Describe chemical and physical mutagens to bring about mutations.

Explanation

Induced Mutations. They are mutations that are produced in response to specific external factors and chemicals. Muller (1927) was the first to produce induced mutations in Drosophila by exposing them to X-rays. The specific factors and chemicals of the environment that induce mutations are called mutagens. Mutagens. Any extracellular physical or chemical factor which can cause mutations or increase the frequency of mutations in organisms is called mutagen. 1. Physical Mutagens. They are of two types, temperature and high energy radiations. (i) Temperature. Increase in temperature increases the rate of mutations with Q = 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 nonionising 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 deaminate and dehydroxylate nitrogen bases, form peroxides and oxidise deoxyribose. Muller (1927) was the first to induce mutations in Drosophila with the help of X-rays. He found 150 fold increase in the rate of mutations. Radiations emitted by nuclear fall-outs and the atom bumbs dropped over Hiroshima 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. (1) Nitrous Acid. It is a deaminating agent which changes cytosine to uracil, guanine to xanthing and adenine to hypoxanthine. Hypoxanthine mispairs with cytosine. Therefore, A-T is replaced by H-C. Similarly C-G is replaced by U-A and C-X. It disturbs replication and transcription. Incomplete or defective polypeptides are produced during translation. (ii) Alkylating Agents. Nitrogen mustards [e.g., RN`(CH_2 Cl)_2`], 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 separation of 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-T is 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 Pharmaceuticals are prepared. Acridines (e.g., acriflavine, proflavine, euflavine, acridine orange) enter the DNA chains in between two base pairs and cause deletion or addition of a few nucleotides. The frame of nucleotide sequence of DNA will be thus disturbed and read differently. It is also known as frame-shift or gibberish mutation.