The structure of proteins can be altered by three types of mutations:
1. Point Mutations:
Point mutations involve addition, deletion or replacement of base pair in a gene.
2. Chromosomal Mutations:
Addition or deletion of DNA cause mutations during crossing over.
3. Jumping genes also cause change in protein structure by shuffling their location from one chromosome to another.
The abnormal mRNA produced by mutations results in a change in protein structure and function. Frame shift mutation caused by addition or deletion of one or two bases will result in the formation of entirely a new polypeptide.
Mutation can change a normal codon to a terminator codon, which will result in the formation of an incomplete polypeptide. The altered or incomplete polypeptide may be inactive and may prove to be fatal for the cell. For example, sickle cell anaemia is caused by a single base substitution. This causes the replacement of glutamic acid by valine at position 6 in the chain of haemoglobin.
But a change in the third base in a triplet may not cause any change in the polypeptide. This is because the codon can interact with the anticodon of the corresponding tRNA. This phenomenon is known as Wobble hypothesis. It was proposed by crick in the year 1966.
According to this hypothesis, only the first two bases of the tRNA anticodon binds with the first two bases of the mRNA codon. The third position is called wobble position. Mutations that do not cause any change in the protein are called silent mutations.