Sheared purine x purine pairing in biology.

The Watson-Crick G x C and A x T base-paired DNA duplex has been the single most important milestone in modem molecular biology. However, it is possible that other types of stable DNA structures besides the double helix might exist, since only about 5% of the human chromosome is transcribed and expressed. Stable, four-stranded G-tetraplex DNA structures occur in the extensive tandem repeated sequences at the telomeres of chromosome. Formation of stable triplexes of the Py x Pu x Py or Pu x Pu x Py type have been implicated at the control regions of certain human genes. We review and discuss the various types of DNA duplex structures containing stable sheared base-pairs and compare their structural characteristics with that of B-DNA. Pu x Pu structural motifs are found in the highly conserved sequences at the replication origins of several single-stranded DNA viruses and in the peri-centromeric regions of human chromosomes, and may be involved in important biological functions, such as viral DNA replication and centromere formation.