Dna is comprised of four different types of nucleotides. each one is made up of a pentose sugar, a phosphate group, and one of four nucleosides. 2. adenine, guanine, cytosine and thymine are nitrogenous bases. 3. adenine and guanine, which have a nine-member double-ring, are called purines. 4. cytosine and thymine, which have a six-member single-ring, are called pyrimidines.