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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.
Answer: Hydrogen bonding, which holds the two DNA strands together, determines which nucleotides will form complementary base-pairs. Adenine is able to form two hydrogen bonds and so it lines up with thymine, which is also able to form two hydrogen bonds. Cytosine, which is a smaller nucleotide like thymine, does not base-pair with adenine because their hydrogen-bonding atoms would not line up correctly. Likewise, guanine is able to form three hydrogen bonds and so it lines up with cytosine. which is also able to form three hydrogen bonds.