Prokaryotic, eukaryotic and mitochondrial DNA sequences of total Length 300 000 
nucleotides have been analyzed to find out whether stretches of alternating 
purines and pyrimidines are unusual in terms of occurrence, composition and base 
sequence. Alternating runs longer than 5 nucleotides are significantly 
under-represented in the natural sequences as compared to random ones. 
Octanucleotides are the most deficient, occurring at only 60% of the frequency 
expected in random sequences. An unexpectedly high proportion of these octamers 
consists of alternating tetramers with the repeat structure (PuPyPuPy)2 or 
(PyPuPyPu)2. DNA stretches containing such sequences can potentially form a S1 
nuclease sensitive slippage (staggered loop) structure, which might serve as a 
locally unstacked intermediate in the B- to Z-DNA conformational transition.