Virology-Microbiology Exam I

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Structure units

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smallest functional equivalent building units of the capsid (also known as protomer)

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Capsomers

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morphological units seen on surface of particles and represent clusters of structure units (union de varios protomers)

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Capsid

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denotes the protein shell that encloses the nucleic acid. It is build of structural units (union de varios capsomeros)

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Nucleocapsid

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capsid together with enclosed nucleic acids

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Envelope

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nucleocapsid may be invested in an envelope which may contain material of host cell as well as viral origin (se les conoce como virus encapsulado)

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Virion

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complete infective virus particle

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Back 7

Enveloped virion= nucleocapsid (a) + glycoprotein membrane (b)

Example: HIV, Herpesvirus, Influenza, Hepatitis B

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Helical symmetry

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• rod-like filamentous structure
• helix of identical protein subunits (protect the nucelic acids)
• allows formation of open structure (any volume of nucleic acids may be stored by varying the lenght)
• organized around a helical axis
• ex. influenza, rabies, tobacco mosaic virus

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Icosahedral Symmetry

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• icosahedron: figure of 20 triangular facets, each an equilateral triangle and 12 vertices and 30 edge.
• allows formation of closed shell (with fixed internal volume)
• axes of rotational symmetry: 5:3:2 symmtery
• requires a multiple of 60 subunits to cover the surface completely
• there are six 5-fold axes of symmetry passing through the vertices, ten 3-fold axes extending through each face and fifteen 2-fold axes passing though the edges of an icosahedron.
• examples: animal DNA viruses EXCEPT poxvirus

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Vertex

Axis of symmetry?

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Where the vertices of 5 triangles meet (12)

axis of symmetry: 5-fold

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Triangular face

Axis of symmetry?

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Each an equilateral triangle (20)

Axis of symmetry: 3-fold

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Edge

Axis of symmetry?

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Where the sides of two triangles meet (30)

Axis of symmetry: 2-fold

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Complex Structure

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• do not exhibit simple cubic (icosahedral) or helical symmetry
• Poxviruses: brick-shaped w/t ridges on external surface and a core and lateral bodies
• T4 virus: es icosahedral y helical asi que se cosidera un virus complejo

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Naked icosahedral

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polio virus, adenovirus, hepatitis A virus

"Naked Icons, Police Are Hectic"

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Naked helical

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tobacco mosaic virus (TMV), no human viruses found

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Enveloped icosahedral

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herpes virus, yellow fever virus, rubella virus

"En-vi her yellow rub"

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Enveloped Helical

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rabies virus, influenza virus, parainfluenza virus, measles virus, mumps virus

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Complex Virus

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poxvirus, T4

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DNA viruses

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1. dsDNA: Asfarviridae, Poxviridae, Iridoviridae (Ranavirus, Lymphocystovirus, Megalocytivirus),Herpesviridae, Polyoaviridae, Papillomaviridae,cAdenoviridae
2. dsDNA (RT): Hepadnaviridae
3. ssDNA: Circoviridae, Anellovirus, Parvoviridae (Parvovirinae)

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RNA viruses

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1. dsRNA: Reoviridae (Orthoreovirus, Orbivirus, Collivirus, Rotavirus, Aquareovirus) and Birnaviridae (Aquabirnavirus and Avibimavirus)

2. ssRNA (-): Orthomyxoviridae, Rhabdoviridae (Lyssavirus, Vesiculovirus, Ephemerovirus, Novirhabdovirus), Deltavirus, Paramyxoviridae, Bornaviridae, Arenaviridae,Filoviridae, and Bunyaviridae (Orthobunyavirus, Hantavirus, Nairovirus, Phlebovirus).

3. ssRNA (+): Caliciviridae, Hepevirus, Picornaviridae, Astroviridae, Flaviviridae, Nodaviridae (Betanodavirus), Coronaviridae, Togaviridae, Arteriviridae.

4. ssRNA (talk RT): Retroviridae

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What virus is this?

Description.

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Adenovirus

Naked, icosahedral, ds linear DNA

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Identify the virus and describe it.

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Papillomavirus

Naked icosahedral, ds circular DNA

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What virus is this?

Description

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Herpesviruses

Enveloped, icosahedral, ds linear DNA

Front 24

identify the virus and describe it

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Influenza virus

Enveloped, helical, negative stranded, segmented RNA

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Identify and describe the virus

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Enteroviruses

Naked, icosahedral, positive stranded RNA

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Identify and describe this virus

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Rotavirus

Naked, icosahedral, double layered capsid, double stranded segmented RNAthe

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This is the general organization of:

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Hepatitis B virion (Hepadnaviridae):

• DNA virus and is enveloped (derived from ER)
• DNA is only partly double stranded and forms circle. When it infects it first needs to convert all of its DNA to double stranded.
• Genome associated with the P (polymerase) protein and is surrounded by core antigens (HBcAg and HBeAg)

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This is the organization of:

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Adenovirus (Adenoviridae)

• non-enveloped icosehedral particles
• contains linear, doble stranded DNA genome with terminal protein (TP) attached covalently to the 5' termini
• DNA wrapped around histone-like protein and has inverted terminal repeats (ITRs) which acts as origin of replication.
• Capsin: build up of 252 capsomers (T=25) of which 240 are hexavalent, 12 (at apices) are pentavalent.
• a "penton fiber" projects from apex--> involved in process of attachment of virus particle to host cell.

