You do need more than 2 bits. DNA can be modified (5-methyl-C is the most well known, but there are lots of other modifications known). You also have degeneracy

You do need more than 2 bits. DNA can be modified (5-methyl-C is the most well known, but there are lots of other modifications known). You also have degeneracy where the exact base at a position is not known and these are represented in the data as additional letters [1]. To cover all these you need 4 bits and if you want to cover the modifications you need at least 5 bits. 1. https://en.m.wikipedia.org/wiki/Nucleic_acid_notation https://en.m.wikipedia.org/wiki/Nucleic_acid_notation