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Topic: Chromosome 21

	Mapping Chromosome 21
		The relative sparseness of genes in chromosome 21 may mean the total number of genes in human DNA is under 40,000 — not the 100,000 or more that had previously been thought, said Andre Rosenthal, a professor at the Institute for Molecular Biotechnology in Jena, Germany.
		The map of chromosome 21 is 99.7 percent complete; technical limitations prevented a complete mapping, said David Patterson, an American scientist involved in the project.
		Chromosome 21 also is linked to certain types of leukemia, a form of manic depression and congenital heart disease.
	www.downsyn.com /mapping21.html (692 words)

	Scientific American: Mapping Chromosome 21
		Chromosome 21, which represents about 1 percent of the entire genome, was expected to be gene-poor, but not as impoverished as it now seems.
		Also on chromosome 21 are genes that are as yet unidentified, but known to be responsible for five so-called monogenic disorders, including two forms of deafness and Usher and Knobloch's syndromes.
		The paper bears good news for scientists hoping to pinpoint the genes on chromosome 21 that lie behind Down syndrome's varied complications: regions of human chromosome 21 share a lot in common with several regions of mouse chromosomes 10, 16 an 17--regions that are called conserved syntenies across species.
	www.sciam.com /print_version.cfm?articleID=0008F64A-4E58-1C75-9B81809EC588EF21 (778 words)

	Chromosome 21 - Genetics Home Reference
		Chromosome 21 is the smallest human chromosome, spanning about 47 million base pairs (the building blocks of DNA) and representing approximately 1.5 percent of the total DNA in cells.
		Genes on chromosome 21 are among the estimated 20,000 to 25,000 total genes in the human genome.
		Trisomy 21 means that each cell in the body has three copies of chromosome 21 instead of the usual two copies.
	ghr.nlm.nih.gov /chromosome=21 (724 words)

	Chromosome 21 - Genetics Home Reference
		Chromosome 21 is one of the 23 pairs of chromosomes in humans.
		Genes on chromosome 21 are among the estimated 20,000 to 25,000 total genes in the human genome.
		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 that is caused by the breakage and reunion of both ends of the chromosome.
	www.ghr.nlm.nih.gov /chromosome=21 (791 words)

	genome.gov \| 2000 Release: Chromosome 21 Sequenced
		It is the second human chromosome whose DNA has been fully deciphered by the Human Genome Project (HGP), the international consortium of scientists dedicated to spelling out, or sequencing, the 3 billion chemical letters of human DNA.
		Chromosome 22 was the first human chromosome to be sequenced.
		With the complete sequence of chromosome 21 available, scientists can better understand where genes are located on the chromosome, how they are expressed, how changes that give rise to disease-causing mutations occur, and how chromosomes are duplicated and inherited.
	www.genome.gov /10001485 (647 words)

	Manbir Online .. Chromosome 21 mapped
		Scientists have mapped chromosome 21, the smallest human chromosome and the one associated with Down syndrome, epilepsy, Lou Gehrig's disease and Alzheimer's.
		The mapping of the chromosome may allow researchers to home in on specific genes in the chromosome that cause mental retardation in people with Down syndrome and then perhaps develop drugs to treat such patients.
		Chromosome map could eventually shed light on why people with Down syndrome have a very low rate of breast, lung and gastrointestinal cancers.
	www.manbir-online.com /news/chromo21.htm (240 words)

	The German-Japanese Consortium deciphers human chromosome 21 - A basis for comprehensive understanding of trisomy 21 ...
		Chromosome 21 is one of the smallest among the 23 different human chromosomes.
		After the successful sequencing of chromosome 22 in December 1999 by a group of international scientists, chromosome 21 is now the second human chromosome which has been deciphered completely.
		Some of them are associated to a number of genetic diseases mapping to chromosome 21 but for which candidate genes are still unknown: several forms of deafness, solid tumors, a form of manic depressive psychosis for instance.
	www.eurekalert.org /pub_releases/2000-05/M-TGCd-0705100.php (811 words)

