Nitrogenous bases are the class of biological molecule to which guanine, adenine, cytosine and thymine belong. These nitrogenous bases combine with a five-carbon sugar and a phosphate group to form nucleic acids such as DNA and RNA.
Guanine and adenine belong to the purines, while thymine and cytosine are pyrimidines. A major difference between the two types of nitrogenous bases is their ring structures: Purines have two rings, whereas pyrimidines have a single ring.
NItrogenous bases make up the rungs of the DNA ladder structure. Chargaff's base-pairing rules dictate that two bases pair with one another — a purine to a pyrimidine. In DNA, adenine usually pairs with thymine and cytosine connects to guanine. Two purines cannot join together because the resulting rung would be too long; in the same vein, two pyrimidines cannot pair up because the rung would then be too short. A purine and pyrimidine pair up as the result of hydrogen bonds. In humans, the relative proportions of adenine and thymine are greater than those of cytosine and guanine; these proportions are also true of many eukaryotes.
DNA has subunits called nucleotides. Each nucleotide consists of a phosphate group, the sugar deoxyribose and a nitrogenous base. Alternating sugar and phosphate molecules bonded covalently make up the backbone of the DNA double helix. A nucleoside consists of only the sugar portion bonded to a nitrogenous base.