Viral ShapeMost viruses have an overall structure that is either helical or isometric. Helical viruses, such as the tobacco mosaic virus, have a rodlike or threadlike appearance. Isometric viruses have a roughly spherical shape whose geometry is revealed only under the highest magnification.The only structural pattern found so far among isometric viruses is the icosahedrons, a structure with 20 equilateral triangular facets, like the adenovirus shown in figure 33.2. Most viruses are icosahedral in basic structure. The icosahedron is the basic design of the geodesic dome. It is the most efficient symmetrical arrangement that linear subunits can take to form a shell with maximum internal capacity.
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