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MUTATIONS

Mutations are the abrupt changes in genetic material that alter the chemical structure of genes at molecular level (gene mutations or point mutations). Mutations occur more frequently in certain regions of DNA that are called hot spots of mutations. The first mutant was isolated by Morgan (1910), that was a white-eyed male Drosophila melanogaster (fruit fly, where the normal eye colour is red) and this character was located on the X-chromosome. Thereafter, about 500 mutants were discovered in Drosophila.

Mutations may occur in somatic tissues and are called somatic mutations, which are non-heritable; whereas germ line mutations occurring in gametes are heritable. Mutations may be lethal or nutritional (auxotrophs requiring additional supplements in the minimal nutrient medium) or may cause some morphological, biochemical or regulatory changes or may impart resistance to antibiotics, pesticides, etc., or silent mutations that do not cause any change or back mutations where the second mutation reverts mutational character to the normal wild type feature.

Mutations that can occur spontaneously or may be induced by certain mutagens that may be caused mainly by any of the following mechanisms (See Fig. 10.1).

1. Base Substitutions. In base substitution, a normal base in the genetic material is replaced by another base, and they are the most common causes of mutations. Base substitutions can be of two types:

(i) Transitions: Where, a purine base is replaced by another purine or a pyrimidine base by a

nother pyrimidine, e.g., 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2. Frame Shift. It occurs due to shifting of reading frame in DNA that leads to change in the pattern of codons in mRNA either by:

(i) Addition of one or a few bases in DNA or
(ii) Deletion of one or a few bases.

Fig.10.1 Different types of mutations.

Fig.10.1 Different types of mutations.