Mutation is defined as a sudden, discontinuous variation in the phenotype (the physical appearance of an individual) and genotype (genetic makeup) of an organism.
Depending upon the causes mutation are of three types:
1. Gene Mutation:
It occurs due to, alteration of DNA due to changes in the nucleotide sequence. It can be due to the change in a single base pair known as a point mutation or changes in more than one nucleotide pair known as gross mutation. Gene mutation can occur due to:
- Deletion: Removal of one or more bases in a nucleotide chain.
- Insertion: Addition of one or more bases in a nucleotide chain.
- Transition: When a purine base (adenine or guanine) is substituted by another purine base or pyrimidine base (thymidine or cytosine) is substituted by another pyrimidine base.
- Transversion: When a purine base is substituted by a pyrimidine base or vice-versa.
2. Chromosomal Mutation:
Chromosomes are made up of protein, DNA and RNA. Each chromatid possesses one DNA helix that is continuous from one end to another. In chromatids, DNA is present in a highly supercoiled form. Therefore, the loss or gain of a segment of DNA results in alteration of chromosomes and as genes are located on chromosomes, so an alteration in chromosomes results in abnormalities or aberrations. The most important aberrations are as follows:
- Deletion: Occurs when a part of a chromosome is lost. It can be of two types-terminal and intercalary. Terminal deletion is the loss of the terminal segment of a chromosome and occurs due to a single break in the chromosome. In intercalary deletion, there is the loss of an intercalary segment of a chromosome due to double break.
- Duplication: Occurs due to the addition of a part of a chromosome, so that a gene or set of genes is represented twice.
- Translocation: It involves shifting of a part of a chromosome to another non-homologous chromosome, so new recombinant chromosomes are formed as this induces faulty pairing of chromosomes during meiosis. An important example of this is Chronic Myelogenous Leukemia (CML) occurs due to translocation of chromosome 22 to chromosome 9.
- Inversion: Change in the linear order of genes by rotating a section of a chromosome by 180 degrees. It can be of two types:
- Pericentric: Inversion involving centromere.
- Paracentric: Inversion without involving centromere.
3. Genomic Mutation:
It is the change in the chromosome number that brings about the visible effects on the phenotype. It is of two types:
In aneuploidy, any change in the number of chromosomes in an organism would be different than the multiple of a basic set of a chromosome. It commonly arises due to the non-disjunction of two homologous chromosomes during cell division due to which one gamete has an extra chromosome (n+1) while the other is deficient in one chromosome (n-1).
- Trisomy: Down’s Syndrome, Klinefelter’s Syndrome
- Monosomy: Turner syndrome
Types of aneuploidy:
- Monosomic condition: 2n-1
- Nullisomic condition: 2n-2
- Trisomic condition: 2n+1
- Tetrasomic condition: 2n+2
In euploidy, any change in the number of chromosomes is in the multiple of no. Of the basic set of chromosomes. It can be of two types:
- Haploidy: One set of chromosomes. Haploids are better suited for mutation studies as all the types of mutation, whether dominant or recessive are immediately expressed in them.
- Polyploidy: More than two sets of chromosomes. Failure of cytokines after telophase stage of cell division results in an increase in the whole set of chromosome. It can be of two types- autopolyploidy (natural) and allopolyploidy (induced).
On the basis of the type of cells in which mutation occurs, there are two types of mutation:
- Somatic Mutation: Occur in somatic cells and don’t have any genetic or evolutionary significance.
- Germinal Mutation: Occur in germ cells. Such mutations are heritable and therefore, have great evolutionary significance.
On the Basis Of Agents Involved:
- Spontaneous Mutation: Occur naturally and are also called background mutations.
- Induced Mutations: Occur due to mutagens like UV light, a variety of chemicals, gamma rays etc..