	Science & Technology at Scientific American.com: Mapping Chromosome 21 -- The smallest human chromosome has so few ... (Site not responding. Last check: 2007-10-12)
		Chromosome 21, which represents about 1 percent of the entire genome, was expected to be gene-poor, but not as impoverished as it now seems.
		Chromosome 22 is considered "gene-rich," but the new information is still making researchers revise their predictions about the genome itself.
		Also on chromosome 21 are genes that are as yet unidentified, but known to be responsible for five so-called monogenic disorders, including two forms of deafness and Usher and Knobloch's syndromes.
	sciam.com /article.cfm?articleID=0008F64A-4E58-1C75-9B81809EC588EF21 (739 words)

	Elucidating the mechanisms of paternal non-disjunction of chromosome 21 in humans -- Savage et al. 7 (8): 1221 -- Human ...
		Most trisomy 21 cases are the result of a maternal meiotic non-disjunction event and, thus, studies of the cause of non-disjunction have focused primarily on the maternal cases.
		Because the altered sex ratio is seen only with free trisomy 21 cases and not with translocations (12), models explaining the phenomenon often include the non-disjoined chromosomes 21 segregating with the Y chromosome.
		Given that paternal cases account for <10% of trisomy 21 cases, it seems that both alterations in the primary sex ratio of male gametes and selection against trisomy 21 female conceptuses must be operating to result in the sex ratio disturbance seen among the DS live born population.
	hmg.oupjournals.org /cgi/content/full/7/8/1221 (5492 words)

	Down Syndrome:Types of Down Syndrome (Site not responding. Last check: 2007-10-12)
		A chromosome study (karyotype) of trisomy 21 is shown in Fig.
		The recipient chromosome is usually chromosome 14 and the combination of 2 chromosomes is called a fourteen, twenty one translocation, written t(14;21) or t(14q21q).
		One is a Robertsonian translocation between two chromosomes 21, t(21;21); this has a 100% risk for DS when transmitted by a carrier parent.
	www.nas.com /downsyn/benke.html (1703 words)

	ABNORMALITIES IN CHROMOSOME STRUCTURE
		Individuals and families have been described with a translocation chromosome abnormality and a concurrent genetic condition; the genetic condition occurring because the chromosome breakpoint is in the midst of a gene.
		The chromosomes are distributed to the daughter cells by the centromeres which are attached to spindle fibers.
		In contradistinction, a numerical chromosome abnormality in a child is presumed to be due to a sporadic cell division error and parental chromosome studies are not indicated.
	www.usd.edu /med/som/genetics/curriculum/1ECHROM3.htm (1824 words)

	Introduction to Chromosome Diseases - WrongDiagnosis.com
		Many chromosome errors cause the fetus to be aborted before birth, but some syndromes can be born and survive, though all typically suffer severe mental and physical defects.
		These conditions are very rare for autosomes (chromosomes 1..22) because body cells without pairs do not seem to survive, but can occur in the sex chromosome (monosomy X is Turner syndrome).
		You are either born with the error in your chromosomes or not Genetic tests can determine whether or not a person has a chromosome disease, even as early as in the fetus by antenatal testing for genetic diseases.
	www.wrongdiagnosis.com /genetics/chromosome.htm (2936 words)

	Gene Sequence of the Down Syndrome Critical Region of Human Chromosome 21, Identified by a New "Alu-Splicing ...
		Chromosome studies of rare patients with partial chromosome 21 trisomy have defined a minimum region for the Down syndrome phenotype which includes approximately 3 megabases (Mb) around the D21S55 marker.
		The identification of genes of chromosome 21 is crucial for understanding the mechanisms involved in the various phenotypic manifestations of DS.
		Transcription maps of chromosome 21 and of the DS critical region have been developed, and work is continuing on obtaining them.sup.14,15,24.
	www.altonweb.com /cs/downsyndrome/5869318.html (5287 words)

