Question

Why are some mutations within a codon more serious than others? What are two major types of mutations?

Answer 1

Mutations occur in two ways:

1. By base (Adenine, Uracil, Guanine, Cytosine) substitution, in which one base is substituted for another

2. By insertion or deletion, in which a base is either incorrectly inserted or deleted from a codon.

The base substitution mutations are of different types. They are

1a. Silent mutations.  Mutations that does not have much effect are called silent mutations. If the codon GAA becomes the codon GAG, this codon will still code for the amino acid glutamate.This is because of the degeneracy of genetic code.

1b.  Missense mutation is a base substitution mut ation in which change in codon changes the amino acid coded by the affected codon. This cause production of defective protein.

1c. Nonsense mutation refers is a type of base substitution in which the changed nucleotide transforms the codon into a stop codon. This leads to a premature termination of translation, which can badly affect the formation of proteins.

2. When a nucleotide is wrongly inserted or deleted from a codon it leds to frameshift mutation, in which the protein formed is completely different from the normal Protein. The frame shift mutation is more drastic because it completely shifts the reading frame of mRNA.

GAU UCC GCU AGG UCC, which codes for the amino acids aspartate, serine, alanine, arginine,serine.

If the U in the GAU were to be deleted, the code would become: GAU CCG CUA GGU CC. This leads to the formation of defective protein.