Chromosome 21 (human)
Encyclopedia
Chromosome 21 is one of the 23 pairs of chromosomes in human
s. People normally have two copies of this chromosome. The trisomy
of the 21st chromosome causes Down Syndrome
. Chromosome 21 is the smallest human chromosome, 47 million nucleotide
s (the building material of DNA
) and 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 pair
s that make up this chromosome. Chromosome 21 was the second human chromosome to be fully sequenced.
Identifying gene
s 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. Chromosome 21 likely contains between 300 and 400 genes
Human
Humans are the only living species in the Homo genus...
s. People normally have two copies of this chromosome. The trisomy
Trisomy
A trisomy is a type of polysomy in which there are three copies, instead of the normal two, of a particular chromosome. A trisomy is a type of aneuploidy .-Description and causes:...
of the 21st chromosome causes Down Syndrome
Down syndrome
Down syndrome, or Down's syndrome, trisomy 21, is a chromosomal condition caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British physician who described the syndrome in 1866. The condition was clinically described earlier in the 19th...
. Chromosome 21 is the smallest human chromosome, 47 million nucleotide
Nucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...
s (the building material of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
) and representing about 1.5 percent of the total DNA in cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
.
In 2000, researchers working on the Human Genome Project
Human Genome Project
The Human Genome Project is an international scientific research project with a primary goal of determining the sequence of chemical base pairs which make up DNA, and of identifying and mapping the approximately 20,000–25,000 genes of the human genome from both a physical and functional...
announced that they had determined the sequence of base pair
Base pair
In molecular biology and genetics, the linking between two nitrogenous bases on opposite complementary DNA or certain types of RNA strands that are connected via hydrogen bonds is called a base pair...
s that make up this chromosome. Chromosome 21 was the second human chromosome to be fully sequenced.
Identifying gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s 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. Chromosome 21 likely contains between 300 and 400 genes
Genes
The following are some of the genes located on chromosome 21:- APPAmyloid precursor proteinAmyloid precursor protein is an integral membrane protein expressed in many tissues and concentrated in the synapses of neurons. Its primary function is not known, though it has been implicated as a regulator of synapse formation, neural plasticity and iron export...
: amyloid beta (A4) precursor protein (peptidase nexin-II, Alzheimer disease) - CBS: cystathionine-beta-synthase
- CLDN14CLDN14Claudin-14 is a protein that in humans is encoded by the CLDN14 gene. It belongs to the group of claudins.-Further reading:...
: claudin 14 - HLCSHLCSHLCS is a human gene that provides instructions for making an enzyme called holocarboxylase synthetase . This enzyme is important for the effective use of biotin, a B vitamin found in foods such as liver, egg yolks, and milk...
: holocarboxylase synthetase (biotin-(proprionyl-Coenzyme A-carboxylase (ATP-hydrolysing)) ligase) - KCNE1KCNE1Potassium voltage-gated channel subfamily E member 1 is a protein that in humans is encoded by the KCNE1 gene.KCNE1 is a gene associated with Long QT syndrome type 5. It can both cause Romano-Wards syndrome and Jervell Lange-Nielsens syndrome -External links:*...
: potassium voltage-gated channel, Isk-related family, member 1 - KCNE2KCNE2Potassium voltage-gated channel subfamily E member 2 is a protein that in humans is encoded by the KCNE2 gene. The protein encoded by this gene is a voltage-gated potassium channel accessory subunit associated with Long QT syndrome....
: potassium voltage-gated channel, Isk-related family, member 2 - LADCD18Integrin beta-2 is a protein that in humans is encoded by the ITGB2 gene.It is the beta subunit of four different structures:* LFA-1 * Macrophage-1 antigen * Integrin alphaXbeta2...
: leukocyte adhesion deficiency (symbols are ITGB2, CD18, LCAMB) - SOD1SOD1Superoxide dismutase [Cu-Zn] also known as superoxide dismutase 1 or SOD1 is an enzyme that in humans is encoded by the SOD1 gene. SOD1 is one of three human superoxide dismutases.- Function :...
: superoxide dismutase 1, soluble (amyotrophic lateral sclerosis 1 (adult)) - TMPRSS3TMPRSS3Transmembrane protease, serine 3 is an enzyme that in humans is encoded by the TMPRSS3 gene.-Further reading:...
: transmembrane protease, serine 3 - PCNTPCNTPericentrin , also known as PCNT, is a protein which in humans is encoded by the PCNT gene.- Function :The protein encoded by this gene binds to calmodulin and is expressed in the centrosome. It is an integral component of the pericentriolar material...
: centrosomal pericentrin - DSCR1DSCR1Down syndrome critical region gene 1, also known as DSCR1, is a protein that in humans is encoded by the DSCR1 gene.- Function :The protein encoded by this gene interacts with calcineurin A and inhibits calcineurin-dependent signaling pathways of genetic transcription, possibly affecting central...
: Down Syndrome critical region 1 - DYRK1ADYRK1ADual specificity tyrosine-phosphorylation-regulated kinase 1A is an enzyme that in humans is encoded by the DYRK1A gene. Alternative splicing of this gene generates several transcript variants differing from each other either in the 5' UTR or in the 3' coding region...
: dual specificity tyrosine-(Y)-phosphorylation regulated kinase 1A - RRP1BRRP1BRRP1B , also known as KIAA0179, is a human gene which is located on Chromosome 21.-Further reading:...
: ribosomal RNA processing 1 homolog B - s100BS100BS100 calcium binding protein B or S100B is a protein of the S-100 protein family.S100 proteins are localized in the cytoplasm and nucleus of a wide range of cells, and involved in the regulation of a number of cellular processes such as cell cycle progression and differentiation...
