Week 2: Virology Flashcards
What is a virus?
Intracellular obligate parasite
- unable to replicate outside of a host cell
- Unable to make own proteins
- Depends on the cell machinery for replication and protein production
- Are NOT living
- must be able to infect a host to survive
- Viral components are assembled - do not replicate by division
How do viruses enter cells?
Via host cell receptors
Viral genes expressed how?
using viral and/or host-encoded functions
What do viral proteins do?
- modify host cell functions to ensure viral replication
- Assemble into caspids that protect viral genomes
How do viruses propagate?
Viral proteins modify host cell functions to ensure viral replication and assemble into capsids that protect viral genomes and then the virus is released to infect new cells.
Range of viral infections?
Demonstrates viral tropism (viruses are found infected certain parts of the body because those tissues have receptors to allow viral entry)

Describe viral entry into the skin
require a breach of the physical integrity of the skin barrier and employs vectors
Describe viral entry into the conjunctiva and other mucous membranes
rather exposed site and relatively unprotected
Describe viral entry into the respiratory tract
possess sophisticated immune defense mechanisms which viruses must overcome
Describe viral entry into the Gastrointestinal Tract
gastric acid, bile salts, etc. provide a difficult environment so viruses would need to be adapted to infect and spread in this hostile environment
Describe viral entry into the genitourinary tract
relatively less hostile than the GI tract but is less frequently exposed to extraneous viruses
Factors affecting viral tissue tropism
- Correct attachment protein to bind to a receptor on the cell surface to gain entry
- Appropriate cellular machinery present within the host cell to allow the virus to replicate
- Are dividing cells with high enough levels of dNTPs for viral DNA synthesis
- A way to exit the host cell to spread the infection
Describe the steps of a typical viral infection
- Primary replication
- Systemic spread
- Secondary replication
Describe the primary replication step of a viral infection
- the place of primary replication where the virus replicated after gaining initial entry into the host cell
- This frequently determines whether the infection will be localized at the site of entry or spread to become a systemic infection
Describe the systemic spread step of a viral infection
Apart from direct cell-to-cell contact, the virus may spread via the bloodstream and the CNS
Describe the Secondary Replication step of a viral infection
Secondary replication takes place at susceptible tissues/organs following systemic spread
Describe acute viral infection outcomes
- Recovery with no residue effects (respiratory viruses)
- Recovery wit residue effects (eg. acute viral encephalitis leading to neurological sequelae)
- Death (Ebola virus)
- Proceed to chronic infection
Describe chronic viral infection outcomes
- Silent subclinical infection for life e.g. CMV, EBV
- A long-silent period before disease e.g. HIV, SSPE, PML
- Reactivation to cause acute disease e.g. herpes and shingles
- Chronic disease with relapses and excerbations e.g. HBV, HCV
- Cancers e.g. EBV, HTLV-1, HPV, HBV, HCV, HHV-8
Describe the possible viral pathogenesis time lines

Describe the virus-host interaction

Describe the ways the cells can respond to viral infections
cells can respond to viral infections in 3 ways
- Failed infection (abortive infection)
- non-permissive cell
- Cell death (lytic infection)
- prevent cellular growth
- disrupt the structure and function of cells
- Replication without cell death (persistent infection)
- chronic - nonlytic viral synthesis
- Latent - limited viral synthesis
- Recurrent - periodic productivity
- Transforming - immortalization of cell
What are prions?
Infectious proteins that causes a very slow infection happening a very long-time before the disease episode
Describe cellular pathogenesis of a persistent viral infection

How are cells damaged by a viral infection?
- Direct cell damage and possibly death from a viral infection
- Indirect cell damage as a result of a viral infection

Describe direct cell damage and possibly death from a viral infection
- Diversion of a cell’s energy
- Shutoff of a cells macromolecular synthesis
- Competition of viral promoters and transcriptional enhancers for cellular transcriptional factors such as RNA polymerases and inhibition of the interferon defense mechanisms
Describe indirect cell damage as a result of a viral infection
- integration of the viral genome
- induction of mutations in the host genome
- inflammation
- host-immune response
Describe the viral life cycle

How are viruses classified?

