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Definition and properties of viruses

•  Viruses cannot make energy or proteins independently of
a host cell, which acts as a “factory”
•  Viruses are obligate intracellular parasites
•  Viral components are assembled and do not replicate by

•  Viruses are filterable agents
•  Genomes may be either DNA or RNA but not both
•  Viruses have a naked capsid or an envelope morphology


Consequences of Viral Properties

•  Viruses are not living organisms
•  Viruses must be infectious to endure in nature
•  Viruses must be able to use cell processes
•  Viruses must encode any required proteins not
provided by the host cell
•  Viral components must self-assemble


How are viruses classified?

* Structure: size, morphology, nucleic acid
•  Biochemical characteristic: structure, mode of replication
•  Disease:
•  Means of transmission:
•  Host cell (host range): animal, plant, bacteria
•  Tissue or organ (tropism)
•  location of first isolation
* members of a particular family


•  Structure: size, morphology, nucleic acid

picornavirus (= small RNA)


•  Biochemical characteristic: structure, mode of replication

togavirus (= mantle, envelope), retrovirus



e.g. encephalitis virus, hepatitis virus, poxvirus


Means of transmission:

e.g. arbovirus (spread by insects)


Host cell (host range): animal, plant, bacteria

(bacterio) phage


Tissue or organ (tropism):

adenovirus, enterovirus


members of a particular family



•  location of first isolation

Marburg virus


What is the basic virus structure?

•  Nucleic acid genome (DNA or RNA)
•  Capsid (protective protein coat
•  Capsids are made of capsomers
•  Envelope is an outer membraneous
layer made of lipids and proteins
•  Not all viruses have envelopes


What is the morphology of a helical caspid?

1. nucleic acid genome protector and attaches there
2. capsomer (proteinaceous subunit)
3. Capsid (protection, host cell attachment)


What are the spikes on the viral structures?

-  protein structures used to host cell binding
-  very specific (narrow host cell spectrum)


What is the morphology of a polyhedral caspid?

•  self-assembly
•  no division
icosahedron - 12 pentamers


What are the environmental properties of the naked capsid?

Environmentally stable to the following:
- temperature
- acid
- proteases
- detergents
- drying


What are the consequence of the naked capsid due to environmental properties?

- can be spread easily (dust, hand-to-hand)
- can dry out and still contain infectivity
- can survive the adverse conditions of the gut
- can be resistant to poor sewage treatment


What are the environmental properties of the envelope?

Environmentally labile and disrupted by:
- acid
- detergent
- drying
- heat


What are the consequences of the environmental properties of the envelope?

- must stay wet
- cannot survive in gastrointestinal tract
- spreads in large droplets, body fluids (blood, saliva,
breast milk, transplants, blood transfusion


How does viral replication occur?

1) interaction of spikes with cell receptors
2) endocytosis
3)get out of the vacuole and uncoats the envelope+capsid to release genome into the cytosol
4) uses cell machinary for replication + proteins
5) Some viruses assemble in the cell and then llyse and release
6) Others assemble on the cell membrane (spike then genome, capsid then the structures bud off creating the structure)


Life cycle of lytic virus

Initially theres a drop in viral numbers as it disassemble in the cell. The numbers stays continues during synthesis and assembly. Then numbers increase during release, which is called the burst size.


What are bacteriophages?

= viruses that infect bacterial cells
have complex structures
Have tail fibres that are specific to specific proteins on species. The virus doesn't get into the cell, rather it injects the DNA into the cell. The end plate attaches to the cell, the tail contracts and DNA goes into the cell.


How do phage replicate?

Binding to a receptor on bacterial cell. Injection of DNA into cell. Phage genes are expressed genome replicated. Phage parts are assembled. Phage escapes by cell lysis.


How does a viral disease progress?

1.  Acquisition (entry into the body)
2.  Initiation of infection (at primary site) - has to recognize specific cell for infection
3.  Incubation period (virus amplifies and spreads)
4.  Replication in target tissue (disease symptoms)
5.  Immune responses (limit and contribute to disease) - disease symptoms are mainly due to the immune system.
6.  Virus production in tissue, release, contagion. When virus is release from the tissue is the person is contagious.
7.  Resolution or persistent infection/chronic disease


How are virus transmitted?

•  aerosols - coughing e.g. influenza virus, picornaviruses
•  food, water, e.g. enteric viruses such as reovirus, hepatitis A
•  fomites - non enveloped viruses (tissues, clothes), e.g. rhinovirus
•  direct contact with secretion (saliva, semen), e.g. cyto-megalovirus, Epstein Barr virus/Kissing virus
•  sexual contact, e.g. herpes simplex virus, papilloma virus, HIV
•  maternal-neonatal, e.g. rubella virus, herpes simplex virus, HIV
•  blood transfusion, organ transplant, e.g. hepatitis B virus, HIV
•  zoonoses (animals, insects), e.g. influenza virus, rabies virus


What are latent viruses?

•  persistent infection
•  virus does not reproduce (no clinical symptoms)
•  virus can occasionally activate and produce symptoms - happens by some stress
•  latent infections are limited by immune response

example chicken pox that is latent even after infection.


How does the latent herpes virus work?

- latent in neurons (nucleic acid integrated in genome)
- activated by fever, stress, sun light
- causing cold sores


How does the latent chicken pox virus work?

- resurfaces when immune system
weakens by disease or old age
- causes shingles


How does the latent HIV virus work?

- latent in T cells, macrophages (integrated in genome)
- causes AIDS


What are oncogenic viruses?

•  viruses that can cause cancer
•  integrate into host genome close to proto-oncogen
•  activation of virus also activated proto-oncogene.