A mutation is a change in a gene’s DNA structure. A mutation is a heritable change in an individual’s phenotype that occurs suddenly. Mutation is defined at the molecular level as a permanent and infrequent change in nucleotides’ number or sequence. Mutagens are substances (chemicals) that can cause mutations.
The mutational alterations in DNA are reflected in replication, transcription, and translation.
V Hugo De Vries developed the term “mutation” in 1890. However, Seth Wright, an English farmer, discovered mutation for the first time in his unusual short-leg male lambs in 1791. He fails to define the process. H. J. Muller conducted artificial mutagenesis studies in 1927. He had introduced mutation in Drosophila using X-rays. In 1946, he was awarded the Nobel Prize for this achievement.
Types of mutation
Based on the site of mutation
i. Chromosomal mutations: Mutations that cause measurable alterations in either chromosomal number or structure.
ii. Gene or pint mutations: Mutations brought about by changes to the base sequences of the relevant genes.
iii. Cytoplasmic mutations: These are mutations caused by alterations in chloroplast DNA (cpDNA) and mitochondrial DNA (mtDNA).
Based on Size and Quality
i. Point mutation
Point mutations are changes inside genes. The simplest is base-pair substitutions, which are alterations to single base pairs. A single base of DNA is altered. It’s also known as single nucleotide polymorphism. Pint mutation is further subdivided into various types they are:
Substitution: A substitution is a type of mutation in which two bases are exchanged. A similar replacement could alter a codon to one that encodes a different amino acid, resulting in a change in the protein generated.
Insertions are mutations that occur when extra base pairs are inserted into a new location in the DNA. The number of base pairs that can be introduced ranges from one to thousands. Huntington’s disease and the Fragile X syndrome are both caused by insertion mutations in which trinucleotide repeats are added into the DNA sequence, resulting in these disorders.
The deletion mutation is a point mutation produced by the loss or deletion of some section (single nucleotide pair) of a triplet codon of a cistron or gene. The deletion of certain nucleotides on chromosome 22 causes 22q11.2 deletion syndrome. Cleft palate, cardiac problems, immunological issues, and other symptoms distinguish this condition.
These arise when one or more base pairs are introduced into or deleted from the DNA, resulting in insertion or deletion mutations. Protein-coding DNA is divided into three-base-long codons; insertions and deletions in these codons can modify a gene, causing its message to be incorrectly decoded. Frameshift mutations are the name given to such mutations.
Frameshift Mutation has been linked to Tay-Sachs Disease, several cancers, Crohn’s Disease, and cystic fibrosis.
ii. Gross mutation
When changes involve more than one nucleotide pair or an entire gene, these are referred to as gross mutations. The gross mutations are caused by gene rearrangements inside the genome.
When some gene sequences are inverted and placed back into the original sequence, this is commonly referred to as inversion.
The movement of a gene to a non-homologous chromosome is referred to as translocation.
Based on the type of cell involved
i. Germinal mutation
Any cell can undergo a mutation. They only affect future generations if they occur in the cells that create gametes: they are referred to as “germinal” or “germline” mutations. As a result, germinal mutations occur in the body’s germ tissues. Mutations can be passed down to offspring.
ii. Somatic mutation
These mutations are found in the body’s somatic tissues.
These mutations are not passed down to offspring. Except in the case of cancer, where the mutant cell proliferates rapidly, mutations in other cells are rarely observed.”Somatic” mutations occur in cells other than germ line cells.
Based on the direction of mutation
i. Forward mutation
A forward mutation is a genetic mutation that occurs when a wild-type allele evolves into a mutant allele. Forward mutation results in the emergence of novel traits in the population.
ii. Reverse or back mutation
Forward mutations are frequently rectified by error-correcting mechanisms, transforming an aberrant phenotype into a wild-type phenotype.
A mutation is a unidirectional process, but certain mutations return the original (wild-type) allele to the population, this kind of mutation is known as a backward or back mutation.
Back mutation is an infrequent and unusual natural phenomenon.
It restores the original phenotype to the population by true back mutation or the occurrence of a secondary mutation.
Based on the phenotypic effect
i. Morphological mutations
Morphological mutations are those that impact an organism’s outwardly observable characteristics (for example, curled ears in cats).
ii. Lethal mutations
Lethal mutations are mutations that damage the organism’s viability (e.g., Manx cat).
iii. Conditional mutations
Lethal mutations occur when the mutant allele generates the mutant phenotype only in specific circumstances (referred to as the restricted condition).
Based on visibility
Macro-mutations cause a distinct morphological change in phenotype (which can be easily observed without any confusion owing to environmental factors), such as flower color, plant height, and so on.
