What are induced mutations ? Write about physical and chemical mutagens responsible for induced mutations.

Explanation

Induced Mutations. They are mutations that are produced in response to specific external factors and chemicals. Muller (1927) was the first to produced induced mutations in Drosophila by exposing them to X-rays. The specific factors and chemicals of the environment that induce mutations are called mutagens.
1. Physical Mutagens. They areoftwo types, temperature and high energy radiations
(i) Temperature. Increase in temperature increases the rate of mutations with `Q_(10)=5` It is further enchanced at higher temperature. Rise in temperature breaks the hydrogen bonding between the two strands of DNA and hence denatures the latter. It disturbs the synthetic process connected with replication and transcription. In Rice, low temperature is known to increase the rate of mutations.
(ii) High Energy Radiations. They include neutrons, alpha particles, cosmic rays, gamma particles, beta rays, X-rays, ultra-violet rays, etc. Ultra-violet rays are nomonismg radiations which affect DNA by forming thymine dimers. It causes bends in DNA duplex that bring about misreplication. Other high energy radiations are ionising radiations. They ionise DNA constituents that can react with several biochemicals. X-rays are known to dearninate and dehydroxylate nitrogen bases, form peroxides and oxidise deoxynbose Muller (1927, was the first to induce mutations in Drosophila with the help of X-rays. IIc found 1 increasc in the rautations. Radiations emitted by nuclear fall-outs and the atom buinbs dropped over Hirosbima and Nagasaki in Japan have caused many mutations,
2. Chemical Mutagens. They are of several types. The common ones are nitrous acid, alkylating agents, base analogues and acridines.
(i) Nitmus Acid. It is a deaminating agent which changes cytosine to uracil, guanine to xanthine and adicnine o hypoxanthine. Hypoxanthine mispairs with cytosine. Thereforė A-T is replaced by Molecular Basis of Inheritance 193 H-C. Similarly. C-G is replaced by U-A and C-X. These unusual or forbidden base pairs disturb replication and transcription. Incomplete or defective polypep-tides are produced during translation.
(ii) Alkylating Agents. Nitrogen mustards diethyl sulphate (DBS) dimethyl nitrosamine (DMN) and other alkylating agents cause methylation or ethylation of nitrogen bases. The latter fail to pair with normal partners as well as prevent separatic two DNA strands.
(iii) Base Analogues. They resemble the normal bases of DNA and, therefore, get incorporated into DNA in place of them. The common mutagens of this type are 5-bromouracil and 5-fluorouracil. They substitute for thymine of DNA and pair with guanine. Thus A-Tis replaced, by G-Bu or Fu. It disturbs replication, transcription and translation.
(iv) Acridines. They are tar derived heteroaromatic flat molecules from which a number of dyes and pharmaceutical are prepared. Acridines (e.g., acriflavine, proflavine, euflavine, acridine orange) enter the DNA chains in between two base pairs and cause deletion c addition of a few nucleotides. The frame of nucleotide sequence of DNA will disturbed and read differently. It is also known as frame-shift or gibberish mutation.