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The Structur e of Viruses
five RNA viruses belonging to different families are shown
INTRODUCTION
in Fig. 2.1. The viruses chosen represent viruses that are
among the largest known and the smallest known, and are
Virus particles, called virions, contain the viral genome
all shown to the same scale for comparison. For each virus,
encapsidated in a protein coat. The function of the coat is to
the top micrograph is of a virus that has been negatively
protect the genome of the virus in the extracellular environ-
stained, the middle micrograph is of a section of infected
ment as well as to bind to a new host cell and introduce the
cells, and the bottom panel shows a schematic representa-
genome into it. Viral genomes are small and limited in their
tion of the virus. The structures of these and other viruses
coding capacity, which requires that three-dimensional viri-
are described next.
ons be formed using a limited number of different proteins.
For the smallest viruses, only one protein may be used to
construct the virion, whereas the largest viruses may use
HELICAL SYMMETRY
30 or more proteins. To form a three-dimensional structure
using only a few proteins requires that the structure must be
Helical viruses appear rod shaped in the electron micro-
regular, with each protein subunit occupying a position at
scope. The rod can be flexible or stiff. The best studied
least approximately equivalent to that occupied by all other
example of a simple helical virus is tobacco mosaic virus
proteins of its class in the final structure (the principle of
(TMV). The TMV virion is a rigid rod 18 nm in diame-
quasi-equivalence), although some viruses are now known
ter and 300 nm long (Fig. 2.2B). It contains 2130 copies
to violate the principle of quasi-equivalence. A regular
of a single capsid protein of 17.5 kDa. In the right-hand
three-dimensional structure can be formed from repeating
helix, each protein subunit has six nearest neighbors and
subunits using either helical symmetry or icosahedral sym-
each subunit occupies a position equivalent to every other
metry principles. In the case of the smallest viruses, the final
capsid protein subunit in the resulting network (Fig. 2.2A),
structure is simple and quite regular. Larger viruses with
except for those subunits at the very ends of the helix. Each
more proteins at their disposal can build more elaborate
capsid molecule binds three nucleotides of RNA within a
structures. Enveloped viruses may be quite regular in con-
groove in the protein. The helix has a pitch of 23 and
struction or may have irregular features, because the use of
there are 161/3 subunits per turn of the helix. The length of
lipid envelopes allows irregularities in construction.
the TMV virion (300 nm) is determined by the size of the
Selected families of vertebrate viruses are listed in Table
RNA (6.4 kb).
2.1 grouped by the morphologies of the virions. Also shown
Many viruses are constructed with helical symmetry and
for each family is the presence or absence of an envelope
often contain only one protein or a very few proteins. The
in the virion, the triangulation number (defined later) if the
popularity of the helix may be due in part to the fact that the
virus is icosahedral, the morphology of the nucleocapsid
length of the particle is not fixed and RNAs or DNAs of dif-
or core, and figure numbers where the structures of mem-
ferent sizes can be readily accommodated. Thus the genome
bers of a family are illustrated. Electron micrographs of
size is not fixed, unlike that of icosahedral viruses.
five DNA viruses belonging to different families and of
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