: calcium binding protein
Diseases & disorders
The following diseases are some of those related to genes on chromosome 21:- Alzheimer's diseaseAlzheimer's diseaseAlzheimer's disease also known in medical literature as Alzheimer disease is the most common form of dementia. There is no cure for the disease, which worsens as it progresses, and eventually leads to death...
- Alzheimer's disease type 1
- Amyotrophic lateral sclerosisAmyotrophic lateral sclerosisAmyotrophic lateral sclerosis , also referred to as Lou Gehrig's disease, is a form of motor neuron disease caused by the degeneration of upper and lower neurons, located in the ventral horn of the spinal cord and the cortical neurons that provide their efferent input...
- Amyotrophic lateral sclerosis type 1
- Down syndromeDown syndromeDown syndrome, or Down's syndrome, trisomy 21, is a chromosomal condition caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British physician who described the syndrome in 1866. The condition was clinically described earlier in the 19th...
- Erondu-Cymet SyndromeErondu-Cymet SyndromeErondu–Cymet syndrome is a syndrome caused by a translocation on the 21st chromosome. The genetic karyotype for people with this condition is 46, XY, inv. Findings in these patients include: hypotension, hypoxemia, seizures, and impairment of cognitive ability...
- Holocarboxylase synthetase deficiencyHolocarboxylase synthetase deficiencyHolocarboxylase synthetase deficiency is an inherited metabolic disorder in which the body is unable to use the vitamin biotin effectively. This disorder is classified as a multiple carboxylase deficiency, a group of disorders characterized by impaired activity of certain enzymes that depend on...
- HomocystinuriaHomocystinuriaHomocystinuria, also known as cystathionine beta synthase deficiency or CBS deficiency, is an inherited disorder of the metabolism of the amino acid methionine, often involving cystathionine beta synthase...
- Jervell and Lange-Nielsen syndromeJervell and Lange-Nielsen syndromeJervell and Lange-Nielsen syndrome, a type of long QT syndrome, causes the cardiac muscle to take longer than usual to recharge between beats. If untreated, the irregular heartbeats, called arrhythmias, can lead to fainting, seizures, or sudden death...
- Leukocyte adhesion deficiencyLeukocyte adhesion deficiencyLeukocyte adhesion deficiency , is a rare autosomal recessive disorder characterized by immunodeficiency resulting in recurrent infections. Leukocyte adhesion deficiency is divided into at least two subtypes: LAD1 and LAD2.-Characteristics:...
- Majewski osteodysplastic primordial dwarfism type IIMajewski osteodysplastic primordial dwarfism type IIMicrocephalic osteodysplastic primordial dwarfism type II is listed as a "rare disease" by the Office of Rare Diseases of the National Institutes of Health...
(MOPD II, or MOPD2) - Nonsyndromic deafnessNonsyndromic deafnessNonsyndromic deafness is hearing loss that is not associated with other signs and symptoms. In contrast, syndromic deafness involves hearing loss that occurs with abnormalities in other parts of the body....
- Nonsyndromic deafness, autosomal recessive
- Romano-Ward syndromeRomano-Ward syndromeRomano-Ward syndrome, is the major variant of long QT syndrome. It is a condition that causes a disruption of the heart's normal rhythm. This disorder is a form of long QT syndrome, which is a heart condition that causes the cardiac muscle to take longer than usual to recharge between beats...
Chromosomal conditions
The following conditions are caused by changes in the structure or number of copies of chromosome 21:- CancerCancerCancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
s: Rearrangements (translocationsChromosomal translocationIn genetics, a chromosome translocation is a chromosome abnormality caused by rearrangement of parts between nonhomologous chromosomes. A gene fusion may be created when the translocation joins two otherwise separated genes, the occurrence of which is common in cancer. It is detected on...
) of genetic material between chromosome 21 and other chromosomes have been associated with several types of cancer. For example, acute lymphoblastic leukemiaAcute lymphoblastic leukemiaAcute lymphoblastic leukemia is a form of leukemia, or cancer of the white blood cells characterized by excess lymphoblasts.Malignant, immature white blood cells continuously multiply and are overproduced in the bone marrow. ALL causes damage and death by crowding out normal cells in the bone...
(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 leukemiaAcute myeloid leukemiaAcute myeloid leukemia , also known as acute myelogenous leukemia, is a cancer of the myeloid line of blood cells, characterized by the rapid growth of abnormal white blood cells that accumulate in the bone marrow and interfere with the production of normal blood cells. AML is the most common acute...
, has been associated with a translocation between chromosomes 8 and 21. - In a small percentage of cases, Down syndromeDown syndromeDown syndrome, or Down's syndrome, trisomy 21, is a chromosomal condition caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British physician who described the syndrome in 1866. The condition was clinically described earlier in the 19th...
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 mental retardationMental retardationMental retardation is a generalized disorder appearing before adulthood, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors...
, 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 proteinAmyloid precursor proteinAmyloid precursor protein is an integral membrane protein expressed in many tissues and concentrated in the synapses of neurons. Its primary function is not known, though it has been implicated as a regulator of synapse formation, neural plasticity and iron export...
(APP) locus (duplicated segment varies in length but includes APP) on Chromosome 21 was found to cause early onset familial Alzheimer's diseaseAlzheimer's diseaseAlzheimer's disease also known in medical literature as Alzheimer disease is the most common form of dementia. There is no cure for the disease, which worsens as it progresses, and eventually leads to death...
in a French family set (Rovelet-Lecrux et al.) and a Dutch family set (Sleegers et al.). 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.