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Nature
of
Viruses
Viruses exist in two different states, the extracellular infectious particle or virion and the intracellular state consisting of viral nucleic acid.
The virion consists of a protein coat or capsid, which encloses a genome of either RNA or DNA. The entire structure is called the nucleocapsid.
The capsid may be a polyhedron or a helix, or a combination of both (in some phages). Viruses are infective microorganisms that show several differences from typical microbial cells.
1. Size. The size range of viruses is from about 20 to 300 nm. On the whole, viruses are much smaller than bacteria. Most animal viruses and all plant viruses and phages are invisible under the light microscope.
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2. Simple structure. Viruses have very simple structures. The simplest viruses are nucleoprotein particles consisting of genetic material (DNA or RNA) surrounded by a protein capsid. In this respect they differ from typical cells which arc made up) of proteins, carbohydrates, lipids and nuc1eicacids.
The more complex viruses contain lipids and carbohydrates in addition to proteins and nucleic acids, e. g. the enveloped viruses.
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These viruses are surrounded by a membranous envelope which is derived from the host cell. It protects the virus and also serves for transmission from one host to another. The envelope consists of a lipid bilayer and proteins with special functions.
The membrane proteins are of two types, glycoproteins and matrix: proteins. Glycoproteins have a hydrophobic end fixed in the lipid bilayer and a hydrophilic glycosylated end which protrudes into the medium.
The spikes on the outer surface of the virions consist of glycoproteins. In the orthomyxoviruses, paramyxoviruses and rhabdoviruses, there is an unglycosylated matrix protein layer on the inner surface of the envelope. This layer appears to connect the envelope with the capsid.
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The envelope and capsid proteins are specified by viral genes. The lipid and carbohydrate of the glycoprotein are derived from the host cell. Since some viruses can be grown in different cell types, they often have different lipid and carbohydrate moieties.
The surface of the virion may therefore contain polysaccharide-determined cellular antigens. The Pseudomonas phage 96 has a lipid envelope, of which the lipid bilayer appears to be assembled de novo, and is not budded from the membranes.
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3. Absence of cellular structure. Viruses do not have any cytoplasm, and thus cytoplasmic organelles like mitochondria, Golgi complexes, lysosomes, ribosomes, etc., are absent.
They do not have any limiting cell membrane. They utilize the ribosomes of the host cell for protein synthesis during reproduction.
4. No independent metabolism.
Viruses cannot multiply outside a living cell. No virus has been cultivated in a cell-free medium. Viruses do not have an independent metabolism. They are metabolically inactive outside the host cell because they do not posses enzyme systems and protein synthesis machinery.
Viral nucleic acid replicates by utilizing the protein synthesis machinery of the host. It codes for the synthesis of a limited number of viral proteins, including the subunits or capsomeres of the capsid, the tail protein and some enzymes concerned with the synthesis or the release of virions.
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5. Nucleic acids. Viruses have only one nucleic acid, either DNA or RNA. Typical cells have both DNA and RNA. Genomes of certain RNA viruses can be transcribed into complementary DNA strands in the infected host cells, e. g. Rous Sarcoma Virus (RSV). Such RNA viruses are therefore also called RNA-DNA viruses.
6. Crystallization. Many of the smaller viruses can be crystallized, and thus behave like chemicals.
7. No growth and division. Viruses do not have the power of growth and division. A fully formed virus does not increase in, size by addition of new molecules. The virus itself cannot divide.
Only
its
genetic
material
(RNA
or
DNA)
is
capable
of
reproduction
and
that
too
only
in
a
host
cell.
It
will
thus
be
seen
that
viruses
do
not
show
all
the
characteristics
of
typical
living
organisms.
They,
however,
possess
two
fundamental
characteristics
of
living
systems.
Firstly,
they
contain
nucleic
acid
as
their
genetic
material.
The
nucleic
acid
contains
instructions
for
the
structure
and
function
of
the
virus.
Secondly,
they
can
reproduce
themselves,
even
if
only
by
using
the
host
cells
synthesis
machinery.
The
debate
as
to
whether
viruses
are
living
or
non-living
is
actually
superfluous.
A
decision
on
this
matter
would
ultimately
depend
upon
the
criteria
adopted
to
distinguish
between
living
and
non-living.
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