BSC 444 Lecture Notes - Lecture 9: Tobacco Mosaic Virus, Symmetry, X-Ray Crystallography
28 Sep 2017
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Capsid assembly: viruses solve these problems by using the rules of symmetry, individual subunits interact in groups to form small symmetrical shapes/units (e. g. , pentons and hexons, small symmetrical units interact to assemble into larger symmetrical units (e. g. , capsid) Helical symmetry: closer examination of the tmx particle by x-ray crystallography revels that the structure of the capsid actually consists of a helix rather than a pile of stacked discs. What is a helix: a helix can be defined mathematically by two parameters, 1. Pitch (the distance covered by each complete turn of the helix) Amplitude and pitch must be constant: helices are rather simple structures formed by stacking repeated components with a constant relationship (amplitude & pitch) to one another. If this simple constraint is broken a spiral forms rather than a helix unsuitable for containing a virus genome. Genome packaging: two basic mechanisms for genome packaging, capsid proteins assemble onto the genome, capsid assembled empty, then genome package into it.
