Point Mutations

Mutation is defined as an alteration in the nucleotide sequence of DNA (deoxyribonucleic acid) of an organism. The structure of DNA consists of nitrogenous bases (purines and pyrimidines), glucose and phosphate. Purine consists of adenine (A) and guanine (G) bases, whereas pyrimidine bases are cytosine(C) and thymine (T).

The bases are arranged as codons that describe the coding of proteins for expression of cells. Mutation occurs when the sequence of nitrogenous base is changed, either due to copying errors during the replication or an exposure to mutagens such as chemicals, pollution, etc. The outcome of mutation is a change in the genetic message coded by the gene.

In general, mutation is classified into two types based on the cells that are getting affected, namely, germ line and somatic. In the former case, mutation occurs in the reproductive cells (egg and sperm), which is then inherited from the parents to the offspring.

This germ line mutation is responsible for causing hereditary diseases and/or genetic disorders. Somatic mutation, on the other hand, takes place in body cells like skin. It is not inherited to the descendants.

Depending upon the effect of mutation on the structure of DNA, mutation can also be divided into several types viz. substitution (exchanging base between two DNA strands), insertion (extra base is inserted in a DNA strand), deletion (loss of base pairs), inversion (reverse of base sequence) and frame-shift (insertion and deletion of one or more base pairs).

When a single base in the nucleotide sequence is replaced by another, then it is known as point mutation. Point mutations also include insertion and/or deletion of a single base in the DNA strand. Usually, they are caused due to error in DNA replication. At times, it occurs after exposure to mutagens like heat and radiation.

Point mutations can be either transitions or transversions. In the former case, a purine base (adenine, guanine) is substituted by another purine or a pyrimidine base (cytosine, thymine). In transversion type of point mutations, purine is substituted by pyrimidine or vice versa. Transition point mutation is more common than transversion type.

The effects of point mutation can vary depending upon the site of mutation on the gene. If point mutation occurs in the coding sequence of DNA or exon, then the protein coded by the altered gene is changed. In case of point mutation in the non-coding sequence or intron, it may result in the change in RNA splicing, which in turn, affects the coding of gene.

Some other possible outcomes of point mutations include a failure of DNA transcription (conversion of messenger RNA to proteins), alteration of regulatory responses and other genetic changes.

Hence, based on the functional changes of the gene, point mutations are divided into three types, namely, nonsense (failure of expression), missense (base code for a different protein) and silent (no significant change). The diseases like sickle cell anemia and thalassaemia are caused due to missense mutation in the coding sequence of a gene.