Mutation and its types

  • Mutation can be defined as any inheritable changes in the sequence of nitrogenous bases of DNA that brings about sudden change in the morphological characteristics of the organism.
  • This change in the base sequence is brought about by addition, deletion or substitution of one or more bases in the nucleotide sequence of DNA.
  • Organisms without mutation are called wild type while the organisms with mutation are called mutants.
  • Mutation results in the insertion of a different amino acid into a protein, resulting in the appearance of an altered phenotype as follows:
    • Drug resistant mutant: In this case, mutant bacteria exhibit resistance to the antibiotics or drugs.
    • Auxotrophic mutant:
      • These are nutritionally deficient mutant which lack or lose their property to synthesize particular food or compounds.
      • These bacteria require the compounds in their growth media without which they can’t and die.
    • Antigenic mutant: The surface antigen is changed in these mutants.
    • Some bacteria exhibit some changes in their morphological characteristics. e.g. some mutant lose their property to produce spore, capsule or flagella.
    • Some mutant show changes in their ability to produce pigments.
    • Some bacteria undergone mutation show altered metabolic activities.
    • Some mutant are killed by mutation, such mutation is called lethal mutation.

Types of mutation:

  • Mutations are a natural event occurring in dividing cells.
  • Mutation occurs spontaneously or enhanced by different mutagens (mutation causing agents).
  • There are three types of mutation:
    • Mutation due to base substitution (Point mutation)
    • Frame-shift mutation
    • Mutation due to transposons and insertion sequence
  1. Point mutation:
  • It occurs as a result of replacement of one nucleotide by other in the specific nucleotide sequence (certain point) of DNA.
  • This occurs during the replication of DNA.
  • Point mutation is also of two types:
    • Transition mutation: Substitution of one purine by another purine or one pyrimidine by other pyrimidine.
    • Transversion mutation: Replacement of one purine by pyrimidine or vice versa.
  • On the basis of translation property, point mutation is of three types:

a. Missense mutation:

  • The altered gene triplet produces a codon on m-RNA which codes the amino acid different from the one present in the normal protein.
  • Such altered protein may be functionally inactive or less active then the normal protein.
  • If this altered amino acid lies in the active site of protein, the entire protein becomes inactive. If it lies any other site, the protein isn’t affected to greater extent.
  • It is further of two types:
    • Conservative mutation: It results in an amino acid change the properties of which remain the same even after mutation and at times, a change to one amino acid in the protein is not detrimental to the organism as a whole
    • Non-conservative mutation: It results in an amino acid change that has different properties than the wild type and the protein thus produced may lose its function.

b. Non-sense mutation:

  • If altered gene triplet produces a stop codon on m-RNA, it is called non-sense mutation.
  • This mutation causes incomplete protein synthesis which is always non-functional.

c. Neutral or silent mutation:

  • The altered gene triplet produces the new codon on m-RNA which codes the same amino acid as the original one.
  • Since the new amino acid is same as the original one, it doesn’t bring about any change in the structure and function of protein.
  1. Frame-shift mutation:
  • It occurs as a result of addition or deletion of one or more nucleotide from the nucleotide sequence of DNA.
  • Addition or deletion of nucleotide causes the shift of the reading frame of m-RNA downstream from the mutation
  • It brings greater change in the protein as compared to the point mutation.
Source: U.S. National Library of Medicine
  1. Mutation due to transposons or insertion sequence:
  • This occurs when transposons or insertion sequences are integrated into the DNA.
  • These newly inserted sequences of DNA cause profound changes in the gene into which they are inserted and also causes changes in the adjacent genes.

Mutation and its types