What's the mechanism of point mutation?

There are several mechanisms of point mutation. The exact mechanism depends on the cause of the mutation. The mechanism that involves substitution of a base pair for another is the most common mechanism. This is because, in most cases, point mutation occurs spontaneously during the DNA replication phase. 

Substitution mechanism of point mutation: 

Substitution mechanism arises due to the hydrogen atoms of a nucleotide base pair moving away in a way that alters the properties of bonding. This is known as a tautomeric shift and the bases involved in DNA replication, which exist in different forms, are called tautomers. The tautomers exist in equilibrium, where DNA houses the keto form while the enol forms are not as common. The change in the structure between the keto and enol form can alter the complementary base pairing of the nucleotides, resulting in point mutation.