1.
Size: viruses are very small retaining infectivity after passing through filter
with pore size
small
enough to hold back the smallest bacteria. Bacteria are measured in terms of
micrometer
(μm, 10-6 of a metre) whereas viruses are measured in nanometer (nm,
10-9
of
a metre). Viruses range in size from 20nm to 300nm. The picornaviruses (e.g.
Footand-
Mouth-Disease
virus) are the smallest viruses (20nm) while the poxviruses are the
largest
viruses (300nm). Viruses can not be seen by light microscope because of their
small
size. They are seen only by the aid of electron microscope. However, poxviruses
can
be seen by light microscope.
2.
Organelles: Viruses do not possess cellular organization and do not have
organelles.
3.
Genome size: The genomes of viruses are smaller than those of bacteria, ranging
from
about
2 kilobase pairs (kbp) to 200 kbp.
4.
Viruses are completely dependent on living cells, either eukaryotes or
prokaryotes for
replication
and existence. Although some viruses possess their own enzymes such as
RNA-dependent
RNA polymerase or reverse transcriptase, they cannot reproduce and
amplify
the information in their genomes without the assistance of the cellular
machinery.
They do not grow in inanimate/non-living media.
5.
Viruses have their genetic information in either DNA or RNA. A virus possesses
only one
species
of nucleic acid either DNA or RNA but never both.
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6.
Viruses have a receptor-binding protein component for attaching to cells so
that they
can
enter such cell and take over the machinery of the cell to their own advantage
in
reproducing
their progeny.
7.
Viruses do not multiply by binary fission but by a complex process involving
protein
synthesis
and nucleic acid production
8.
Viruses are unaffected by antibiotics
9.
Viruses induce the production of interferon by infected host cell and are
sensitive to the
interferons.
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