Mutation

MUTATION

A heritable change in the structure, content and organization of the genetic material that can be passed down to the next is termed a mutation. Mutation may occur in one gene when it is termed point mutation or may affect a number of genes on a part of chromosomes when it is termed chromosomal mutation.

Chromosomal Mutation

Involves a number of genes. It is of two types, due to 1. Change in number of chromosomes and 2. Change in structure of chromosomes.

The number of chromosomes in individuals of a species is fixed. For example humans have 2n = 46 chromosomes. But sometimes one or more chromosomes may be lost or more chromosomes may be lost or added and such a change in number is termed Aneuploidy. Sometimes the whole set of chromosomes may be duplicated so that instead of 2n, an individual way possess 3n or 4n chromosomes. This is euploidy. Chromosomal change in structure is also termed chromosomal aberrations. It is of four types 1. Deletion, in which a piece of a chromosome may be lost. 2. Inversion, a piece of a chromosome breaks off and rejoins in the reverse direction. 3. Duplication a part of the chromosome may get represented twice and 4. A piece from another chromosome may get attached translocation.

Genes mutation or Point Mutation

A change which affects only one gene is called gene mutation or point mutation you already know that gene is a segment of DNA and is made a sequence of nucleotides. Whenever one nucleotide is changed within a gene, it may cause a change in the phenotype. When a purine base is replaced by another purine base or a pyrimidine base by another pyrimidine

ATGCATGC>AGGCAGGC

2. Transversion 3. Frameshift

When a purine base is replaced by the another pyrimidine base and similarly a pyrimidine base by another purine ATGC ATGC > ATGT ATGC

Sometimes due to loss or gain of one nucleotide the reading frame of the genetic code for an entire gene changes CAT CAT CAT CAT > CAT ATC ATC ATC

lost

4. Missense 5. Nonsense

A change in the genetic code due to replacement of a nucleotide (base) may give rise to a different protein e.g. sickle cell haemoglobin.

If a genetic code changes such that it becomes a stop codon mid way, no protein is formed e.g. GAAGAAGAAGAAUAAAA synthesis stops

6. Silent : When the change in a nucleotide does not bring about any phenotypic change.

Mutagens

Agent that cause mutation in the genetic material are called mutagens. Mutagens belong to two categories