The association of DNA bases on two strands, A:T and C:G, is called base pairing.  The two strands of DNA are complementary.  Knowing the base sequence of one strand automatically provides the sequence of the other strand.  The duplex DNA of the human genome consists of about 3 billion base pairs, and about 99.9 percent of those bases are the same in all people. Note that individuals inherit a complete genome from their father and another complete genome from their mother, so an individual's total complement of DNA is 6 billion base pairs.