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1. The DNA double helix is held together by two types of bonds, covalent andhydrogen. Covalent bonds occur within each linear strand and strongly bond thebases, sugars, and phosphate groups (both within each component and betweencomponents).Hydrogen bonds occur between the two strands and involve a basefrom one strand with a base from the second in complementary pairing.ovalent bonds occur within each linear strand and strongly bond thebases, sugars, and phosphate groups (both within each component and betweencomponents). Hydrogen bonds occur between the two strands and involve a basefrom one strand with a base from the second in complementary pairing.
The formation of stable hydrogen bonds depends on the distance between two strands, the size of the bases and geometry of each base. Stable pairings occur between guanine and cytosine and between adenine and thymine (or adenine and uracil in RNA). Three hydrogen bonds form between guanine and cytosine. Two hydrogen bonds form between adenine and thymine or adenine and uracil. Complementary pairs always involve one purine and one pyrimidine base. Pyrimidine-pyrimidine pairings do not occur because these relatively small molecules do not get close enough to form hydrogen bonds.