Mutation & Its Types | Microbiology


  • Mutation is a random, undirected heritable variation caused by change in nucleotide sequence of the DNA of the cell.
  • The frequency of mutation ranges from 10-2 to 10-10 per bacterium per division.
  • Mutation occurs spontaneously but its frequency may be enhanced by mutagens such as UV rays, alkylating agents, 5-bromouracil and acridine dyes.
  • Mutation is a natural event, taking place all the time, in all dividing cells. Most mutants go unrecognised as the mutation may be lethal or may involve some minor function that may not be expressed. 


There are broadly two types of mutations:

1. Point mutation

2. Multisite mutations

1. Point mutation

Point mutation may occur by

(i) Base pair substitution

(ii) Frame shift mutations

(i) Base pair substitution

A single base pair has been substituted for another pair. It can be subdivided into

(a) Transition

(b) Transversion

(a) Transition:

This happens by replacement of one pyrimidine by another pyrimidine, and one purine by another purine ie. AT replaced by GC. Transition is the most frequently occurring mutation

(b) Transversion

When purine is replaced by pyrimidine and vice versa, it is named transversion, e.g. GC changes to CG.

Pont mutation, Base pair substitution, Transition, transversion,
Point Mutation: Base pair substitution→ Transition & Transversion mutation

(ii) Frame shift mutations

Sometimes, during DNA replication, one or a few base pairs are inserted into or deleted from the DNA. This shifts the normal ‘reading frame’ of the coded message forming new set of triplet codon.

The coded message is read correctly up to the point of addition or deletion, but the subsequent codons will specify the incorrect aminoacids

Frame shift, point, mutation, microbiology
Frame shift mutation (point mutation)

2. Multisite mutations

Large number of base pairs are altered in DNA. This take place by four types such as

(i) Addition (gain)

(iI) Deletion (loss)

(iIi) Duplication

(iv) Inversion

Multisite mutation, Addition, deletion, duplication, Inversion, mutation, Microbiology
Multisite mutation
Online, Test Series, Previous Year, Question Paper


I-Induced mutation

Although mutation occurs spontaneously, its frequency is greatly enhanced by certain agents called mutagens. These may be physical or chemical 

1. Physical agents

(i) UV light

(ii) Ionising radiation e.g. X-rays

(iii) Visible light

(iv) Heat

2. Chemical agents

(i) 5-Bromouracil

(ii) 2-Aminopurine

(iii) Nitrous acid

(iv) Acridine dyes

II-Nonsense mutation

When a a non-sense codon (UAG, UAA or UGA) is formed within a gene by mutation, the protein synthesis is terminated prematurely and a partial  polypeptide is produced during translation. This is called a nonsense mutation.

III- Missense mutation

Both substitution and frame shift mutation give rise to an altered codon which specifies different amino acid from that normally located at a particular position in the protein. This is called missense mutation. 

IV- Suppression mutation

A second mutation that restores the function of a gene altered by previous mutation (first) is called suppression mutation.

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