In Micro-mutations, phenotypic changes are undetectable (and thus easily confused with environmental influences).
Based on the character affected
i. Morphological mutation
A mutation that affects an individual’s morphological features.
ii. Biochemical mutation:
A mutation that changes an individual’s biochemical function.
Based on the source
i. Spontaneous mutation
Spontaneous mutations occur for unknown reasons. It occurs as a result of metabolic mistakes, replication problems, or development errors. Spontaneous mutations are uncommon and occur for no apparent reason. Because larger genes have a higher likelihood of replication errors, they are more prone to spontaneous mutation.
ii. Induced mutation
Induced mutation occurs when an organism is exposed to mutagenic chemicals. Radiation, UV light, and chemicals are common mutagenic agents.
UV light induces xeroderma pigmentation and skin cancer by entering the skin and causing uncontrolled cell division. Common chemical mutagens include alkylating agents and base analogs. Furthermore, climatic change and lifestyle play a significant impact in the acquisition of mutations.
Characteristics of mutation
- Mutations are more or less permanent and heritable changes in an individual’s phenotype.
- The occurrence of spontaneous mutations is quite rare. The use of physical and chemical mutagens, on the other hand, can greatly increase the mutation rate.
- The rate of mutation varies from gene to gene. Some genes are more prone to mutation than others. Mutable genes, such as the white eye in Drosophila, are examples of such genes.
- Usually, mutations happen from a wild type to a mutant allele or from a dominant to a recessive allele. However, reversal mutations, such as notch wing and bar eye in Drosophila, are also known.
- In general, mutations are damaging to the organism. Only around 0.1% of induced mutations can be used to improve crops.
- The site of mutation is a muton, which is a sub-division of a gene. A typical gene has 500 to 1000 mutational sites. Some locations in a gene are much more alterable than others. These are commonly known as hot spots.
- Mutations occur at random. They can arise in any gene, in any cell, and at any stage of an individual’s development.
- The same sort of mutation may occur frequently or repeatedly in different members of the same population. Therefore, mutations are recurring in nature.
Consequences of point mutation
A single base sequence change in a point mutation may result in one of the following:
- Silent mutation:
It lacks expression—a new codon codes for the same amino acid as the wild-type one in silent mutation. The altered base in the codon (of mRNA) may code for the same amino acid. For example, UCA codes for serine, and a change in the third base (UCU) codes for serine. This is related to the genetic code’s degeneracy. As a result, there are no visible impacts in silent mutation.
- Missense mutation
A codon is the result of a nucleotide alteration that codes for various amino acids. It can cause protein alterations or loss of function. In this example, the altered base could encode for a different amino acid. UCA, for example, codes for serine, whereas ACA codes for threonine. In terms of protein molecule function, the incorrect amino acid may be acceptable, slightly acceptable, or unsatisfactory. A famous example of a missense mutation is sickle cell anemia.
- Nonsense mutation
Sometimes, the codon with the changed nucleotide could turn into a termination (also known as a nonsense codon). The premature protein is given a stop codon. Because a stop codon terminates protein synthesis, it results in a premature or truncated protein. For example, a mutation in the second base of a serine codon (uCN) can result in UAA. At that point, the changed codon acts as a stop signal, resulting in the stoppage of protein synthesis.
Advantages of mutation
- Some of the variety caused by mutations can be extremely beneficial to an organism or species. Mutations frequently result in the production of proteins with new or altered activities that can be beneficial to organisms in distinct or changing environments. Mutations, for example, have helped many insects withstand chemical pesticides.
- Microorganisms have been able to adapt to new materials in the environment due to some mutations.
- Polyploid plants are typically larger and more resilient than diploid plants.
- Important crop plants, such as bananas and limes, have been created in this manner.
- Polyploidy occurs naturally in citrus plants as well, frequently as a result of spontaneous mutations.
Disadvantages of mutation
- Some of the most harmful mutations are those that alter protein structure or gene function drastically.
- These mutations produce faulty proteins, which can impair normal biological activity and result in hereditary diseases.
- Some cancers, for example, are caused by mutations that induce uncontrolled cell growth.
- Alterations in the morphology of red blood cells characterize sickle cell disease. Red blood cells are normally spherical. Sickle cells are long and pointed in appearance.
- Anaemia, intense pain, recurrent infections, and stunted growth are all indications of the condition.
- Ferrier D. R. & Harvey R. A. (2014). Lippincott’s illustrated reviews: biochemistry (6th ed.). Wolters Kluwer Health : Lippincott Williams & Wilkins.