Gene Mutations: Point Mutation, Frame-shift Mutation, Non-sense Mutation, Missense Mutation, Silent Mutation

A mutation can be defined as an alteration in DNA sequence. Any base-pair change in any part of a DNA molecule is a mutation. A mutation may comprise a single base-pair substitution, a deletion or insertion of one or more base pairs, or a major alteration in the structure of a chromosome. Mutations may occur within regions of a gene that code for protein or within non-coding regions of a gene like introns and regulatory sequences.

Mutations can occur in somatic cells or within germ cells. Those that occur in germ cells are heritable. These are the basis for the transmission of genetic diversity and evolution, as well as genetic diseases. Those that occur in somatic cells are not transmitted to the next generation. But these may lead to altered cellular function or tumors. Geneticists classify mutations according to several different schemes.

Spontaneous and Induced Mutations

Spontaneous mutations:

Mutations can be classified as either spontaneous or induced. Spontaneous mutations are changes in the nucleotide sequence of genes without known cause. No specific agents are associated with their occurrence. They are generally accidental. Many of these mutations arise as a result of normal biological or chemical processes in the organism. These processes alter the structure of nitrogenous bases. Often, spontaneous mutations occur during the enzymatic process of DNA replication.

Induced mutations:

The mutations that result from the influence of external factors are called induced mutations. Induced mutations may be the result of either natural or artificial agents. For example, radiation from cosmic and mineral sources and ultraviolet radiation from the sun are energy sources. These may be factors that cause induced mutations. The earliest demonstration of artificial induction of mutations occurred in 1927. Hermann J. Muller reported that X rays can cause mutations in Drosophila. Numerous natural and synthetic chemical agents are also mutagenic.

Classification based on location of mutation:

Mutations may be classified according to the cell type or chromosomal locations.

Somatic mutations:

Somatic mutations are those occurring in any cell in the body except germ cells. Mutations arising in somatic cells are not transmitted to future generations. A recessive autosomal mutation may occur in a somatic cell of a diploid organism. But it unlikely to result in a detectable phenotype.

Autosomal mutations:

Autosomal mutations are mutations within genes located on the autosomes.

X-linked and Y-linked mutations:

X-linked and Y-linked mutations are those within genes located on the X or Y chromosome.

Mutations in germ cells are of greater significance. They may be transmitted to offspring as gametes. They can be expressed in all cells of an offspring. Inherited dominant autosomal mutations will be expressed phenotypically in the first generation. X-linked recessive mutations arise in the gametes of a homogametic female. It may be expressed in hemizygous male offspring. This will occur provided that the male offspring receives the affected X chromosome.

Classification based on type of molecular change:

Geneticists often classify gene mutations in terms of the nucleotide changes.

1.   Point mutation:

A change of one base pair to another in a DNA molecule is known as a point mutation, or base substitution.

2.   Frame-shift mutation:

This type of mutation occurs when addition or loss of DNA bases changes a gene’s reading frame. A reading frame consists of groups of 3 bases. Each group is called codon which codes for an amino acid. Frame shift mutation shifts the grouping of these bases and thus changes the code for amino acids. Resulting protein is usually non-functional.

3.   Non-sense mutation:

In this type of mutation one DNA base pair changes. This change of base pair forms a codon which is actually a stop codon. It stops formation of proteins.

4.   Missense mutation:

This type of mutation is a change in one DNA base pair that results in substitution of one amino acid for another in the protein.

5.   Silent mutation:

If the point mutation alters a codon but does not result in a change in the amino acid, it is called silent mutation.

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