The basic idea here is simple: the sequence of bases on just one of the two strands determines the entire structure. On the other strand, the sequence is given by the complementary bases to those on the first strand – swap A and T, and swap C and G. If you could pull DNA apart into its two strands, each of them would contain the necessary ‘information’ to reconstruct the other. So all you have to do is make two complementary strands, and fit the pairs back together to get two perfect copies of the original.