THE STRUCTURE OF B-DNA IN ORIENTED FIBERS

Native, general sequence B-form DNA in uniaxially oriented fibers is a ten-fold helix with identical antiparallel strands: this is to say th e molecular symmetry is 2 2 10(1). The diffraction patterns indicate t hat local variations, however significant, must be modest. This is tru e also for the lithium salt of calf thymus DNA in fibers that are poly crystalline as well as oriented. The contents of its orthorhombic unit cells are arranged with P2(1)2(1)2(1) symmetry which permits the mole cular symmetry to be merely two-fold. The molecular structure of DNA i n such conditions resembles, conformationally and molecularly, that of B-type DNA in oligonucleotide single crystals and in oriented polycry stalline fibers of polyoligonucleotides, and therefore provides a basi s for evaluating the variations that may be due to sequence effects in polyoligonucleotides in fibers and oligonucleotides in single crystal s.