Mutation and Variation. In genetics , a mutation is a change of the nucleotide sequence of the genome of an organism , virus , or extrachromosomal genetic element .
In genetics, a mutation is a change of the nucleotide sequence of the genome of an organism, virus, or extrachromosomal genetic element.
Mutations result from unrepaired damage to DNA or to RNA genomes (typically caused by radiation or chemical mutagens), errors in the process of replication, or from the insertion or deletion of segments of DNA by mobile genetic elements.
Mutations may or may not produce discernible changes in the observable characteristics (phenotype) of an organism.
Mutations play a part in both normal and abnormal biological processes including: evolution, cancer, and the development of the immune system.
(a) Initial (mRNA) sequence and translated product.
(b) Deletion of a single base alters the subsequent reading frame producing a different amino acid sequence and encountering a stop codon.
(c) Addition of a base at a different position restores the original reading frame and may suppress the mutation
There are many genes that do not affect resistance to antibiotics or bacteriophages,
biosynthesis of essential metabolites or utilization of carbon sources.
Some of these genes are indispensable and any mutants defective in those activities would die (or fail to grow).
A base substitution changes CAG to the stop codon UAG, causing premature termination of translation. This can be suppressed by a separate mutation in a tRNA gene, giving rise to a tRNA that can recognize the UAG codon
general or homologous recombination
site-specific recombinational mechanisms
Spontaneous mutation occurs through errors in the replication of DNA
Nitrous acid causes oxidative deamination of bases
Nitrous acid causes an oxidative deamination in which amino groups are converted to keto groups and thus cytosine
residues for example will be converted to uracil
Structure of thymine dimers
The best defence that is mounted against damage by UV irradiation is known as photoreactivation.
This is catalysed by an enzyme (photolyase) within the cells that in the presence of visible light can break the covalent bonds linking the two pyrimidine residues, thus re-establishing the original nature of the base sequence at that point
In photoreactivation repair, the PRE enzyme activated by blue light breaks the dimer, restoring the normal base pairing. Note that blue light is at the same end of the spectrum as UV radiation.
(b) In excision repair, the uvr system excises the dimer, and the gap is filled in by the proof-reading activity of DNAPol
Non-SOS repair is basically error-free, but SOS repair is error-prone. This is why UV is a mutagen. May be due to RecA binding ssDNA in lesions, which could then bind to DNA Pol III complex passing through this area of the DNA and inhibit 3'>5' exonuclease (proofreading) ability. This makes replication faster but also results in more mutations. This affect on proofreading seems to involve UmuD'-UmuC complex as well. RecA facilitates proteolytic cleavage of UmuD to form UmuD'. The UmuD'-UmuC complex may bind to the RecA-Pol III complex and promote error-prone replication