In the helical structure of DNA, the two strands are held together by hydrogen bonds between specific pairs of bases. Cytosine from hydrogen bond with guanine, while adenine forms hydrogen bond with thymine. As a result , the two strands are complementary to each other.

DNA consists of two strands of nucleic acid chains coiled around each other in the form of a double helix. The base of one strand of DNA is paired with bases on other strand by means of hydrogen bonding. This hydrogen bonding is very specific as the bases can only base pair in a complementary manner. Adenine pairs with only thymine via 2 hydrogen bonds and guanine pairs with cytosine through 3 hydrogen bonds. Thus, the two strands of DNA are complementary to each other in the sense that the sequence of bases in one strand automatically determines that of the other. So the DNA stands cannot be identical, but they are complementary to each other.