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Viruses are obligate intracellular
organism, contain DNA or RNA within a cylindrical or spherical protein
coat or capsid, which may be surrounded by a lipid bilayer (envelope).
Viruses are organisms that can be
characterized as having two distinct phases in their life cycle - an
intracellular and an extracellular phase.
Intracellular phase is the replicative
phase during which the virus multiplies in the infected cell.
There it borrows the metabolic
machinery of the cell to direct the synthesis of proteins coded by the
viral genome.
The structural and nonstructural virion
components are synthesized independently, and the structural proteins
are assembled into whole virions during the final stage of
reproduction.
When virions leave the cell, they are
particles of uniform size, shape and chemical composition that in some
cases can crystallize.
This is the extracellular phase of the
virus.
The viral particles can initiate the
infectious process of new cells, and hence they constitute the
infectious form of the virus.
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