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This is the organization of:

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Retroviridae (HIV-1 virion)

• genome is one long strand of +RNA
• viral membrane derived from cytoplasmic membrane encloses the particle and has gp160 spikes embedded (involved in binding and membrane fusion when virus attaches to the cell)
• inner core contains viral RNA and some enzymes (reverse transcriptase, protease, integrase)
• Components: surface glycoprotein (gp120), transmembrane glycoprotein (gp41), gag membrane associated (matrix) protein MA (p17), gag Capsid CA (Core shell) p24, RNA (2 molecules-exactly the same), pol (Protease PR p9, Polymerase RT, RNAse H RNH p66, integrase IN p32)

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This is the organization of:

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Hepatitis A virus (Picornaviridae)

• single strand, 3'-polyadenylated, +RNA genome surrounded by naked (non-enveloped) icosahedral capsid that is approx. 28nm in diameter
• at 5' end of RNA strand is a viral protein called VPg

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This is the organization of:

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Influenza virus (Orthomyxoviridae)

• virions roughlt spherical (80-120nm in diameter)
• envelope (made of cytoplasmic membrane to which the virus inserts hamagglutin and neuramidase proteins) contains rigid "spikes" of hemmaglutin and neuramidase which form characteristic halo projections
• viral genome: eight segments of negative sense ssRNA
• contains helical ribonucleoprotein

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This is the organization of:

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Unconventional/Atypical virus-like agents (Viroids) ej. potato spindle tuber viroid

• subviral particle with RNA only
• small (less than 400nucleotides), single stranded, circular RNAs with some double stranded regions.
• RNA not packaged, does not code for proteins, and have only been shown to be associated with plants disease.
• gets name b/c of oblong tubers produced from infected plants

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Prions

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• PROteinaceous INfectiouS Particles: contain protein only
• examples prion-caused human diseases: Kuru, Creutzfeldt-Jakob, Gerstmann-Straussler
• Prions also cause scrapie in sheep and mad cow disease (bovine spongiform encephalitis) in cows

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Prion Hypothesis

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suggests that abnormal conformer (PrPsc) of the cellular prion protein (PrPc) can induce PrPc to undergo change of conformation into PrPsc (acumulates inside cell and affects cell function).

• abnormally folded PrPsc forms of prion protein may arise: spontaneously, hereditary, or from an exogenous infective source.
• once a misfolded PrPsc has arisen other PrPsc molecules are converted, propagating the disease in a form of chain rxn.

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Prion disease in humans:

Kuru

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1. source: canibalism
2. age of onset: 4-20 yrs after ingestion
3. mean time of death: 6 months after symptom onset

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Prion disease in humans:

Creutzfelt-Jakob

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1. Source: sporadic, familial or corneal transplant
2. Age of onset: 6th-7th decade of life
3. mean time of death: 4-7 months

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Prion disease in humans:

Variant C-J

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Source: ingestion of infected bovine neural tissue or bone marrow

age of onset: variable after ingestion

mean time of death: 14 months

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Prion disease in humans:

Gerstmann- Straussler-Scheinker syndrome

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Source: Familial or sporadic

Age of onset: 4th-5th decade of life

mean time of death: 5yrs

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Prion disease in humans:

Fatal familial insomnia

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source: familial

Age of onset: 35-61yrs

Mean time of death: 13-25 months

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Defective viruses

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• has viral nucleic acid and proteins but can't replicate without a "helper virus" (to infect it need co-infection with another virus), which provides a missing function
• some use unrelated viruses as helper
• ex. hepatitis delta virus (RNA virus) uses the envelope of hepatitis B virus (DNA virus) (need co-infection with HepB in order to infect)

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Pseudovirus

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• contain host DNA (instead of viral) within capsid
• formed during infection with certain viruses when host DNA is fragmented and pieces of it are incorporated within the capsid
• can infect cells but cant replicate
• used a mechanism to distract the host immune system- it looses resources attacking these "viruses" and not the real ones.

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DNA Viral Genomes

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• Range: 3.2Kbp (hepadnavirus) to 375kbp (poxviruses)
• single molecules of linear or circular configuration

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RNA viral genomes

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• Range: 7kb (some picornaviruses) to 30kb (coronavirus)
• single linear molecules (picornaviruses) to several segments (orthomixoviruses)
• Positive sense: INFECTIOUS (picornaviruses, togaviruses)
• Negative sense: not infectious (rhabdoviruses, orthomixoviruses), virions carry RNA polymerase
• double stranded RNA: not infectious (Rotavirus)

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Lipid Envelopes

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Acquisition of envelope occurs where virus-specific proteins have been inserted into host cell membranes:

• nuclear: herpes viruses
• ER: hepatitis B viruses
• Cytoplasmic membrane: influenza virus

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Routes of Viral entry into body

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1. Oral transmission: contaminated food and drink, saliva
2. direct skin contact
3. trans-placental
4. droplet transmission: inhalation
5. direct inoculation: injections, trauma, insect bites
6. sexual transmission
7. transplants
8. blood transfusions/blood products
9. surgical treatment

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Determinants of severity of viral disease

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1. cytopathic ability of the virus
2. immune status:

• competence of immune system
• prior immunity to virus

3. Immunopathology: hypersensitivity and inflammatory rxns

4. virus inoculum size

5. general health status of person: nutrition and other diseases influencing immune status

6. genetic make-up

7. age: ability for body to repair damage