thymine

Thymine is the DNA letter that always partners with adenine, written simply as T. If adenine is a key, thymine is the lock it fits — wherever there is an A on one strand, there is a T directly across from it on the other.

Chemically, thymine is a pyrimidine: a smaller base with a single ring, in contrast to the double-ringed purines. It pairs with adenine through two hydrogen bonds. This A-T pairing, together with G-C pairing, is the rule that makes the two strands of DNA complementary.

Thymine is one feature that distinguishes DNA from RNA. RNA uses uracil where DNA uses thymine; thymine is essentially uracil with an extra methyl group attached. That small chemical tag is thought to help cells recognize and protect DNA, and to spot certain kinds of damage.

Because A pairs only with T, counting the bases in any DNA sample shows roughly equal amounts of adenine and thymine — one half of the regularity later summarized as Chargaff's rules.