Pyrimidine-purine DNAs with repeating sequences can be made to undergo a 
reversible transition to possibly a tetra-stranded complex. Physicochemical 
characterization of the new structures and model building are consistent with, 
in the case of d(TC)n-d(GA)n, a tetra-stranded complex forming by the addition 
of d(GA)n to the remaining space in the major groove of the triple-stranded 
complex d(TC)n-d(GA)n-d(CT)n. A possible role for tetra-stranded complexes in 
chromosome condensation is suggested by the natural occurrence of repeating 
sequence pyrimidine-purine DNAs and the properties of condensed chromosomes.