Gene Mutations and Types of Genetic Mutation

The changes to the genetic material (either DNA or RNA) are called mutation. Their are different types of Gene Mutations. The mutations can be caused by following reasons.

1. Copying errors in the genetic material during cell division
2. By exposure to radiation, chemicals (mutagens).
3. Viruses may cause mutations.
4. Mutations can occur under cellular control during processes such as meiosis or hypermutation.

In multicellular organisms, mutations can be subdivided into two types:

Germline Mutations: These mutations can be passed on to descendants.

Somatic Mutations: The somatic mutations cannot be transmitted to descendants in animals. Plants sometimes can transmit somatic mutations to their descendants asexually or sexually (in case when flower buds develop in somatically mutated part of plant.

Effects of Mutations

Mutations create variation in the gene pool. Therefore, mutations can be:

Deleterious: These mutations can be less favorable (or deleterious) mutations. These are removed from the gene pool by natural selection.

Beneficial: But more favorable (beneficial or advantageous) tend to accumulate through evolution.

Neutral Mutations: Neutral mutations are defined as mutations whose effects do not influence the fitness of either the species or the individuals of the species. These can accumulate over time due to genetic drift. DNA repair is able to revert most changes before they become permanent mutations. Therefore, majority of mutations have no significant effect. Many organisms have mechanisms for eliminating permanently mutated somatic cells.

Classification of Gene Mutations

Classification by Effect on Structure

The DNA sequence of a gene can be altered in a number of ways. Gene mutations have varying effects on health. It depends on where they occur and whether they alter the function of essential proteins. Structurally, mutations can be classified as:

(a) Small-Scale Gene Mutations: These mutations affecting one or a few nucleotides. These are:

Point mutations: The mutations caused by exchange of single nucleotide are called point mutations. These are often caused by chemicals or malfunction of DNA replication. Most common is the transition that exchanges a purine for a purine (A <-> G) or a pyrimidine for a pyrimidine, (C <-> T). A transition can be caused by nitrous acid, base mispairing, or mutagenic base analogs such as 5-bromo-2-deoxyyuridine (BrdU). Less common is a transversion, which exchanges a purine for a pyrimidine or a pyrimidine for a purine (C/t <-> A/G). a point mutation can be reversed by another point mutation, in which the nucleotide is changed back to its original state (true reversion) or by second-site reversion. Point mutation that occur within the protein coding region of a gene are classified into three kinds:

• Silent Mutation: Which code for the same amino acid.
• Missense Mutation: Which code for a different amino acid.
• Nonsense Mutation: Which code for a stop codon.

Insertions: The mutation in which one or more extra nucleotides are added into the DNA is called insertion. They are usually caused by transposable elements or errors during replication of repeating elements (e.g. AT repeats). It has two types:

• Splice Site Mutation: In this case, the coding region of a gene may alter splicing of the mRNA (splice site mutation).

• Frameshift Mutations: These mutations cause a shift in the reading frame (frameshift). It significantly alters the gene product.

Deletions: The removal of one or more nucleotide from the DNA is called deletion. Like insertions, these mutations can alter the reading frame of the gene. They are irreversible.

(b) Large-scale Gene Mutations: These mutations cause change in chromosomal structure, including:

Amplification (or gene duplications) leading to multiple copies or chromosomal regions. It increases the dosage of the genes located within them.

Deletions of large chromosomal regions, leading to loss of the genes within those regions.

Mutations whose effect is to juxtapose previously separate pieces of DNA. It brings together separate genes to form functionally distinct fusion genes. These include:

• Chromosomal Translocation: Interchange of genetic parts from nonhomologous chromosomes.
• Interstitial Deletions: Removing regions of DNA from a single chromosome, thereby apposing previously distant genes.
• Chromosomal Inversions: Reversing the orientation of a chromosomal segment.

Loss of Heterozygosity: Loss of one allele, either by a deletion or recombination event, in organisms which previously had two.

Classification of Gene Mutation by Effect on Function

Amormphic Mutations or Loss-of-Function Mutations are the result of gene product having less or no function. When the allele has a complete loss of function (null allele) it is often called an amorphic mutation. Phenotypes associated with such mutations are most often recessive.

Neomorphic Mutations or Gain-of-Function Mutations change the gene product such that it gains a new and abnormal function. These mutations usually have dominant phenotypes. These are also called a neomorphic mutation.

Actinomorphic Mutations or Dominant Negative Gene Mutations have an altered gene product that acts antagonistically to the wild-type allele. These mutations usually result in an altered molecular function (often inactive). They are characterized by a dominant or semi-dominant phenotype.

Lethal mutations are mutations that lead to a phenotype incapable of effective reproduction.

Classification of Mutations on the Basis of Causing Agents

There are two classes of mutations are spontaneous mutations (molecular decay) and induced mutations caused by mutagens.

Spontaneous Mutations: The mutations caused due to molecular decay are called spontaneous mutations. These include:

• Tautomerism – A base is changed by the repositioning of a hydrogen atom.
• Depurination – Loss of a purine base (A or G).
• Deamination – Changes a normal base to an atypical base; C -> U, (which can be corrected by DNA repair mechanisms), or spontaneous deamination of 5-methycytosine (irreparable), or A -> HX (hypoxanthine).
• Transition – A purine changes to another purine, or a pyrimidine to a pyrimidine.
• Transversion – A purine becomes a pyrimidine or vice versa.

Induced Mutations: The mutations which are cause by mutagens are called induced mutations. Induced mutations may be caused chemicals like Nitrosoguanidine (NTG) or by radiation like ultra violet radiations.

Effects of Mutations

1. Changes in DNA caused by mutation can cause errors in protein sequence, creating partially or completely non-functional proteins.
2. If mutation is present in a germ cell, this can give rise to offspring that carriers the mutation in all of its cells. This is case in heredity diseases.
3. Often, gene mutations that could cause a genetic disorder are repaired by the DNA repair system of the cell. Each cell has a number of pathways through which enzymes recognize and repair mistakes in DNA.
4. A very small percentage of all mutations actually have a positive effect. These mutations lead to new versions of proteins. These proteins help an organism and its future generations better adopt to changes in their environment.