Point mutation



A point mutation, or single base substitution, is a type of mutation that causes the replacement of a single base nucleotide with another nucleotide of the genetic material, DNA or RNA. Often the term point mutation also includes insertions or deletions of a single base pair. One can categorize point mutations as follows:


 * transitions: replacement of a purine base with another purine or replacement of a pyrimidine with another pyrimidine
 * transversions: replacement of a purine with a pyrimidine or vice versa.

Transition mutations are about an order of magnitude more common than transversions. Point mutations can also be categorized functionally:


 * nonsense mutations: code for a stop, which can translate the protein
 * missense mutations: code for a different amino acid
 * conservative mutations: result in an amino acid change; however, the properties of the amino acid remain the same (e.g. hydrophobic, hydrophilic, etc).
 * non conservative mutations: result in an amino acid change that has different properties than the wild type.


 * silent mutations: code for the same amino acid

For example, sickle-cell disease is caused by a single point mutation (a missense mutation) in the beta-hemoglobin gene that converts a GAG codon into GUG, which encodes the amino acid valine rather than glutamic acid. This is an example of a non-conservative (missense) mutation.

Point mutations that occur in non-coding sequences are most often without consequences, although there are exceptions. If the mutated base pair is in the promoter sequence of a gene, then the expression of the gene may change. Also, if the mutation occurs in the splicing site of an intron, then this may interfere with correct splicing of the transcribed pre-mRNA.

Sometimes the term point mutation is also used to describe insertions or deletions of a single base pair (which has more of an adverse effect on the synthesized protein due to the nucleotides' still being read in triplets, but in different frames: a mutation called a frameshift mutation).

A "point mutant" is an individual which is affected by a point mutation.

Causes
Point mutations may arise from spontaneous mutations that occur during DNA replication. The rate of mutation may be increased by mutagens. Mutagens can be physical, such as radiation from UV rays, X-rays or extreme heat, or chemical (molecules that misplace base pairs or disrupt the helical shape of DNA). Mutagens associated with cancers are often studied to learn about cancer and its prevention.

There is a systematic difference in rates for transitions (Alpha) and transversions (Beta).