[SagarikaGhosh]

Mutations in a broad sense include all those heritable changes which alter the phenotype of an individual. The term mutation was first used by Hugo de Vries; thus characterizing phenotypic changes as separate from environmental variations. However, the term mutation, as it is used now refers to only those changes that alter the chemical structure of the gene at the molecular level. These mutations are usually called point mutations. Theoretically it may be easy to differentiate structure changes and mutations, but it is not so easy at the molecular level. This is because some structural changes may have the same phenotypic effects as gene mutations. Hence, scientists use cytology to differentiate between the two. Also, gene mutations are able to give reverse mutations.

The earliest record of point mutation was in 1791 when Seth Wright noticed a lamb with unusually short legs in his flock of sheep. He bred an entire flock of these short legged sheep and realized that individuals with short legs were homozygous recessive. He confirmed his results by perform a back cross.

Mutations can be artificially induces with the help of mutagenic agents which are of two types- physical mutagens and chemical mutagens. Physical mutagens are radiations, change in pH etc. Ionizing radiations like X rays and gamma rays are the main physical mutagens. There use in radiation therapy, nuclear reactors has made us prone to radiation exposure. Acute or chronic exposure to highly penetrable irradiation like gamma rays can be lethal if not treated immediately. Chemical mutagens like Mustard Gas, MMS, EMS, and DMN have been used since the beginning of world war II for their delayed mutagenic effect. Scientists like C. Auerbach are credited to be the pioneers in this field.

The molecular mechanism for mutation depends greatly on the type of mutagen. Physical mutagens like ionizing radiations are primarily known to cause thymine dimerisation. They may also be responsible for backbone breakage, depurination etc. Chemical mutagens cause a wider variety of chemical changes in the DNA molecule- incorporation of base analogues, inhibition of nucleic acid precursors, changing the nucleic acid resting state structure. All these chemical reactions alter the reactivity of the purine and pyrimidines molecule and therefore the DNA.

Due to the seriousness of the effects of mutagens and the threat of lethal mutations, scientists all over the world are working on radiation counter measure agents while focusing on compounds with antioxidant properties.