The rules of base pairing explains the phenomenon that whatever the amount of adenine (A) in the DNA of an organism, the amount of thymine (T) is the same. It's also like this with guanine (G) and cytosine (C). Whatever the amount of guanine (G) in the DNA of an organism, the amount of cytosine (C) is the same. This is Chargaff's rule of base pairing. I hope this helps!