In this article we will discuss about:- 1. Meaning of Mutations 2. Occurrence of Mutations 3. Expression.
Meaning of Mutations:
Inheritance is based on genes that are faithfully transmitted from generation to generation in microorganisms (in all organisms). Despite the biochemical mechanisms that facilitate such transmissions faithfully, sudden change in the sequence of nucleotide bases in genes can and do occur.
These changes, called mutations introduce variability into the gene pool and are heritable. Therefore, a mutation can be defined as “a sudden and heritable change in the nucleotide sequence of a gene”.
Mutations always change the genotype (the genetic constitution of the cell) and thus the phenotype (the expression of the genotype in observable properties) of the microorganisms. A microbial cell exhibiting change in its phenotype due to the effect of a mutation is a mutant (mutant strain). Any physical or chemical agent that increases the mutation-rate is called mutagen or mutagenic agent.
Mutation is an important phenomenon because it is the ultimate source of genetic variation and provides the raw material for evolution. All the genes would exist in only one form and no mutants (alleles) would be produced without mutation. Thus the mutations are essential to provide new genetic variability to allow microorganisms to evolve and adapt to environmental changes.
At the same time, if mutations occurred too frequently they would totally disrupt the transmission of genetic information from generation to generation.
However, mutations occur only rarely in nature; a microbial population typically contains about one in 108 cells that carries a detectable mutation in any given gene. But, exposure of microorganisms to certain powerful mutagens increases dramatically the frequency of their mutant cells, by as much as 105-fold.
Occurrence of Mutations:
Mutations occur in one of the two ways – spontaneous or induced.
(i) Spontaneous Mutations:
Spontaneous mutations are those that arise occasionally in the absence of a known cause, i.e., without exposure to external agents. These mutations may result from errors in DNA replication, or from the action of transposons, or even from the effect of some mutagenic agents present in the environment.
Some most commonly operating spontaneous mutation mechanisms are as follows:
1. Transition and Transversion Mutations (Errors in DNA Replication):
The bases of nucleotides in DNA strands exist in tautomeric form, i.e., they exist as structural isomers that are in chemical equilibrium and readily change into one another. Bases typically exist in keto form and seldom they can take on either an imino or enol form by error resulting in a rare tautomeric form (Fig. 29.9A).
These tautomeric shifts alter the hydrogen bonding characteristics of the bases allowing purine for purine or pyrimidine for pyrimidine substitutions which can eventually result in a stable alteration of the nucleotide sequence (Fig. 29.9B).
Such substitutions are called transition mutations. In transversion mutation, a pyrimidine substitutes a purine or a purine substitutes a pyrimidine. These mutations are rarer due to the steric problems of pairing pyrimidine with pyrimidine and purine with purine.
2. Frame-Shift Mutations:
Spontaneous mutations also arise from frame-shifts, usually caused by the deletion of DNA segments resulting in an altered codon reading frame. It occurs where there is a short repeated nucleotide sequence.
In such a location, the pairing of template and its complementary strand (new strand) can be displaced by the distance of the repeated sequence leading to additions or deletions of bases in the complementary strand (Fig. 29.10).
These spontaneous frame-shift mutations originate from lesions in DNA as well as from replication errors. For example, it is possible for purine nucleotides to be depurinated, i.e., to lose their base.
This results in the formation of an aprinic site, which will not base pair normally and may cause a transition type mutation after the next round of replication. Cytosine can be deaminated to uracil, which is then removed to form an apyrimidinic site.
(ii) Induced Mutations:
Any agent which directly damages DNA, changes its chemistry, or interferes with repairs mechanisms will induce mutations and the agents which cause such actions are known as mutagens. They can be conveniently classified according to their mechanism of action.
Following are the modes of mutagens actions:
1. Base Analogs:
These are similar to normal nitrogenous bases and can be incorporated into the growing polynucleotide chain during replication. Once in place, these compounds typically exhibit base pairing properties different from the bases they replace and can eventually cause a stable mutation. 5-bromouracil (5- BU), is a widely used base-analog of thymine (Fig. 29.11). It undergoes a tautomeric shift from the normal keto form to an enol much more frequently than does a normal base.
The enol forms hydrogen bonds like cytosine and directs the incorporation of guanine rather than adenine. The mechanism of action of other base analogs is similar to that of 5-bromouracil.
2. Specific Mispairing:
This action is caused when a mutagen changes a base’s structure and, therefore, alters its base pairing characteristics. Certain mutagens in this category are selective. They preferentially react with some bases and produce a specific kind of DNA damage. Methyl- nitrosoguanidine an alkylating agent that adds methyl groups to guanine, causing it to mispair with thymine.
A subsequent round of replication could then result in a GC-AT transition. Ethylmethane- sulfonate and hydroxylamine are other examples of mutagens with similar mode of action.
3. Intercalating Agents:
These agents distort DNA to induce single nucleotide pair—insertions and deletions. These mutagens are planar and insert themselves (intercalate) between the stacked bases of the helix. This results in a mutation, possibly through the formation of a loop in DNA. Acridines (proflavin) and acridine orange are important intercalating agents.
4. Other Mutagenes:
Several carcinogens directly damage DNA bases severely (hydrogen bonding between base pairs is impaired or prevented and the damaged DNA can no longer act as a template).
UV radiation generates cyclobutane type dimers (thymine dimers), between adjacent primidines. Aflatoxin B1 and other benzo (a) pyrene derivatives cause damage to DNA which is generally lethal but may trigger a repair mechanism that restores much of the damaged genetic material.
The Expression of Mutations:
The expression of a mutation can be observed if it produces a detectable, altered phenotype. A mutation from the most prevalent gene form, the wild type, to a mutant form is called a forward mutation.
Later, a second mutation may make the mutant appear to be a wild-type organism again. Such a mutation is called a reversion mutation or back mutation. In this the organism revertes back to its original phenotype. A true mutation converts the mutant nucleotide sequence back to the wild-type arrangement.
The wild-type phenotype also can be regained by a second mutation in a different gene, a suppressor mutation, which overcomes the effect of the first mutation. Sometimes, however, mutations may take place and do not alter the phenotype for a variety of reasons. When a large deletion and insertion mutations exist and affect only one base pair in a given location, they are called point mutations.
Following are important types of point mutations:
1. Silent Mutation:
It is the kind of point mutation that could not be detected without use of nucleic acid sequencing techniques. If a mutation is an alteration of the nucleotide sequence of DNA, mutations can occur and have no visible effect because of code degeneracy. When there is more than one codon for a given amino acid, a single base substitution could result in the formation of a new codon for the same amino acid.
For example, if the codon CGU were changed to CGC, it would still code for arginine even though a mutation had occurred. The expression of this mutation would not be detected except at the level of the DNA or mRNA. Since there would be no change in the protein, there would be no change in the phenotype of the organism.
2. Missense Mutation:
It is a second type of point mutation that involves a single base substitution in the DNA that changes a codon for one amino acid into a codon for another. For example, the codon GAG, which specifies glutamic acid, could be changed to GUG, which codes for valine.
3. Nonsense Mutations:
It is a third type of point mutation and involves the early termination of translation and therefore results in a shortened polypeptide. Such mutations involve for conversion of a sense codon to a non-sense or stop codon. Depending on the relative location of mutation, the phenotypic expression may be more or less severely affected.
4. Frame-shift Mutation:
It is a fourth type of point mutation that arise from the insertion or deletion of one or two base pairs within the coding region of the gene. These mutations normally are very deleterious and yield mutant phenotypes resulting from the synthesis of nonfunctional proteins (Fig. 29.12).