A free virus particle may be thought of as a packaging device by which viral genetic material can
be introduced into appropriate host cells, which the virus can recognize by means of proteins on
its outermost surface. A bacterial virus infects the cell by attaching fibers of its protein tail
to a specific receptor site on the bacterial cell wall and then injecting the nucleic acid into the
host, leaving the empty capsid outside. In viruses with a membrane envelope the nucleocapsid
(capsid plus nucleic acid) enters the cell cytoplasm by a process in which the viral envelope merges
with a host cell membrane, often the membrane delimiting an endocytic structure in which the virus
has been engulfed.
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