Mutation and Its Types

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The gradual change in the biological characteristics of every organism has been tagged for ages as the process of “evolution”. As stated by Darwin, we all evolve from a primate version of ancient organisms. The theory of evolution could be argumentative, however, the fact remains that all of us undergo different biological and hereditary changes eventually. The best example of this would be a mutation.

Typically mutation could be defined as the change in the structure of the gene, or DNA to be specific, resulting in a variant that may be transmitted to subsequent generations caused by the alteration of single base units in DNA or the deletion, insertion, or rearrangement of larger sections of genes or chromosomes. It could be a result of DNA copying mistakes made during cell division, exposure to ionizing radiation, or any sort of mutagens. Mutagen can be any chemical element that causes a mutation in the body.

Mutation could be somatic or germline. Germline mutation occurs in the eggs and sperm and could be passed onto offspring. Somatic mutations occur in body cells and are not passed on. Mutations may or may not produce detectable 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, including junctional diversity. The mutation is the ultimate source of all genetic variation, providing the raw material on which evolutionary forces such as natural selection can act.

Mutation can result in many different types of change in sequences. Mutations in genes can have no effect, alter the product of a gene, or prevent the gene from functioning properly or completely. Mutations can also occur in non-genic regions. 

Types of Mutations

There are different types of mutations i.e.,

  • Spontaneous Mutation
  • Error-Prone Replication Bypass Mutations
  • Error Introduced During DNA Repairs
  • Induced Mutations Caused By Mutagens
  • Mutations and Polymorphisms

1. Spontaneous Mutation

Mutations can also occur spontaneously. For instance, depurination, in which a purine base is lost from a nucleotide through hydrolysis even though the sugar-phosphate backbone is unaltered, can occur without an explicit insult from the environment. If uncorrected by DNA repair enzymes, depurination may result in the incorporation of an incorrect base during the next round of replication.

Deamination, or the removal of an amine group from a base, may also occur. Deamination of cytosine converts it to uracil, which will pair with adenine instead of guanine at the next replication, resulting in a base substitution. Repair enzymes can recognize uracil as not belonging to DNA, and they will normally repair such a lesion.

However, if the cytosine residue in question is methylated (a common modification involved in gene regulation), deamination will instead result in conversion to thymine. Because thymine is a normal component of DNA, this change will go unrecognized by repair enzymes.

2. Error-Prone Replication Bypass Mutations

Lesion bypass is an important cellular response to unrepaired DNA damage during replication. Two modes of lesion bypass are known error-free and error-prone bypass. The former mechanism does not introduce mutations opposite the lesion, whereas the latter mechanism is frequently accompanied by mutations. Therefore, error-prone lesion bypass constitutes a major mechanism of DNA damage-induced mutagenesis in cells.

3. Error Introduced During DNA Repairs

Most mistakes during replication are corrected by DNA polymerase during replication or by post-replication repair mechanisms. In proofreading, the DNA pol reads the newly-added base before adding the next one so a correction can be made. The polymerase checks whether the newly-added base has paired correctly with the base in the template strand.

If it is the correct base, the next nucleotide is added. If an incorrect base has been added, the enzyme cuts the phosphodiester bond and releases the incorrect nucleotide. This is performed by the exonuclease action of DNA pol III. Once the incorrect nucleotide has been removed, a new one will be added again.

4. Induced Mutations Caused By Mutagens

There can be mutations caused by external agents such as environmental factors. DNA interacts with the environment, and sometimes that interaction can be detrimental to genetic information. Every time we go outside, we put our DNA in danger, because ultraviolet (UV) light from the Sun can induce mutations in human skin cells.

One type of UV-generated mutation involves the hydrolysis of a cytosine base to a hydrate form, causing the base to mid-pair with adenine during the next round of replication and ultimately be replaced by thymine. Indeed, researchers have found an extremely high rate of occurrence of this UV-induced C-to-T fingerprint type mutation in genes associated with basal cell carcinoma, a form of skin cancer.

UV light can also cause covalent bonds to form between adjacent pyrimidine bases on a DNA strand, which results in the formation of pyrimidine dimers. Repair machinery exists to cope with these mutations, but it is somewhat prone to error, which means that some dimers go unrepaired.

Oxidizing agents, commonly known as free radicals, are substances that can chemically modify nucleotides in ways that alter their base-pairing capacities. For instance, dioxin intercalates between base pairs, disrupting the integrity of the DNA helix and predisposing that site to insertions or deletions.

5. Mutations and Polymorphisms

While mutation could be defined as just an alteration in the DNA, sequence biologists sometimes use the term single nucleotide polymorphism(SNP) to refer to a single pair alteration. Moreover, the cutoff of at least 1% prevalence for a variation to be classified as polymorphism is somewhat arbitrary; if the frequency is lower than this, the allele is typically regarded as a mutation.

With the minute difference, it can be quite common to compare both SNP and mutations.

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