Waston-Crick" pairing The strictness of the rules for Waston-Crickpairing derives from the complementarity both of shape and of hydrogen bonding properties between adenine and thymine and between guanine and cytosine. sugar N一H…○ sugar ximal hydrogen bondi H N3 A sugar sugarThe strictness of the rules for “Waston-Crick” pairing derives from the complementarity both of shape and of hydrogen bonding properties between adenine and thymine and between guanine and cytosine. “Waston-Crick” pairing Maximal hydrogen bonding