Hydrogen bonds hold DNA strands together. Nitrogenous bases are between these two strands that link together in a specific manner with different types of hydrogen bonds.
A DNA molecule is made up of the sugar deoxyribose, a phosphate group and complementary nitrogenous base pairs. The double strand of the helix consists of the sugar-phosphate backbone. The nitrogenous base pairs, which are linked by hydrogen bonds that also hold the strands together, are between this backbone.
However, the nitrogenous bases, which are adenine, thymine, cytosine and guanine, pair up so that cytosine only pairs with guanine and adenine links up with thymine. There are three hydrogen bonds between guanine and cytosine, while adenine and thymine have two hydrogen bonds. Similarly, cytosine and thymine are called pyrimidines, and guanine and adenine are the purines.