What it means to have a mutation and what role mutations play in Huntington’s disease.
What are mutations?
In the broadest definition of the word, a mutation is a change in the genetic composition of an organism. Many different phenomena fit this definition. For instance, chromosomal mutations (or chromosomal rearrangements) result from breaks in the chromosome. In these mutations, a segment of the chromosome is deleted, and may even attach onto another chromosome. Since chromosomes are made of millions of pieces of DNA all strung together, breaks in these chromosomes can mean the loss of many important genes, with very harmful results for the individual. (To learn more about chromosomes, click here.) Point mutations are also potentially harmful. They occur when one nitrogenous base is substituted for another (for example, when adenine (A) is substituted for thymine (T)). In some instances this can result in a silent mutation, in which no noticeable change occurs. In others, however, the code alteration can disturb the gene, with the ultimate result of creating either an altered protein or no protein at all. Both of these results can be quite harmful to the individual.
Point and chromosomal mutations have been identified as the cause of many human diseases. With regard to HD, however, they do not appear to play a role. Instead, the mutation involved in HD is known as an expansion. Expansion refers to the increase from one generation to the next (parent to child) in the number of copies of a certain codon. This codon is normally repeated a certain number of times, composing what is called a repeat region of the DNA. When the mutation increases the number of copies of this codon, the length of the repeat region is expanded, hence the name "expansion." In HD, the codon involved is CAG and the repeat region is located on the Huntington gene.
Last Modified: 1-28-04
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Point and chromosomal mutations have been identified as the cause of many human diseases. With regard to HD, however, they do not appear to play a role. Instead, the mutation involved in HD is known as an 
