a nucleotide is composed of a nucleobase (nitrogenous base), a five-carbon sugar (either ribose or 2-deoxyribose), and one phosphate group. nucleotides contain either a purine or a pyrimidine base. ribonucleotides are nucleotides in which the sugar is ribose. deoxyribonucleotides are nucleotides in which the sugar is deoxyribose.