Chromosome 21
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Chromosome 21 is one of the 23 pairs of chromosomes in humans. Chromosome 21 is both the smallest human autosome and chromosome,[1] 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.[2] 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.[3]
| Estimated by | Protein-coding genes | Non-coding RNA genes | Pseudogenes | Source | Release date |
|---|---|---|---|---|---|
| CCDS | 212 | — | — | [4] | 2025-01-15 |
| HGNC | 215 | 190 | 194 | [5] | 2025-01-15 |
| Ensembl | 221 | 450 | 184 | [6] | 2024-05-13 |
| UniProt | 244 | — | — | [7] | 2024-11-27 |
| NCBI | 254 | 637 | 246 | [8][9][10] | 2025-01-19 |
Gene list
Script error: No such module "Category see also".Template:Category see also/Category pair check 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.
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Diseases and disorders
The following diseases and disorders are some of those related to genes on chromosome 21:
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- Acute myeloid leukemia
- Alzheimer's disease[11]
- Amyotrophic lateral sclerosis
- Atrial fibrillation, familial
- Autoimmune polyendocrine syndrome type 1
- Basal ganglia calcification
- Bartsocas–Papas syndrome
- Bethlem myopathy
- Closed angle glaucoma
- Cataract
- CHAND syndrome
- Down syndrome
- Epilepsy
- Erondu–Cymet syndrome
- Ewing sarcoma
- Galloway Mowat syndrome
- Glucocorticoid deficiency
- Hepatitis B (susceptibility to)
- Hereditary motor and sensory neuropathy
- Holocarboxylase synthetase deficiency
- Homocystinuria
- Hyperhomocysteinemia
- Hypotrichosis
- Immunodeficiency
- Inflammatory bowel disease
- Intellectual developmental disorder
- Jervell and Lange-Nielsen syndrome
- Keppen–Lubinsky syndrome
- Knobloch syndrome
- Leukocyte adhesion deficiency-1
- Majewski osteodysplastic primordial dwarfism type II (MOPD II, or MOPD2)
- Non-small cell lung carcinoma[12]
- Neurodevelopmental disorder
- Nonsyndromic deafness
- Parkinson disease
- Peripheral neuropathy
- Phosphofructokinase deficiency
- Primary ciliary dyskinesia
- Primary immunodeficiency
- Primitive neuroectodermal tumor
- Prostate cancer
- Romano–Ward syndrome
- Spastic quadriplegia
- Ullrich congenital muscular dystrophy
- Unverricht–Lundborg disease, a form of progressive myoclonus epilepsy
- ZTTK syndrome
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[13] and a Dutch family set.[11] 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
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| Chr. | Arm[15] | Band[16] | ISCN start[17] |
ISCN stop[17] |
Basepair start |
Basepair stop |
Stain[18] | Density |
|---|---|---|---|---|---|---|---|---|
| 21 | p | 13 | 0 | 311 | Script error: No such module "val". | Script error: No such module "val". | gvar | |
| 21 | p | 12 | 311 | 683 | Script error: No such module "val". | Script error: No such module "val". | stalk | |
| 21 | p | 11.2 | 683 | 1056 | Script error: No such module "val". | Script error: No such module "val". | gvar | |
| 21 | p | 11.1 | 1056 | 1274 | Script error: No such module "val". | Script error: No such module "val". | acen | |
| 21 | q | 11.1 | 1274 | 1367 | Script error: No such module "val". | Script error: No such module "val". | acen | |
| 21 | q | 11.2 | 1367 | 1584 | Script error: No such module "val". | Script error: No such module "val". | gneg | |
| 21 | q | 21.1 | 1584 | 2019 | Script error: No such module "val". | Script error: No such module "val". | gpos | 100 |
| 21 | q | 21.2 | 2019 | 2144 | Script error: No such module "val". | Script error: No such module "val". | gneg | |
| 21 | q | 21.3 | 2144 | 2330 | Script error: No such module "val". | Script error: No such module "val". | gpos | 75 |
| 21 | q | 22.11 | 2330 | 2485 | Script error: No such module "val". | Script error: No such module "val". | gneg | |
| 21 | q | 22.12 | 2485 | 2610 | Script error: No such module "val". | Script error: No such module "val". | gpos | 50 |
| 21 | q | 22.13 | 2610 | 2703 | Script error: No such module "val". | Script error: No such module "val". | gneg | |
| 21 | q | 22.2 | 2703 | 2858 | Script error: No such module "val". | Script error: No such module "val". | gpos | 50 |
| 21 | q | 22.3 | 2858 | 3200 | Script error: No such module "val". | Script error: No such module "val". | gneg |
References
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- ↑ "p": Short arm; "q": Long arm.
- ↑ For cytogenetic banding nomenclature, see article locus.
- ↑ a b These 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.
- ↑ gpos: 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.
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External links
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