	ScienceDaily: German-Japanese Consortium Deciphers Human Chromosome 21
		Chromosome 21 is one of the smallest among the 23 different human chromosomes.
		Some of them are associated to a number of genetic diseases mapping to chromosome 21 but for which candidate genes are still unknown: several forms of deafness, solid tumors, a form of manic depressive psychosis for instance.
		By extrapolation from the number of 225 genes encoded by chromosome 21 and from the 545 genes encoded by chromosome 22, and considering the size of the human genome, the total number of human genes may be as little as 40,000.
	www.sciencedaily.com /releases/2000/05/000510165648.htm (917 words)

	Human Genetics - Chromosomal Inheritance 1
		To identify chromosomes, they are arrested in late prophase or early metaphase of mitosis, when the chromosomes are duplicated and condensed, but the centromere has not yet divided.
		Chromosome replication is different in humans than it is in bacteria, although the method of DNA replication is the same.
		Meiotic nondisjunction, the failure of the chromosomes to disjoin and pass to opposite poles, in either the first or second meiotic division is the major cause of chromosomal abnormalities.
	www.uic.edu /classes/bms/bms655/lesson9.html (2625 words)

	Down Syndrome (Trisomy 21) - Lucile Packard Children's Hospital
		For example, if a baby is born with three #21 chromosomes, rather than the usual pair, the baby would be said to have "trisomy 21." Trisomy 21 is also known as Down syndrome.
		Sometimes, an error occurs when the 46 chromosomes are being divided in half and an egg or sperm cell keeps both copies of the #21 chromosome instead of just one copy.
		Occasionally, the extra chromosome 21 is attached to another chromosome in the egg or sperm; this may result in what is called "translocation" Down syndrome (3 to 4 percent of cases).
	www.lpch.org /diseasehealthinfo/healthlibrary/genetics/downs.html (1828 words)

	Biology 442, Human Genetics - Chromosome Abnormalities
		Chromosome 21 is shorter than chromosome 22 but was misnamed so long ago that they left the number as 21.
		For example, 45,XY,der(14;21)(q10;q10) indicates a male with 45 chromosomes, in whom one normal chromosome 14 and one normal chromosome 21 have been replaced by a derivative chromosome arising from the translocation of the long arm (q) of chromosome 21 to the long arm of chromosome 14.
		Centric fusion translocation of chromosomes 13 and 14.
	www.carolguze.com /text/442-5-chromosome_abnormalities.shtml (8615 words)

	LICR \| News \| Chromosome 21
		Out of the 23 pairs of chromosomes that act as a storage spot for all of our genes, chromosome 21 has been one of the most intensively studied ever since it was discovered that Down syndrome occurs in those who have an extra copy of this chromosome.
		Chromosome 21 is associated with several other disorders besides Down syndrome, including bipolar disease and certain types of cancer.
		After using this approach to locate a number of suspected new genes on chromosome 21, the team ran further tests to ensure that their suspicions proved correct.
	www.licr.org /C_news/020604_chromosome21.php (544 words)

	Chromosome 21 Sequence Completed
		Chromosome 21 contains relatively few genes, but they are laid out in a complex tangle.
		The map of Chromosome 21 is 99.7 percent complete; "technical limitations prevented a complete mapping," said Dr. David Patterson, an American scientist involved in the project.
		Chromosome 21 is the second human chromosome sequence to be documented, encompassing more than 33 million base pairs of DNA, and its publication therefore marks a major scientific milestone.
	www.grg.org /Chrom21.htm (909 words)

	Genome Biology \| Full text \| Chromosome 21 sequenced
		The publication of the sequence of chromosome 21 represents a second milestone for the public Human Genome Project and attests to the strength of the map-directed sequencing approach and international collaboration (this study involves researchers from six different countries).
		Chromosome 21 has been a focus of genome mapping efforts for at least two reasons: it is the smallest human autosome, and trisomy 21 causes Down syndrome, the most common form of mental retardation, which affects one in 700 births.
		Finally, they predict that the chromosome 21 sequence and gene catalog will be valuable for hypothesis-driven selection of candidate genes that might explain the effects of trisomy 21 and other diseases associated with chromosome 21.
	genomebiology.com /2000/1/2/reports/0058 (623 words)

