sábado, 17 de agosto de 2013

Chromosome 13 - Genetics Home Reference

Chromosome 13 - Genetics Home Reference

Genetics Home Reference: your guide to understanding genetic conditions

Chromosome 13
Reviewed April 2009

What is chromosome 13?

Humans normally have 46 chromosomes in each cell, divided into 23 pairs. Two copies of chromosome 13, one copy inherited from each parent, form one of the pairs. Chromosome 13 is made up of about 115 million DNA building blocks (base pairs) and represents between 3.5 and 4 percent of the total DNA in cells.
Identifying genes on each chromosome is an active area of genetic research. Because researchers use different approaches to predict the number of genes on each chromosome, the estimated number of genes varies. Chromosome 13 likely contains 300 to 400 genes that provide instructions for making proteins. These proteins perform a variety of different roles in the body.
Genes on chromosome 13 are among the estimated 20,000 to 25,000 total genes in the human genome.

How are changes in chromosome 13 related to health conditions?

Many genetic conditions are related to changes in particular genes on chromosome 13. This list of disorders associated with genes on chromosome 13 provides links to additional information.
Changes in the structure or number of copies of a chromosome can also cause problems with health and development. The following chromosomal conditions are associated with such changes in chromosome 13.
8p11 myeloproliferative syndrome
A rearrangement (translocation) of genetic material involving chromosome 13 has been identified in most people with a rare blood cancer called 8p11 myeloproliferative syndrome. This condition is characterized by an increased number of white blood cells (myeloproliferative disorder) and the development of lymphoma, a blood-related cancer that causes tumor formation in the lymph nodes. The myeloproliferative disorder usually develops into another form of blood cancer called acute myeloid leukemia. 8p11 myeloproliferative syndrome most commonly results from a translocation between chromosome 13 and chromosome 8, written as t(8;13)(p11;q12). This genetic change fuses part of the ZMYM2 gene on chromosome 13 with part of the FGFR1 gene on chromosome 8. The translocation occurs only in cancer cells. The protein produced from the normal FGFR1 gene can turn on cellular signaling that helps the cell respond to its environment, for example by stimulating cell growth. The protein produced from the fused ZMYM2-FGFR1 gene leads to constant FGFR1 signaling. The uncontrolled signaling promotes continuous cell growth and division, leading to cancer.
Retinoblastoma, a cancer of the light-sensing tissue at the back of the eye (the retina) that affects mostly children, is caused by abnormalities of a gene called RB1. This gene is located on a region of the long (q) arm of chromosome 13 designated 13q14. Although most retinoblastomas are caused by mutations within the RB1 gene, a small percentage of retinoblastomas result from a deletion of the 13q14 region. In addition to retinoblastoma, deletions of the 13q14 region may cause intellectual disability, slow growth, and characteristic facial features such as prominent eyebrows, a broad nasal bridge, a short nose, and ear abnormalities. A loss of several genes is likely responsible for these developmental problems, although researchers have not determined which other genes in the deleted region are involved.
trisomy 13
Trisomy 13 occurs when each cell in the body has three copies of chromosome 13 instead of the usual two copies. Trisomy 13 can also result from an extra copy of chromosome 13 in only some of the body's cells (mosaic trisomy 13). In some cases, trisomy 13 occurs when part of chromosome 13 becomes attached (translocated) to another chromosome during the formation of reproductive cells (eggs and sperm) or very early in embryonic development. Affected individuals have two copies of chromosome 13, plus extra material from chromosome 13 attached to another chromosome. People with this genetic change are said to have translocation trisomy 13. The physical signs of translocation trisomy 13 may be different from those typically seen in trisomy 13 because only part of chromosome 13 is present in three copies. Researchers believe that extra copies of some genes on chromosome 13 disrupt the course of normal development, causing the characteristic features of trisomy 13 and the increased risk of medical problems associated with this disorder.
other cancers
Changes in chromosome 13 have been associated with several types of cancer. These genetic changes are somatic, which means they are acquired during a person's lifetime and are present only in certain cells. The loss of genetic material from the middle of chromosome 13 is common in cancers of blood-forming cells (leukemias), cancers of immune system cells (lymphomas), and other related cancers.
other chromosomal conditions
Partial monosomy and partial trisomy of chromosome 13 occur when a portion of the long (q) arm of this chromosome is deleted or duplicated, respectively. The effect of missing or extra chromosome material varies with the size and location of the chromosome abnormality. Affected individuals may have developmental delay, intellectual disability, low birth weight, and other physical abnormalities.

Is there a standard way to diagram chromosome 13?

Geneticists use diagrams called ideograms as a standard representation for chromosomes. Ideograms show a chromosome's relative size and its banding pattern. A banding pattern is the characteristic pattern of dark and light bands that appears when a chromosome is stained with a chemical solution and then viewed under a microscope. These bands are used to describe the location of genes on each chromosome.

Where can I find additional information about chromosome 13?

You may find the following resources about chromosome 13 helpful. These materials are written for the general public.
You may also be interested in these resources, which are designed for genetics professionals and researchers.

Where can I find general information about chromosomes?

The Handbook provides basic information about genetics in clear language.
These links provide additional genetics resources that may be useful.

What glossary definitions help with understanding chromosome 13?

References (10 links)

The resources on this site should not be used as a substitute for professional medical care or advice. Users seeking information about a personal genetic disease, syndrome, or condition should consult with a qualified healthcare professional. See How can I find a genetics professional in my area? in the Handbook.

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