Mutagenesis and Errors in Transcription

When coding information is inaccurately copied, this leads to an alteration of base sequences and a deleterious modification of the newly synthesised protein. 

A reproducible transmissible alteration in the code is called a mutation. Agents which produce such mutations are called mutagens. 

Ultraviolet, x-rays and ionizing radiations readily break DNA chains and in this way can induce mutations. In general, mutations can be induced by chemical attack and by accidental factors as well as by radiation. 

The most frequently observed alterations are changes in the base sequences, known as transitions, which involve the replacement of one purine base by another, or one pyrimidine base by another. When a purine base replaces a pyrimidine base or vice versa, this is known as transversion, but it is of much rarer occurrence. 

A simple transition mutation can be carried out by treating RNA with nitrous acid. This deami-nates amino groups to hydroxyl groups and converts cytosine into uracil (Chapter 10.4). The latter then pairs with adenine, resulting in the replacement of CG pairs with UA pairs. Sodium bisulphite will also effect deaminations of this kind. 

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