The monomers of DNA and RNA are nucleotides, which are made up of a phosphate group, a five-carbon sugar and a nitrogenous base. In DNA, the nitrogenous bases are adenine, cytosine, guanine and thymine. In RNA, the nitrogenous bases are adenine, guanine, cytosine and uracil.
DNA and RNA is a polymer, or macromolecule, made up of many similar smaller molecules covalently bonded together. These smaller molecules are called monomers, and the specific monomers vary depending on the macromolecule (protein, carbohydrates or nucleic acids). Because DNA and RNA are nucleic acids, the monomers that form them are nucleotides, which are molecules made up of a nitrogenous base, a five-carbon sugar and a phosphate group. According to About, these nucleotides are covalently bonded together when the nitrogenous base of one nucleotide corresponds with the nitrogenous base of another nucleotide.
According to How Stuff Works, the four nitrogenous bases fall into two categories: purines and pyrimidines. The purines are double-ringed, and the pyrimidines are single-ringed. When a nucleotide bonds to another nucleotide, a purine base and a pyrimidine base hook up. Adenine pairs up with thymine, and cytosine always pairs up with guanine. After they bond, the nucleotides form the double helix of DNA.