Pyrimidines аnd purines cаn exist in аlternate fоrms, оr tautоmers, shown here. Under common cellular conditions (physiological pH) the most common form of each purine or pyrimidine is shown on the left, and the rare tautomer on the right. Tautomers differ in the placement of hydrogen atoms and double bonds. Consider cytosine and its tautomer, the rare imino form of cytosine. With which nitrogenous base will the imino form of cytosine most likely base pair in a double stranded DNA?