2. What Are
Viruses?
Viruses are microscopic parasites,
generally much smaller than bacteria.
They lack the capacity to thrive and
reproduce outside of a host body.
4. StructureofViruses
Viruses are inert outside the host cell.
Small viruses, e.g., polio and tobacco
mosaic virus, can even be crystallized.
Viruses are unable to generate energy. As
obligate intracellular parasites,during
replication, they fully depend on the
complicated biochemical machinery of
eukaryotic or prokaryotic cells. The main
purpose of a virus is to deliver its genome
into the host cell to allow its expression
(transcription and translation) by the host
cell.
This Photo by Unknown author is licensed under CC BY-SA.
5. ClassificationofViruses
Viruses are classified on the basis of morphology, chemical composition, and mode
of replication. The viruses that infect humans are currently grouped into 21 families,
reflecting only a small part of the spectrum of the multitude of different viruses
whose host ranges extend from vertebrates to protozoa and from plants and fungi to
bacteria.
6. General
Properties of
Viruses
1.Viruses contain either DNA or RNA as their
genetic material,but not both.This nucleic acid
usually has unique chemical and/or physical
features which makes it distinguishable from
human nucleic acid.
2.Viral nucleic acid is enclosed in a capsid
made up of protein subunits called protomeres.
5. All viruses undergo a replication cycle in their
human host cell consisting of adsorption,
penetration, uncoating,nucleic acid replication,
maturation and release stages.
7. Example ofaVirus
Poliovirus,the causativeagent of polio,is a serotype of the species
Enterovirus C,in the family of Picornaviridae.Poliovirus is
composed of an RNA genome and a protein capsid.The genome is a
single-strandedpositive-sense RNAgenome that is about 7500
nucleotideslong.The viral particleis about 30 nm in diameter with
icosahedral symmetry.Becauseof its short genome and its simple
composition—only RNAand a nonenveloped icosahedralprotein
coat that encapsulates it,poliovirusis widelyregarded as the
simplest significant virus
