The DNA double helix is comprised of several nucleotides. The nucleotides are adenine, guanine, cytosine and thymine. Adenine and guanine are purines. Cytosine and thymine are pyrimidines. The purines present on one strand form a hydrogen bond with the pyrimidines on the complementary strand. The adenine forms a pair with thymine and the cytosine forms a pair with guanine on the complementary strand. This hydrogen bonding between the several nucleotides result in the force that holds the two complementary strands together.