A mutation is a change in the DNA (deoxyribonucleic acid) genetic code. Geneticists usually qualify the definition of a mutation to mean that the change in the genetic code is a permanent change. Changes in the DNA can include something as small as a single nucleotide base to substantial changes in entire chromosomes. In modern scientific usage, scientists mostly refer to a mutation as a change in a gene, which is a basic inheritable unit of DNA that represents a trait.
There are different kinds of mutations that are named according to the changes that can occur in the DNA molecule.
Base insertion is when a single extra nucleotide base is added to the sequence of a gene.
Base deletion is when a single nucleotide base is missing in the sequence of a gene.
Base substitution is when a single nucleotide base is substituted for another.
Point mutations occur at certain specific sites in genes and include an insertion, deletion, or substitution of one nucleotide base for another. At times, more than one nucleotide base can be inserted. Huntington's Disease is a dominant autosomal inherited disorder that is caused by numerous repeats of CAG bases in chromosome 4.
Because the changes in the gene sequences can be passed to offspring, a mutation can result in new traits. The traits can have negative consequences to survival. Occasionally, a mutation provides a new trait that is a survival advantage to an organism (a mutant) and over time the resulting trait increases in a population. In humans, mutations usually cause deleterious effects to metabolic pathways. Diseases like tyrosinemia, phenylketonuria, and Tay-Sachs disease are the result of mutations.
Other kinds of mutations are changes in chromosomes. Chromosomal mutations can include: monosomy (one less than the diploid number of chromosomes), trisomy (an extra chromosome in a diploid set, as in Downs Syndrome), and polyploidy (an entire extra set of chromosomes). In translocation, a piece of a chromosome breaks off and attaches to another non homologous chromosome. Transposons (jumping genes) are the mobile genetic elements (units) discovered by Dr. Barbara McClintock in Indian corn. She found that the transposons move from one location to another in a chromosome. In the usual situation, transposons may turn on or off adjacent genes with no resulting problem. It is thought that transposon movement causes most of the naturally occurring mutations in eukaryotic organisms.
Factors that induce mutations are mutagens. The following are known to cause changes in DNA: ionizing radiation, specific chemicals, and ultra-violet rays from sunlight, viruses, and high temperatures.
McClintock, Barbara.1983. Autobiography. http://nobelprize.org/nobel_prizes/medicine/laureates/1983/mcclintock-autobio.html
Peterson, Thomas. 1992. A celebration of the life of Dr. Barbara McClintock.