Ring chromosomes usually occur when a chromosome breaks in two places, typically at the ends of the p and q arms, and then the arms fuse together to form a circular structure. The ring may or may not include the centromere, depending on where on the chromosome the breaks occur. In many cases, genetic material near the ends of the chromosome is lost.
Many cancer cells also have changes in their chromosome structure. These changes are not inherited; they occur in somatic cells cells other than eggs or sperm during the formation or progression of a cancerous tumor. Other chapters in Help Me Understand Genetics. Genetics Home Reference has merged with MedlinePlus. Learn more. The information on this site should not be used as a substitute for professional medical care or advice.
Contact a health care provider if you have questions about your health. Can changes in the structure of chromosomes affect health and development? From Genetics Home Reference. Changes in chromosome structure include the following: Translocations A translocation occurs when a piece of one chromosome breaks off and attaches to another chromosome.
Deletions Deletions occur when a chromosome breaks and some genetic material is lost. Duplications Duplications occur when part of a chromosome is abnormally copied duplicated. Inversions An inversion occurs when a chromosome breaks in two places; the resulting piece of DNA is reversed and re-inserted into the chromosome.
Isochromosomes An isochromosome is a chromosome with two identical arms. Dicentric chromosomes Unlike normal chromosomes, which have one centromere, a dicentric chromosome contains two centromeres. And it can be something small, just a base pair; it can be something larger; it can be part of a gene; it can be even larger; it can be an entire gene; or yet larger again, it can be part of the chromosome.
And depending upon what it is, you have to look at it in different ways. You can find a deletion in a chromosome just by doing a cytogenetic or chromosome analysis, or a deletion in a gene you can find out by sequencing the DNA. So when you have a deletion, depending upon the size, it can have different effects. Eichler and his colleagues suspected the piece of DNA that was missing in the new syndrome was important because it contains six genes, including one previously linked to seizure disorders.
That part of chromosome 15 has many repeated DNA segments, Eichler points out. When a cell divides to produce germ cells, these duplicated segments can misalign through a process known as nonallelic homologous recombination, which can cause large segments of DNA to be missing in one of the daughter cells.
The newly discovered syndrome accounts for approximately 3 of every 1, cases of mental retardation. That may not seem like much, but Eichler estimates that similar deletions and rearrangements of DNA may account for 15 to 20 percent of mental retardation. In the paper, he and his colleagues recommend that people with mental retardation and seizures be tested to determine if their condition is caused by the deletion on chromosome Eichler expects that future genetic analyses will uncover other structural variants that cause physical and mental disorders.
But many variants will be less common, which will require that more people be screened to find the variant. When we can do that in a cost-effective way, we will find all kinds of rare variants associated with disease. Skip to navigation Skip to main content Skip to footer. UGP Home. About About HHMI is a science philanthropy whose mission is to advance basic biomedical research and science education for the benefit of humanity.
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