nonsense mutation (redirected from Nonsense codon)

nonsense mutation,

a mutation in which one of the three terminator codons in the mRNA used to signal the end of a polypeptide appears in the middle of a genetic message and causes premature termination of transcription and release of incomplete, generally nonfunctional polypeptides from the ribosome. The three types are amber mutation, ochre mutation, opal mutation.

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mutation

1. a nucleotide change, including base substitutions, insertions or deletions in DNA, or RNA in the case of some viruses, that gives rise to the mutant phenotype.

2. an animal exhibiting such change. Called also a sport.

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back mutation

see reverse mutation (below).

base substitution mutation

may be a transition in which a purine-pyrimidine pair is substituted by the other purine-pyrimidine pair, or transversion in which a purine-pyrimidine pair is replaced by one of the two pyrimidine pairs.

chain termination mutation

one in which the new base sequence introduces a stop codon and thereby prematurely terminates synthesis of the polypeptide; the three mutations are also called amber (UAG), ochre (UAA) and opal (UGA).

deletion mutation

one produced by loss of nucleotides from a DNA sequence.

frame shift mutation

occur as a result of either the insertion of a new base pair or the deletion of a base pair or a block of base pairs from the DNA base sequence; these, unless they occur in 3 or multiples of 3, are most serious in that the message to the right of the frame shift is garbled.

leaky mutation

one in which the amino acid substitution only partially disrupts the function of the protein; in bacteria this is usually manifested by reduced growth rate.

mis-sense mutation

one causing an amino acid substitution in the protein.

nonsense mutation

one in which a stop codon is substituted for a codon that specifies an amino acid.

operator constitutive mutation

one or more base changes in the operator region (originally defined for the lactose operon) which stop the repressor protein from tightly binding to sequence such that it is less effective in preventing RNA polymerase from inhibiting transcription.

point mutation

a single changed base pair in the DNA of an organism which may be a base substitution, base insertion or base deletion.

mutation rate

the frequency of mutations in the population over time.

repressor-constitutive mutation

in regulation of gene expression, a mutation in the repressor protein that decreases the binding affinity of the repressor protein for the operator which leaves the gene permanently turned on.

reverse mutation

one in which the wild-type phenotype is restored; such organisms are called revertants. Called also back mutation, reversion mutation.

second-site mutation

see suppressor mutation.

silent mutation

one in which there is a base change but because of the redundancy of the genetic code the same amino acid is coded, or one in which there is an amino acid substitution in the protein which has no detectable effect on the phenotype.

somatic mutation

a change in the DNA sequence that occurs in somatic cells, i.e. not gametes. The mechanism underlying the generation of diversity of antigen recognition by immunoglobulins and T cell receptor molecules. The fundamental cause of cancer, in which the mutation occurs spontaneously or is induced by carcinogens, such as sunlight, chemicals or viruses.

suppressor mutation

a particular type of reversion mutation in which a mutation at a second site restores the original phenotype; most simply a mutation produced by a base deletion may be restored to wild type by a proximate but independent base substitution. Called also second-site mutation.

temperature-sensitive (ts) mutation

one in which there is an altered protein that is active at one temperature, typically 86°F (30°C) and inactive at a higher temperature, usually 104 to 108°F (40 to 42°C), e.g. ts mutant virus and bacteria.

transdominant mutation

occur in genes producing diffusible products, in contrast to cis-dominant mutation in which mutations occur in regulatory sequences that are recognized by other proteins.

transition mutation

one in which the base change does not change the pyrimidine-purine orientation. See also base substitution mutation (above).

transposition mutation

one produced by the insertion of a transposable genetic element.

transversion mutation

one in which the purine-pyrimidine orientation is changed to pyrimidine-purine or vice versa. See also base substitution mutation (above).