	InteliHealth:
		When the egg with two copies of chromosome 21 combines with a sperm (which carries one copy of chromosome 21), the baby inherits three copies of the chromosome.
		The parent could then pass on the remaining, normal chromosome 21 plus the translocated copy, and the other parent would contribute a normal chromosome 21, resulting in a total of three copies of chromosome 21 in the embryo.
		Therefore, a baby could inherit a regular chromosome 21 from each parent in addition to this rearranged chromosome, resulting in a total of three copies of chromosome 21.
	www.intelihealth.com /IH/ihtIH/WSIHW000/32193/35641.html (2604 words)

	chrom21.html (Site not responding. Last check: 2007-10-12)
		Research of chromosome 21 is involved for the form of AD known as Early Onset Familial (FAD) which is the result of an autosomal dominant mutation.
		There seems to be an abnormality on chromosome 21 that is common in a few of the families with a history of FAD.
		Down's Syndrome is similar to AD in the respect that people with Down's Syndrome have an extra version of chromosome 21, and those who live to 30 or 40, usually develop plaques and tangles like those found in AD and develop similar symptoms.
	www.macalester.edu /~psych/whathap/UBNRP/alzheimer/chrom21.html (281 words)

	Genomics\|Info\|Reports\|Research\|Trisomy 21 and Oral Contraceptives (Site not responding. Last check: 2007-10-12)
		Ninety-five percent of the cases are due to an extra, free-standing chromosome 21 (trisomy 21) which results from an error in meiosis (meiotic nondisjunction) or rarely, from an error in post-zygotic mitosis.
		A lack of recombination or recombination that only occurs distally on chromosome 21 is associated with an MMI error, whereas recombination that occurs near the centromere increases the chance of an MMII error.
		Parental origin of the extra chromosome was determined by examining the contribution of chromosome 21 alleles from each parent to his or her offspring.
	www.cdc.gov /genomics/info/reports/research/trisomy21.html (5216 words)

	CNN.com - Genetic researchers announce big step in sequencing smallest chromosome - May 8, 2000
		Despite showing that Chromosome 21 has so few genes, scientists are interested in its content because it is the only one linked to Down Syndrome.
		According to Waterston -- whose team contributed to the sequencing of Chromosome 22 -- the significance of deciphering Chromosome 21 is "that is came so fast" after sequencing Chromosome 22.
		The next chromosomes to be sequenced are likely to be Chromosome 20 and the male Y chromosome, according to Waterston.
	www.cnn.com /2000/HEALTH/05/08/chromo.21/index.html (695 words)

	Chromosome 21 from Reuters (Site not responding. Last check: 2007-10-12)
		Dr. Sakaki and co-authors say that "the chromosome 21 gene catalogue will open new avenues for deciphering the molecular bases of Down syndrome and of aneuploidies in general." The findings also may have implications for several cancers, including cancers of the head and neck, breast, pancreas, mouth, stomach, esophagus, and lung.
		Sakaki's group points out that "the observed loss of heterozygosity [with these cancers] indicates that there may be at least one tumor suppressor gene" on Chromosome 21.
		In addition, the research team notes, Chromosome 21 is thought to play a role in the development of several monogenic disorders, such as one form of Alzheimer's disease and a predisposition to leukemia, so the study findings may have implications for these conditions as well.
	www.grg.org /Chrom21Reuters.htm (290 words)

	CNN.com - Genetic researchers announce big step in sequencing smallest chromosome - May 8, 2000
		Despite showing that Chromosome 21 has so few genes, scientists are interested in its content because it is the only one linked to Down Syndrome.
		Down Syndrome is caused by the presence of three copies of genes on Chromosome 21 instead of two.
		The next chromosomes to be sequenced are likely to be Chromosome 20 and the male Y chromosome, according to Waterston.
	archives.cnn.com /2000/HEALTH/05/08/chromo.21 (695 words)

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