The secondary structure of DNA in solution and in nucleohistone

Abstract

Solutions and gels of calf thymus DNA and nucleohistone were studied using wide-angle X-ray scattering. The experimental scattering curves were compared to those calculated by computer from various atomic models. The structure of DNA in solution (0.05 to 0.15 m-monovalent salt) was found to be of the B kind. A structure which is derived from the B form (as determined from fiber diffraction by decreasing the turn angle to 0.575 radian from 0.628 radian per residue (to give a pitch = 37 Å)) is most consistent with the results. A comparison of the scattering for DNA and nucleohistone, and the scattering calculated from the B form, show that there is a family of structures of the B kind. The results are in good agreement with the Watson-Crick base-pairing scheme and the atomic co-ordinates given by Arnott, Dover & Wonacott (1969). We find that the secondary structure of the DNA in nucleohistone is also of the B family. A turn angle per residue of 0.65 radian (pitch, 32 Å) gives the best agreement with our data.