The type of bond that holds nitrogen bases together is called a hydrogen bond. Nitrogen bases occur in nucleotides, which are the basic chemical building blocks that make up the chemical structure of DNA.
There are two categories of nitrogen-containing bases (ring compounds) found in DNA. They are the purines and pyrimidines. Purines are double-ringed and include guanine (G) and adenine (A). Pyrimidines are single-ringed structures and include cytosine (C) and thymine (T). The bases form pairs by bonding with another, complementary base (purines pair with pyrimidines, G with C and A with T). The bond formed between the two bases is called a hydrogen bond, which is a weak bond that occurs over short distances and can be easily broken or formed.