Small spherical virus is made up of a large number of identical protein subunits and a nucleic acid (RNA or DNA) . These proteins are arranged icosahedrally around the nucleic acid which is the core part of a virus particle. In some cases, viruses crystallize and make it possible to get high resolution X-ray diffraction pattern. Southern Bean Mosaic Virus (SBMV) now going to be mentioned is the typical example. The virus Particle with molecular weight 6.6×10
6 daltons has a shell of 180 protein subunits, arranged with T=3 icosahedral symmetry around a core of RNA with molecular weight 1.4×10
6 daltons. The crystal of space group R32 with cell dimensions
a=318Å, α=64°0' diffracts X-rays beyond 3Å.
Here are reported the symmety of spherical virus and the determination of the orientation of virus particle in the unit cell using characteriotic pattern of X-ray diffraction from the virus molecule with very high symmetry, as well as low resolution structure study of SBMV
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