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.