Mutations are heritable changes in genotype that can occur spontaneously or be induced by chemical or physical treatments. (Organisms selected as reference strains are called wild type, and their progeny with mutations are called mutants.) The process of mutation is called mutagenesis and the agent inducing mutations is called mutagen. Changes in the sequence of template DNA (mutations) can drastically affect the type of protein end product produced. For a particular bacterial strain under defined growth conditions, the mutation rate for any specific gene is constant and is expressed as the probability of mutation per cell division.
Spontaneous mutation occurs naturally about one in every million to one in every billion divisions. Mutation rates of individual genes in bacteria range from 10-2 to 10-10 per bacterium per division. Most spontaneous mutations occur during DNA replication.
Mechanisms of mutation
a. Substitution of a nucleotide: Base substitution, also called point mutation, involves the changing of single base in the DNA sequence. This mistake is copied during replication to produce a permanent change. If one purine [A or G] or pyrimidine [C or T] is replaced by the other, the substitution is called a transition. If a purine is replaced by a pyrimidine or vice-versa, the substitution is called a transversion. This is the most common mechanism of mutation.
b. Deletion or addition of nucleotides: deletion or addition of a nucleotide during DNA replication. When a transposon (jumping gene) inserts itself into a gene, it leads to disruption of gene and is called insertional mutation.
Results of mutation
a. Missense mutation: Missense mutations are DNA mutations which lead to changes in the amino acid sequence (one wrong codon and one wrong amino acid) of the protein product. This could be caused by a single point mutation or a series of mutations.
b. Nonsense mutation: A mutation that leads to the formation of a stop codon is called a nonsense mutation. Since these codon cause the termination of protein synthesis, a nonsense mutation leads to incomplete protein products.
c. Silent mutation: Sometimes a single substitution mutation change in the DNA base sequence results in a new codon still coding for the same amino acid. Since there is no change in the product, such mutations are called silent.
d. Frameshift mutation: Frameshift mutations involve the addition or deletion of base pairs causing a shift in the “reading frame” of the gene. This causes a reading frame shift and all of the codons and all of the amino acids after that mutation are usually wrong. Since the addition of amino acids to the protein chain is determined by the three base codons, when the overall sequence of the gene is altered, the amino acid sequence may be altered as well.
e. Lethal mutation: Sometimes some mutations affect vital functions and the bacterial cell become nonviable. Hence those mutations that can kill the cell are called lethal mutation.
f. Suppressor mutation: It is a reversal of a mutant phenotype by another mutation at a position on the DNA distinct from that of original mutation. True reversion or back mutation results in reversion of a mutant to original form, which occurs as a result of mutation occurring at the same spot once again.
g. Conditional lethal mutation: Sometimes a mutation may affect an organism in such a way that the mutant can survive only in certain environmental condition. Example; a temperature sensitive mutant can survive at permissive temperature of 35oC but not at restrictive temperature of 39oC.
h. Inversion mutation: If a segment of DNA is removed and reinserted in a reverse direction, it is called inversion mutation.
Based on extent of base pair changes, mutations can be of two types; microlesion and macrolesion. Microlesions are basically point mutations (affecting single base pairs) whereas macrolesions involve addition, deletion, inversion or duplication of several base pairs.
The mutations in DNA can occur spontaneously or can be caused by an external force or substance called a mutagen. Mutagens can be chemicals such as nitrous acid, which alters adenine to pair with cytosine instead of thymine. Other chemical mutagens include acridine dyes, nucleoside analogs that are similar in structure to nitrogenous bases, benzpyrene (from smoke and soot) and aflatoxin. Radiation can also be a cause of DNA mutations. High energy light waves such as X-rays, gamma rays, and ultraviolet light have been shown to damage DNA. UV light is responsible for the formation of thymine dimers in which covalent links are established between the thymine molecules. These links change the physical shape of the DNA preventing transcription and replication.
Significance of mutation:
• Discovery of a mutation in a gene can help in identifying the function of that gene.
• Mutations can be induced at a desired region to create a suitable mutant, especially to produce vaccines.
• Spontaneous mutations can result in emergence of antibiotic resistance in bacteria.
• Mutations can result in change in phenotype such as appearance of novel surface antigen, alternation in physiological properties, change in colony morphology, nutritional requirements, biochemical reactions, growth characteristics, virulence and host range.
Tests to detect or select mutations:
• Replica plating
• Penicilin enrichment
• Fluctuation test
• Ames test
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