In reproduction your Adenine can only match with the Thyminc .Also the Guanine and the Cytosine can only be together. This four thing are called Nitrogenous Bases. When DNA Replication seperates it uses the same rule by forming two complementary strands. The two complementary strands are models of the DNA.This is caused because the enzymes that were carried out were "unzipped" by a molecule. 
