Nonenveloped Viruses

All nonenveloped animal viruses have an icosahedral structure The structural proteins of simple icosahedral viruses associate spontaneously to form capsomers, which undergo self-assembly to form capsids into which viral nucleic acid is packaged Completion of the virion often involves proteolytic cleavage of one or more species of capsid protein The best studied example, that of poliovirus, is depicted in Fig. 3-8

The mechanism of packaging viral nucleic acid into a preassembled empty procapsid has been elucidated for adenovirus. A particular protein binds to a nucleotide sequence at one end of the viral DNA known as the packaging sequence, this enables the DNA to enter the procapsid bound to basic core proteins, after which some of the capsid proteins are cleaved to make the mature virion

Most nonenveloped viruses accumulate within the cytoplasm or nucleus and are released only when the cell eventually lyses.

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