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.<br>