Mutations are heritable changes in genetic information. Somatic mutations occur in somatic cells; germ-
The simplest type of mutation is a base substitution, a change in a single base pair of DNA. Transitions are base substitutions in which purines are replaced by purines, or pyrimidines are replaced by pyrimidines. Transversions are base substitutions in which a purine replaces a pyrimidine, or a pyrimidine replaces a purine.
Insertions are additions of nucleotides, and deletions are removals of nucleotides; these mutations often change the reading frame of the gene.
Expanding nucleotide repeats are mutations in which the number of copies of a set of nucleotides increases with the passage of time; they are responsible for several human genetic diseases.
A missense mutation alters the coding sequence so that one amino acid substitutes for another. A nonsense mutation changes a codon that specifies an amino acid into a stop codon. A silent mutation produces a synonymous codon that specifies the same amino acid as does the original sequence, whereas a neutral mutation alters the amino acid sequence but does not change the functioning of the protein.
A suppressor mutation reverses the effect of a mutation at a different site and may be intragenic (within the same gene as the original mutation) or intergenic (within a different gene).
The mutation rate is the frequency with which a wild-
Some mutations occur spontaneously. These mutations include the mispairing of bases in replication and spontaneous depurination and deamination.
Insertions and deletions can arise from strand slippage in replication or from unequal crossing over.
Base analogs can become incorporated into DNA in the course of replication and pair with the wrong base in subsequent replication events. Alkylating agents, deaminating chemicals, and hydroxylamine lead to mutations by modifying the chemical structure of bases. Intercalating agents insert themselves into the DNA molecule and cause single-
Ionizing radiation is mutagenic, altering base structures and breaking phosphodiester bonds. Ultraviolet light produces pyrimidine dimers, which block replication.
The Ames test uses bacteria to assess the mutagenic potential of chemical substances.
Transposable elements are mobile DNA sequences that insert into many locations within a genome and often cause mutations and DNA rearrangements.
Most transposable elements have two common characteristics: terminal inverted repeats and the generation of short direct repeats in DNA at the point of insertion.
A transposon can be transposed as a DNA molecule or through the production of an RNA molecule that is then reverse transcribed into DNA. Transposition may be replicative, in which the transposable element is copied and the copy moves to a new site, or nonreplicative, in which the transposable element excises from the old site and moves to a new site.
Transposons are mutagenic and have played an important role in genome evolution.
Damage to DNA is often corrected by DNA-
Defects in DNA repair are the underlying cause of several genetic diseases.