DNA triple helix

In the major groove of the double helix in B-DNA, some funcional groups from nitrogen bases stick out which may establish hydrogen bonds if a third base approaches the groove. This structure is favoured in particular when the first strand is rich in pyrimidines, the second strand in purines and the third in pyrimidines. It is sometimes named as the H form of DNA.

In this model, the 3rd strand is actually RNA, but the structure would be the same if it were DNA.

DNA: 5′ .. .. 3′

5′-TTCTCTCT-3′
   ::::::::
3′-AAGAGAGA-5′
   ::::::::
3′-UUCUCUCU-5′
Style:

just one trio of paired bases.
trio.

This is the structure of the base trios:

structural formula of a cytidine-guanosine-cytidine trio
structural formula of a uridine-adenosine-thymidine trio