A protein
coat that covers the
nucleoprotein core or nucleic
acid of a virion. Commonly shows
icosahedral symmetry and may itself be enclosed in an
envelope (as in the Togaviridae). The
capsid is built up of subunits (some integer
multiple of 60, the
number required to
give strict
icosahedral symmetry) that
self assemble in a
pattern typical of a special virus. The subunits are often packed, in less capsids,
into 5 or 6 membered rings (pentamers or hexamers) that constitute the
morphological unit (capsomere). The
packing of subunits is not perfectly symmetrical in most cases and some
units may have strained interactions and are said to have quasi
equivalence of
bonding to
adjacent units.
This
entry appears with alow from the Vocabulary of Cell and Molecular Biology