosome to be fully sequenced.


The estimated number of genes on chromosome 21 which houses is between 200 and 400 genes. Some of these genes are:
APP: beta amyloid precursor protein (A4) (peptidase nexin-II, Alzheimer disease)
CBS: cystathionine-beta-synthase
CLDN14: claudin 14
HLCS: holocarboxylase synthetase (biotin-(proprionil-Coenzyme A-carboxylase (ATP-hydrolysing)) ligase)
KCNE1 potassium channel activated by voltage, Isk-related family, member 1
KCNE2: activated potassium channel voltage, Isk-related family, member 1
LAD: leukocyte adhesion deficiency (symbols: ITGB2, CD18, LCAMB)
SOD1: superoxide dismutase 1, soluble (amyotrophic lateral sclerosis, type 1 (adult))
TMPRSS3: transmembrane protease, serine 3
[Edit] Diseases and disorders

The following are some of the diseases related to genes on chromosome 21:
Alzheimer's Disease
Alzheimer's disease type 1
Amyotrophic Lateral Sclerosis
Amyotrophic lateral sclerosis type 1
Down Syndrome
Holocarboxylase synthetase deficiency
Homocystinuria
Jervell and Lange-Nielsen
Leukocyte adhesion deficiency
Nonsyndromic Deafness
Nonsyndromic deafness, autosomal recessive
Romano-Ward syndrome
[Edit] Conditions chromosomal

The following conditions are caused by changes in the structure or number of copies of chromosome 21:
Cancers: Reorganizations (translocations) of genetic material from chromosome 21 and other chromosomes have been associated with various cancers. For example, acute lymphoblastic leukemia (a type of blood cancer most commonly diagnosed in childhood) has been associated with a translocation between chromosomes 12 and 21. Another way leicemia, acute myeloid leukemia, has been associated with a translocation between chromosome 8 and 21.
In a small percentage of cases, the syndrome is caused by a chromosomal rearrangement of material between chromosome 21 and chromosome another. As a result, the person has the usual two copies of chromosome 21, plus extra material from chromosome 21 attached to another chromosome.
Researchers believe that the extra copies of genes on chromosome 21 corrupt the course of normal development, causing the typical features of Down syndrome and increased risk of medical problems associated with this disorder.
Other changes in the number or structure of chromosome 21 can have a variety of effects, including mental retardation, developmental delay, and certain facial features. In some cases, signs and symptoms are similar to those of Down syndrome. Changes in chromosome 21 include the loss of a segment of chromosome in each cell (partial monosomy 21) and a circular structure called ring chromosome 21. An annular chromosome occurs when both ends of the broken chromosome are joined.
It was found that a doubling in the locus of the amyloid precursor protein (duplicate segment varies in length, but includes APP) on chromosome 21 causing premature Alzheimer's disease in a set of French families (Rovelet-Lecrux et al) and Dutch ( Sleegers et al). Compared with Alzheimer's disease caused by nonsense mutations in APP, the frequency of Alzheimer's disease caused by duplication of APP is significant. All patients who have an extra copy of the APP gene due to duplication gene locus suffering with Alzheimer severe cerebral amyloid angiopathy.