In a dna double helix an adenine of one strand always pairs with a(n) _____ of the complementary strand, and a guanine of one strand always pairs with a(n) _____ of the complementary strand. in a dna double helix an adenine of one strand always pairs with a(n) _____ of the complementary strand, and a guanine of one strand always pairs with a(n) _____ of the complementary strand. uracil ... cytosine cytosine ... thymine guanine ... adenine thymine ... cytosine cytosine ... uracil