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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.ames Watson and Francis Crick proposed a structure for the DNA molecule that suggested the basic mechanism of DNA replication. The model proposes that DNA is composed of two strands of DNA running anti-parallel (remember the 5' and 3' ends!!) to each other.