- 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
- 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.
- 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.
- 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.