In DNA, the two purines are Adenine and Guanine and the two pyrimidines are Thymine and Cytosine. NOw...in DNA...Adenine pairs with Thymine and Guanine pairs with Cytosine. A purine pairs with a pyrimidine. That explains why Adenine cannot pair with Guanine and why Thymine cannot pair with Cytosine. However, I do not know why Adenine cannot pair with Cytosine or Guanine with Thymine... Does anyone know why this is? If so I would greatly appreciate some insight on this. Thanks!