The four types of nucleotides found in DNA are guanine, cytosine, adenine and thymine. These are nitrogenous bases and are subdivided into purines and pyrimidines. The purines are adenine and guanine, and the pyrimidines are cytosine and thymine.
These four bases form complementary pairs in the DNA polymer, where guanine only pairs with cytosine and adenine links up with thymine. There are two hydrogen bonds between adenine and thymine, and three hydrogen bonds between cytosine and guanine.
The nucleotide in DNA consists of three main structures, which are deoxyribose, a phosphate group and one of four nitrogenous bases. Deoxyribose is a five-carbon sugar in which the carbons link up with the phosphate group. These two structures form the DNA backbone, or two-strand helix, while the nitrogenous bases are found inside the DNA helix in a ladder-type arrangement.