Dna replication is facilitated by the fact that the base on one strand of the double helix (t, for example) always has the same partner (a, in this case) on the other strand. this feature of dna is called:

This feature of DNA is termed complimentary base pairing, in that the base that compliments a certain base, say A, is always paired with base T, and likewise every T has its pair partner always as A. The same holds true with C with G, and vice versa.