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Virology
Definition
Viruses are particles consisting of an RNA or DNA genome ,covered by protein
coat called capsid . some viruses have outer lipoprotein membrane ,called
envelope external to coat .They lack cellular structure(nucleus ,cytoplasm,
mitochondria or ribosome) and independent metabolic processes. They replicate
within living cells because they cannot generate energy or synthesis proteins ,so
viruses are obligate intracellular parasites.
Essential Characteristics of Viruses
Size : Viral size very small ranges from 20 to 300 nm. 20nm (polio virus) to 300
nm ( small pox virus)
Genome : DNA or RNA. Double-stranded or single-stranded nucleic acid,
depending on the species.
Structure : protein and nucleic acid; some viral species have envelope
Reproduction: Only in living cells.
Morphology and Structure
A mature virus particle is also known as a virion. It consists of either two or three
basic components:
— A genome of virus is located internally and can be either DNA or RNA,
double-stranded or single-stranded, linear or circular, and in some cases
segmented.
—Virus-coded proteins
ONE protein ,The capsid ---enclosing the nucleic acid of the virus and
determining its antigenicity; the capsid can have a cubic, helical or complex
symmetry and is made up of subunits called capsomers . Each capsomer
,consisting of one or several proteins .Arrangement of capsomers gives virus
structure of symmetry .there are three forms of symmetry in virus capsides
1- Icosahedral symmetry,in which capsomers are arranged in 20 triangles
that forma symmetric (Icosahedron) with outline sphere. ex : polio virus.
2- Helical symmetry, in which capsomers are arranged in a hollow coil that
appears rod. e x: influenza virus
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3- Complex
symmetry, in which
capsomers are arranged in more
complicated structure . e x : small pox
# SECOND protein are enzymes
— In some cases an envelope that surrounds the capsid and is always derived
from cellular membranes. Some viruses may have spikes to help attach to the
host cell and depend on spikes ,Most viruses infect only SPECIFIC host cells.
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The capsid or envelope of viruses functions
(1) to protect the nucleic acid genome from damage during the extracellular
passage of the virus from one cell to another.
(2) attach & entry into the cell.
(3) in some cases to package enzymes essential for the early steps of the
infection process.
Types of Viruses
 Animal viruses: Rabies, Polio, Mumps, Small pox, and Influenza.
 Plant Viruses: Tobacco mosaic virus (TMV), Banana virus, Carrot virus.
 Bacterial Virus: Bacteriophages ( T1, T2, T3, and T4.)

Atypical Virus like agent
Two classes of infectious agents exist that are structurally simpler than viruses.
Viroids are infectious circular RNA molecules that lack protein shells; they are
responsible for a variety of plant diseases.
Prions, are infectious agents composed only of protein, that appear to be
responsible for some transmissible and inherited spongiform
encephalopathies .
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comparison of viruses and cells
Classification
The taxonomic system used for viruses is based on the following morphological
properties:
— Genome: DNA or RNA genome as well as configuration of nucleic acid
structure: single-stranded (ss) or double-stranded(ds).
— Capsid symmetry: cubic, helical, or complex symmetry.
— Presence or absence of an envelope.
— Diameter of the virion.
Viral Taxonomy
Family names end in -viridae
Genus names end in -virus
Viral species: A group of viruses sharing the same genetic information
and host. Common names are used for species
Ex:---Herpesviridae -------Herpesvirus -------Human herpes virus 1, 2,3
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DNA VIRUSES
NON ENVELOPE
DOUBLE
STRAND
GENOME
ENVELOPE
SINGLE
STRAND
GENOME
Adeno virus
DOUBLE
STRAND
GENOME
Parvo virus
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Pox, Herpes
viruses
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RNA VIRUSES
ENVELOPE
NON ENVELOPE
DOUBLE
STRAND
GENOME
SINGLE
STRAND
GENOME
Reo viruses
SINGLE
STRAND
GENOME
Picorna, calici
viruses
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Orthomyxo ,
paramyxo ,corona,
,rhabdo viruses
HIV
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