Virus

Virus
Characteristics
- smaller than bacteria
- not retained by bacterial filters
- not visible in the light microscope
- obligately intracellular parasites
- virion : intact, fully assembled, infective virus
- viroid : piece of RNA without a protein coat
Host
Range
- infect invertebrates, vertebrates,
plants, protest, fungi or bacteria
- narrow host range
- determined by presence of
specific receptors on the cell
- some are very specific
Size
- smaller than bacteria
- wide range
General
Morphology
(capsid
architecture)
- helical viruses
- polyhedral viruses
- enveloped viruses
- complex viruses
Bacteriophages
- lytic cycle
- host cells lyse and die
- lysogenic cycle
- host cell remains alive
Isolation
and Cultivation
- growth of bacteriophages
- plaque method : mix
bacteriophages with host bacteria
and nutrient agar
- quantifying viruses
- plaque assay : the number of plaque directly
relates to numbers of infectious virus particles
applied to the plate
- growth of animal viruses
- in living animal hosts
- in embryonated eggs
- in cell cultures
- viral infections
- infection by individual human viral pathogens
- patterns of viral infection or disease
- latent
- persistent
Taxonomy
- used to be classified based on
host preferences
- animal viruses
- plant viruses
- bacterial viruses
- International Commitee for
Taxonomy of Virus (ICTV)
- nucleic acid type
- nucleic acid strandedness
- the sense (+ve/-ve) of single stranded RNA
- presence/absence of envelope
- type of host
- names of viruses
- family : ends with viridae
- subfamily : ends with virinae
- genus and sp. : virus
- characteristics to divide viruses
into taxonomic groups
- nature of host
- nucleic acid characteristics
- capsid symmetry
- presence of envelope
- diameter of the virion or nucleocapsid
- number of capsomers in icosahedral viruses
- type of virus release
- with/without tails
- with/without envelope
- presence/absence DNA intermediate
Viral Structure
- nucleic acid
- contain either DNA/RNA but never both
- DNA
- linear – single stranded , double stranded
- circular – single stranded, double stranded
- RNA
- single linear, segmented linear,
segmented double stranded
linear
- capsid
- protein coat surrounding the nucleic acid
- composed of subunits, capsomers
- envelope
- covered the capsid
- spikes : structures that protrude
out of the envelope
- protein envelope
- lipid envelope
- carbohydrate envelope
Family
- DNA Virus
- DNA is released into the
nucleus of the host cell
- transcription and translation
produce viral DNA
- capsid protein which is synthesis
in the cytoplasm
- example : Adenoviruses,
Herpesviruses, Papovaviruses and
Hepadnaviruses
- RNA Virus
- multiplication occurs in the cytoplasm
- dependent RNA polymerases
synthesizes the double stranded
RNA
- after maturation, viruses are released by
budding or through ruptures in the host cell
membrane
- example : Picornaviruses,
Togaviruses, Rhabdoviruses and
Retroviruses
Viruslike Agents
- viroids
- an infectious RNA particle
smaller than a virus
- plant infected
- differ from viruses in six ways
- consists of a single circular RNA
molecule of molecular weight
- exist inside cells as particles
- do not require a helper virus
- does not produce proteins
- copied in the host cell nucleus
- viroid particles are not apparent in infected
tissue without the use of special
techniques
- prions
- resistant to inactivation by
heating to 90 degree celcious
- not sensitive to radiation treatment
- sensitive to protein denaturing agents
- smaller than bacteria
- not retained by bacterial filters
- not visible in the light microscope
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