Virus particles, called virions, contain the viral genome
encapsidated in a protein coat. The function of the coat is to
protect the genome of the virus in the extracellular environment
as well as to bind to a new host cell and introduce the
genome into it. Viral genomes are small and limited in their
coding capacity, which requires that three- dimensional virions
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
30 or more proteins. To form a three-dimensional structure
using only a few proteins requires that the structure must be
regular, with each protein subunit occupying a position at
least approximately equivalent to that occupied by all other
proteins of its class in the final structure (the principle of
quasi-equivalence), although some viruses are now known
to violate the principle of quasi-equivalence. A regular
three-dimensional structure can be formed from repeating
subunits using either helical symmetry or icosahedral symmetry
principles. In the case of the smallest viruses, the final
structure is simple and quite regular. Larger viruses with
more proteins at their disposal can build more elaborate
structures. Enveloped viruses may be quite regular in construction
or may have irregular features, because the use of
lipid envelopes allows irregularities in construction.
Virus particles, called virions, contain the viral genomeencapsidated in a protein coat. The function of the coat is toprotect the genome of the virus in the extracellular environmentas well as to bind to a new host cell and introduce thegenome into it. Viral genomes are small and limited in theircoding capacity, which requires that three- dimensional virionsbe formed using a limited number of different proteins.For the smallest viruses, only one protein may be used toconstruct the virion, whereas the largest viruses may use30 or more proteins. To form a three-dimensional structureusing only a few proteins requires that the structure must beregular, with each protein subunit occupying a position atleast approximately equivalent to that occupied by all otherproteins of its class in the final structure (the principle ofquasi-equivalence), although some viruses are now knownto violate the principle of quasi-equivalence. A regularthree-dimensional structure can be formed from repeatingsubunits using either helical symmetry or icosahedral symmetryprinciples. In the case of the smallest viruses, the finalstructure is simple and quite regular. Larger viruses withmore proteins at their disposal can build more elaboratestructures. Enveloped viruses may be quite regular in constructionor may have irregular features, because the use oflipid envelopes allows irregularities in construction.
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