Nonenveloped Viruses With More Complicated Structural Features

In addition to the nonenveloped viruses that possess relatively straightforward icosahedral symmetry or helical symmetry, many viruses possess more complicated symme tries made possible by the utilization of a large number of structural proteins to form the virion, The tailed bacterio-phages are prominent examples of this (Fig, 2,13), Some of the tailed bacteriophages possess a head that is a regular icosahedron (or, in at least one case, an octahedron) connected to a tail that possesses helical symmetry, Other appendages, such as baseplates, collars, and tail fibers, may be connected to the tail, Other tailed bacteriophages have heads that are assembled using more complicated patterns, For example, the T-even bacteriophages have a large head, which can be thought of as being formed of two hemi-icosahedrons possessing regular icosahedral symmetry, which are elongated in the form of a prolate ellipsoid by subunits arranged in a regular net connecting the two icosa-hedral ends of the head of the virus,

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