104 R. MARKHAM 



Carlisle in their work. Using some large single crystals of the nucleoprotein, 

 they took precession photographs, and found that the unit cell when viewed 

 at low resolution appeared to be a body centred cubic cell of side 350 A, and 

 that the centers of the virus particles occupied the lattice points. This 

 observation is of interest, inasmuch as it implies that the particles themselves 

 must have cubic symmetry. The true unit cell is however 8 times the volume, 

 and so has 16 particles per cell instead of the 8 which one would expect from 

 the diamond lattice, which is rather full of spaces. 



The fact that the 700-A cells did contain 16 particles, and not 8 as deduced 

 by Bernal and Carlisle, was confirmed in a direct way by measuring the 

 ultraviolet light absorption of single virus crystals of known thickness, and 

 comparing it with the known ultraviolet absorption spectrum of solutions 

 of the virus. In this way it was shown (with the collaboration of Dr. P. M. B. 

 Walker) that the unit ceU contained 18 ± 3 virus particles. 



A satisfactory solution of the X-ray crystaUographic anomalies seems to 

 have been reached by the assumption that the crystal lattice consists of 

 two interpenetrating diamond lattices in which the particles are orientated 

 alternately at 90° rotation. This arrangement gives a pattern superficially 

 similar to that of a 350-A body-centered cubic cell, but it will be noted that 

 the central particle of such a cell will be rotated by 90° with respect to those 

 particles which (partially) occupy the cube apices. The symmetry of such a 

 structure demands that the virus particles should be built up from at least 

 12 equivalent subunits, but the question of the particle symmetry has been 

 pursued further by studying the top component crystals, where there is less 

 possibility of interference by the scattering of the nucleic acid component, 

 which contributes about half of the scattering of X-rays and which may weU 

 have different symmetry itself. 



4. The Symmetry of the Top Component 



The top component has been studied mainly as a powder because large 

 crystals are diiBEicult to obtain. The patterns indicate that the particles are 

 hollow spheres with internal and external diameters of 210 and 280 A. There 

 is an indication that the spheres are not uniform in their scatter, but that 

 they have a periodicity of about 60 A. A model of such a hollow sphere has 

 been constructed, using 60 subunits of 60 A diameter arranged in the form 

 of a snub dodecahedron (Fig. 17), and this type of model would effectively 

 satisfy the X-ray requirements. It would also mean that one might expect 

 60 chemical subimits (or a multiple of 60) in the protein component of the 

 virus. This wiU be discussed later. 



The fact that the virus particles seem to be built up of regular subunits 

 is also suggested by electron micrographs made by Cosslett (1956) and by 

 Steere (1957). These pictures do not, however, appear to show as many 



