Changes that affect the structure of chromosomes can cause problems with growth, development, and function of the body’s systems. These changes can affect many genes along the chromosome and disrupt the proteins made from those genes.
Structural changes can occur during the formation of egg or sperm cells, in early fetal development, or in any cell after birth. Pieces of DNA can be rearranged within one chromosome or transferred between two or more chromosomes. The effects of structural changes depend on their size and location, and whether any genetic material is gained or lost. Some changes cause medical problems, while others may have no effect on a person’s health.
Changes in chromosome structure include:
Translocations (illustration: balanced), (illustration: unbalanced)
A translocation occurs when a piece of one chromosome breaks off and attaches to another chromosome. This type of rearrangement is described as balanced if no genetic material is gained or lost in the cell. If there is a gain or loss of genetic material, the translocation is described as unbalanced.
Deletions occur when a chromosome breaks and some genetic material is lost. Deletions can be large or small, and can occur anywhere along a chromosome.
Duplications occur when part of a chromosome is copied (duplicated) too many times. This type of chromosomal change results in extra copies of genetic material from the duplicated segment.
An inversion involves the breakage of a chromosome in two places; the resulting piece of DNA is reversed and re-inserted into the chromosome. Genetic material may or may not be lost as a result of the chromosome breaks. An inversion that involves the chromosome’s constriction point (centromere) is called a pericentric inversion. An inversion that occurs in the long (q) arm or short (p) arm and does not involve the centromere is called a paracentric inversion.
An isochromosome is a chromosome with two identical arms. Instead of one long (q) arm and one short (p) arm, an isochromosome has two long arms or two short arms. As a result, these abnormal chromosomes have an extra copy of some genes and are missing copies of other genes.
Dicentric chromosomes (illustration)
Unlike normal chromosomes, which have a single constriction point (centromere), a dicentric chromosome contains two centromeres. Dicentric chromosomes result from the abnormal fusion of two chromosome pieces, each of which includes a centromere. These structures are unstable and often involve a loss of some genetic material.
Ring chromosomes (illustration)
Ring chromosomes usually occur when a chromosome breaks in two places and the ends of the chromosome arms fuse together to form a circular structure. The ring may or may not include the chromosome’s constriction point (centromere). 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.
For more information about structural changes to chromosomes:
The National Human Genome Research Institute provides a list of questions and answers about chromosome abnormalities, including a glossary of related terms.
Chromosome Deletion Outreach offers a fact sheet on this topic titled Introduction to Chromosomes. This resource includes illustrated explanations of several chromosome abnormalities.
The Centre for Genetics Education provides fact sheets about changes in chromosome number or size and chromosomal rearrangements (translocations).
EuroGentest offers fact sheets about chromosome changes and chromosome translocations.
The University of Leicester’s Virtual Genetics Education Center provides an explanation of structural chromosome aberrations.
More technical information is available from the textbook Human Molecular Genetics (second edition, 1999) in the section about structural chromosome abnormalities.
The Atlas of Genetics and Cytogenetics in Oncology and Haematology provides a technical introduction to chromosomal aberrations and a detailed discussion of ring chromosomes, particularly their role in cancer.
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