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They hold the two strands of DNA together. The nitrogenous bases in DNA are A (adenine), C (cytosine), G (guanidine), and T (thymine). In each of these bases, the nitrogen of the secondary amine bonds to the deoxyribose in the backbone. DNA consists of two strands running in opposite directions. The hydrophobic nitrogenous bases are on the inside, hidden from water. Hydrogen bonds between the nitrogenous base hold the two strands of DNA together. These hydrogen bonds are base specific. That is, A (adenine) can form hydrogen bonds only with T (thymine). C (cytosine) can form hydrogen bonds only with G (guanine). The thymine-adenine pair interacts through two hydrogen bonds.
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.