Mutation

A mutation is a permanent change in the DNA sequence of an organism.

It can involve a single nucleotide or large segments of DNA and may occur spontaneously or due to external factors(e.g., radiation, chemicals, viruses).

Types of mutations- 

1. Point mutation: Mis-Sense, Non-sense, Silent mutation

2. Frame shift mutation 

Point mutaion- 

• Transversion: A purine is replaced by a pyrimidine or vice versa.

• Transition: A purine is replaced by another purine (A ↔ G) or a pyrimidine is replaced by another pyrimidine (C ↔ T).

1. Mis-Sense mutation: This type of mutation results in a single amino acid change in the protein sequence. It occurs when a change in a single nucleotide results in the substitution of one amino acid for another in the protein.

Example, Misense mutation where the second codon changes from GAG (Glutamic acid) to GUG (Valine)- as seen in sickle cell anemia (SCA).

Sickle cell anemia (SCA): 
• Autosomal recessive inheritance 
• In the β-globin gene (HBB gene) on chromosome 11,

One nucleotide is changed (A → T in the DNA sequence). 
• This leads to a change in the mRNA codon from GAG (glutamic acid) to GTG (valine).
(Mnemonic: Glutamic Go, Valine Welcome)

2. Non-Sense mutation: This mutation changes a codon that codes for an amino acid into a stop codon (UAA, UAG, UGA).

Example, UAC (which codes for the amino acid tyrosine) is mutated to UAA, which is one of the three stop codons in mRNA.

Example: Beta-thalassemia (absent production of the beta-globin chains of hemoglobim)

3. Silent mutation: This is a change in the DNA sequence that does not alter the amino acid sequence of the protein. It occurs because of the redundancy in the genetic code (multiple codons can code for the same amino acid).

Frame shift mutations-

• Deletion: Removal of 1 or 2 nucleotides from the DNA sequence.
• Addition: Insertion of 1 or 2 nucleotides into the DNA sequence.