iGen3 02-08

Double-ringed Purine / Single-ringed Pyrimidine bases

The purines are G & A. Note that the main 6-membered ring is identical with a pyrimidine ring.

The pyrimidines in DNA are C & T. In RNA, U replaces T; thymine is  5-methyl-uracil.

The numbering system is different in the purine and pyrimidine rings, following rules from organic chemistry.

Figure © 2010 PJ Russell, iGenetics 3rd ed.; all text material © 2010 by Steven M. Carr