Explain the mechanism of the formation of bonds between the nucleotides of two polynucleotide chains of the DNA double helix.

Each base on one of the strands binds to one specific base on the second strand. This specific binding is called complementary. Purine bases are complementary to pyrimidine bases (that is, they are capable of forming hydrogen bonds with them): adenine forms two hydrogen bonds only with thymine, and cytosine forms three hydrogen bonds only with guanine. In a double helix, DNA strands are also linked by hydrophobic bonds.

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