What is the alphabet of genetics?

The alphabet of genetics consists of four letters ATC and G each representing the corresponding nitrogenous base, e.g., A for adenine, T for thiamine, C for cytosine and G for guanine (in case of RNA, T is replaced by U which stands for uracil). These are constituents of a nucleotide monomer along with pentose sugar and phosphoric acid. In the double helical structure of the DNA, the two nucleotide polymer chains are linked by H-bondings between these bases. These four letters constitute all kinds of genetic codes in the entire biological world.