Chromosome 21 is one of the 23 pairs of chromosomes in humans. Chromosome 21 is both the smallest human autosome and chromosome, with 46.7 million base pairs (the building material of DNA) representing about 1.5 percent of the total DNA in cells. Most people have two copies of chromosome 21, while those with three copies of chromosome 21 (trisomy 21) have Down syndrome.

Researchers working on the Human Genome Project announced in May 2000 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.

Genes Number of genes The following are some of the gene count estimates of human chromosome 21. Because researchers use different approaches to genome annotation, their predictions of the number of genes on each chromosome varies (for technical details, see gene prediction). Among various projects, the collaborative consensus coding sequence project (CCDS) takes an extremely conservative strategy. Thus CCDS's gene number prediction represents a lower bound on the total number of human protein-coding genes.

Estimated by Protein-coding genes Non-coding RNA genes Pseudogenes Source Release date CCDS 212 — — 2025-01-15 HGNC 215 193 194 2025-09-26 Ensembl 221 450 184 2024-05-13 UniProt 244 — — 2024-11-27 NCBI 254 637 246 2025-01-19

Gene list

The following is a partial list of genes on human chromosome 21. For complete list, see the link in the infobox at the top of the article.

In addition, the chromosome has many genes for keratin-associated protein, with symbols: KRTAP6-1, KRTAP6-2, KRTAP6-3, KRTAP7-1, KRTAP8-1, KRTAP10-1, KRTAP10-2, KRTAP10-3, KRTAP10-4, KRTAP10-5, KRTAP10-6, KRTAP10-7, KRTAP10-8, KRTAP10-9, KRTAP10-10, KRTAP10-11, KRTAP10-12, KRTAP11-1, KRTAP12-1, KRTAP12-1, KRTAP12-2, KRTAP12-3, KRTAP12-4, KRTAP13-1, KRTAP13-2, KRTAP13-3, KRTAP13-4, KRTAP15-1, KRTAP19-1, KRTAP19-2, KRTAP19-3, KRTAP19-4, KRTAP19-5, KRTAP19-6, KRTAP19-7, KRTAP19-8, KRTAP20-1, KRTAP20-2, KRTAP20-3, KRTAP20-4, KRTAP21-1, KRTAP21-2, KRTAP21-3, KRTAP22-1, KRTAP22-2, KRTAP23-1, KRTAP24-1, KRTAP25-1, KRTAP26-1, KRTAP27-1.

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

Chromosomal conditions

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

Cancers: Rearrangements (translocations) of genetic material between chromosome 21 and other chromosomes have been associated with several types of cancer. For example, acute lymphoblastic leukemia (a type of blood cancer most often diagnosed in childhood) has been associated with a translocation between chromosomes 12 and 21. Another form of leukemia, acute myeloid leukemia, has been associated with a translocation between chromosomes 8 and 21. In a small percentage of cases, Down syndrome is caused by a rearrangement of chromosomal material between chromosome 21 and another chromosome. As a result, a person has the usual two copies of chromosome 21, plus extra material from chromosome 21 attached to another chromosome. These cases are called translocation Down syndrome. Researchers believe that extra copies of genes on chromosome 21 disrupt the course of normal development, causing the characteristic features of Down syndrome and the 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 intellectual disability, delayed development, and characteristic facial features. In some cases, the signs and symptoms are similar to those of Down syndrome. Changes to chromosome 21 include a missing segment of the chromosome in each cell (partial monosomy 21) and a circular structure called ring chromosome 21. A ring chromosome occurs when both ends of a broken chromosome are reunited. Duplication in amyloid precursor protein (APP) locus (duplicated segment varies in length but includes APP) on chromosome 21 was found to cause early onset familial Alzheimer's disease in a French family set and a Dutch family set. Compared to Alzheimer's caused by missense mutations in APP, the frequency of the Alzheimer's caused by APP duplications is significant. All patients that have an extra copy of APP gene due to the locus duplication show Alzheimer's with severe cerebral amyloid angiopathy.

Cytogenetic band

+ G-bands of human chromosome 21 in resolution 850 bphs Chr. Arm"p": Short arm; "q": Long arm. BandFor cytogenetic banding nomenclature, see article locus. ISCNstartThese values (ISCN start/stop) are based on the length of bands/ideograms from the ISCN book, An International System for Human Cytogenetic Nomenclature (2013). Arbitrary unit. ISCNstop Basepairstart Basepairstop Staingpos: Region which is positively stained by G banding, generally AT-rich and gene poor; gneg: Region which is negatively stained by G banding, generally CG-rich and gene rich; acen Centromere. var: Variable region; stalk: Stalk. Density 21 p 13 0 311 gvar 21 p 12 311 683 stalk 21 p 11.2 683 1056 gvar 21 p 11.1 1056 1274 acen 21 q 11.1 1274 1367 acen 21 q 11.2 1367 1584 gneg 21 q 21.1 1584 2019 gpos 100 21 q 21.2 2019 2144 gneg 21 q 21.3 2144 2330 gpos 75 21 q 22.11 2330 2485 gneg 21 q 22.12 2485 2610 gpos 50 21 q 22.13 2610 2703 gneg 21 q 22.2 2703 2858 gpos 50 21 q 22.3 2858 3200 gneg

References

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