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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.
The DNA polymer chains are held together when the bases attached to the sugar groups of the nucleotides are bound together by hydrogen connections, known as base pairing.