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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.
See Fig. 1. The nucleotides within each strand are held together by the phosphodiester bonds between the 5' carbon of one nucleotide and the 3 ' carbon of the adjacent nucleotdie. These strong covalent bonds form the the glue that holds the sugar/phosphate backbone together and thus act to keep each strand together.The two strands of DNA are held together by weak hydrogen bonds (remember, something must hold them together!) between the nitrogenous bases.The interesting (and very important) aspect of the Watson-Crick model is that these hydrogen bonds are base specific.hat is, A (adenine) can only form hydrogen bonds with T (thymine) and C (cytosine) can only form bonds with G (guanine). This means, that if you know the sequence of bases of one strand, you know the sequence of bases of the other strand. The strands is such a molecule are said to be complementary.