Nucleic acids are all made up of building blocks called nucleotides, which are compounds that are made of nitrogen, a carbon sugar ring and one or more phosphate groups. The five nucleotides are called cytosine, guanine, adenine, thymine and uracil. Uracil can only be used in the construction of ribonucleic acid, or RNA.
Deoxyribonucleic acid, also known as DNA, is often represented with four letters: A, T, C and G. These four letters stand for the four nucleotides that compose it. Similarly, other types of nucleic acids, like RNA, mRNA and tRNA can also be represented by letters. The sequence of nucleotides in nucleic acids is responsible for communicating important messages to other parts of the cell.