CD spectroscopy and PAGE were used to cooperatively analyze melting conformers 
of DNA strands containing GA and TA dinucleotide repeats. The 20mer (GA)10 
formed a homoduplex in neutral solutions containing physiological concentrations 
of salts and this homoduplex was not destabilized even in the terminal (GA)3 
hexamers of (GA)3(TA)4(GA)3, although the central (TA)4 portion of this 
oligonucleotide preserved the conformation adopted by (TA)10. This observation 
demonstrates that homoduplexes of alternating GA and TA sequences can co-exist 
in a single DNA molecule. Another 20mer, (GATA)5, adopted as a whole either the 
AT duplex, like (TA)10, or the GA duplex, like (GA)10, and switched between them 
reversibly. The concentration of salt controlled the conformational switching. 
Hence, guanine and thymine share significant properties regarding 
complementarity to adenine, while the TA and GA sequences can stack in at least 
two mutually compatible ways within the DNA duplexes analyzed here. These 
properties extend our knowledge of non-canonical structures of DNA.