Adenine cannot pair with Cytosine because the purine and pyrimidine bases pair only in certain combinations. Adenine pairs with thymine (A:T) and guanine with cytosine (G:C). Adenine and thymine are joined by two hydrogen bonds through atoms attached to positions 6' and 1'. Cytosine and guanine are joined by three hydrogen bonds through positions 6', 1' and 2'.
