Within the cell the virus nucleic acid uses the host machinery to make copies of the viral nucleic
acid as well as enzymes needed by the virus and coats and enveloping proteins, the coat proteins of
the virus. The details of the process by which the information in viral nucleic acid is expressed and
the sites in the cell where the virus locates vary according to the type of nucleic acid the virus
contains and other viral features. As viral components are formed within a host cell, virions are created
by a self-assembly process; that is, capsomere subunits spontaneously assemble into a protein coat around
the nucleic core. Release of virus particles from the host may occur by lysis of the host cell, as in
bacteria, or by budding from the host cell's surface that provides the envelope of membrane-enveloped forms.
Usually the protein coat, or capsid, of an individual virus particle, or virion, is composed of multiple
copies of one or several types of protein subunits, or capsomeres. Some viruses contain enzymes, and some
have an outer membranous envelope. Many viruses have striking geometrically regular shapes, with helical
structure as in tobacco mosaic virus, polyhedral (often icosahedral) symmetry as in herpes virus, or more
complex mixtures of arrangements as in large viruses, such as pox viruses and the larger bacterial viruses,
or bacteriophages. Certain viruses, such as bacteriophages, have complex protein tails. The inner viral
genetic material-the nucleic acid-may be double stranded, with two complementary strands, or single stranded;
it may be deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). The nucleic acid specifies information for
the synthesis of from a few to 50 different proteins, depending on the type of virus.
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