Describe the difference between RNA and DNA viruses

What is viral symmetry?
Symmetry refers to the way in which capsomere units are arranged in viral capsid. Two kinds of symmetry are recognized in the viruses which corresponds to two primary shape ie. Rod and spherical shape of virus. Rod shaped virus have helical symmetry and spherical shaped virus have icosahedral symmetry
What types of viral symmetry are there?
5 types
- Naked icosahedral
- Naked helical
- Enveloped icosahedral
- Enveloped helical
- Complex
Examples of Naked Icosahedral viruses
- Poliovirus
- Adenovirus
- Hepatitis A virus
Examples of Naked helical viruses
No known human viruses
Examples of Enveloped Icosahedral viruses
- herpes virus
- yellow fever virus
- Rubella virus
Examples of Enveloped Helical viruses
- Rabies virus
- Influenza virus
- Parainfluenza virus
- Mumps virus
- Measles virus
Examples of complex viruses
e.g. Poxvirus
Naked icosahedral viruses
Poliovirus
Adenovirus
Hepatitis A virus
Naked helical viruses
No known human viruses
Enveloped icosahedral viruses
Herpes virus
Rubella virus
Yellow fever viruses
Enveloped helical viruses
Rabies virus
Influenza virus
Parainfluenza virus
Mumps virus
Measles virus
Complex viruses
poxvirus
Capsid virus assembly
Individual proteins -> larger units
Capsid viruses environmental stability
Environmentally stable
Enveloped viruses environmental stability
Environmentally liable
DS DNA viruses
Polyoma
Papilloma
Adeno
Herpes
Pox
SS DNA viruses
Parvo
+RNA viruses
Picorna
Noro
Toga
Flavi
Corona
-RNA viruses
Rhabdo
Paramyxo
Orthomyxo
Bunya
Filo
DSRNA viruses
Reovirus
Retroviruses
Retrovirus
DNA virus replication
Viral DNA resembles host DNA
Replication requires a DNA-dependent DNA polymerase
DNA virus early genes
Encode DNA-binding proteins
DNA virus Late genes
Encode structural proteins for assembly
DNA virus transcription location
Transcription occurs in the nucleus
*Except for Poxviruses* which encode their own polymerase and transcription occurs in the cytoplasm
DS DNA viruses
Herpesviruses
Adenoviruses
Papovaviruses
Poxviruses (Complex DNA virus)
SS DNA viruses
Parvovirus
Describe replication of simple DNA virus
DNA virus uses host cell machinery to replicate
DNA replication occurs in the nucleus

Describe replication of Complex DNA virus: Poxvirus
Replication occurs in the cytoplasm
Provide their own mRNA and DNA synthetic machinery

SS +RNA enveloped icosahedral viruses
Flaviviridae
Togaviridae
Retroviridae
SS +RNA enveloped helical viruses
Coronaviridae
SS +RNA nonenveloped icosahedral viruses
Picornaviridae
Calciviridae
SS -RNA enveloped helical viruses
Orthomyxoviridae
Paramyxoviridae
Rhabdoviridae
Filoviridae
Bunyaviridae
Arenaviridae
DS -RNA nonenveloped icosahedral viruses
Reoviridae
RNA viruses characteristics
- Must encode an RNA-dependent RNA-polymerase
- Host cells have no mechanism of replicating RNA
- RNA polymerases are error-prone
- RNA is very liable and can be degraded by the cell
- All RNA viruses must carry their own polymerase
- *Except (+) RNA viruses
Describe replication of simple RNA viruses

DS RNA viruses mechanism of replication
Reoviruses
Birnaviruses
The virion RNA is DS and so cannot function as mRNA
Viruses package an RNA polymerase to make their mRNA after infection of the host cell
SS + RNA viruses
Picornaviruses
Togaviruses
Coronaviruses
Flaviviruses
SS +RNA viruses transcription
In these viruses the genomic RNA functions as mRNA
This mRNA can be translated immediately upon infection of the host cell using host cell machinery-> it is infectious
SS -RNA viruses
Orthomyxoviruses
Rhabdoviruses
Paramyxovirus
Filovirus
Bunyavirus
SS -RNA viruses transcription
The virion RNA is negative sense -> must be copied into the plus-sense mRNA before proteins can be made
Packaged in the virion so that they can make mRNAs upon infecting the cell
Transcription and replication occurs in the cytoplasm *Except for influenza virus*
SS -RNA viruses transcription location
Transcription and replication occurs in the cytoplasm *Except for influenza virus*
Retroviruses genome
Two identical (+)sense ssRNAs
Neither directly transcribed nor translated
Viral RNA-dependent DNA polymerase (reverse transcriptase) co-packaged with the viral genome
Converts ssRNA into ds DNA
Proviral cDNA integrated into the host genome
Proviral genes transcribed by cellular RNA polymerase II
Human pathogens: HIV-1, HIV-2, human T-cell leukemia virus (HTLV)-1 and HTLV-2
Describe Influenza virus genetics in action
- Surrounding the helical nucleocapsids is a lipid envelope with two viral glycoproteins: Hemagglutinin (H or HA) and neuraminidase (N or NA)
- Genome consists of 8 individual segments of SS negative polarity RNA
- Segments assemble at random into virions, some of which are infectious
- Viral RNA polymerase is error-prone due to lack of proofreading activity so there is a high-mutation rate
How are influenza virus strains identified?
Strains are identified by the specific H and N antigen combination (eg H3N1)
Describe influenza virus RNA polymerase
polymerase is error-prone due to lack of proofreading activities, like most RNA viruses thus there is a high mutation rate
Epidemiology of influenza viruses
- May be endemic epidemic or pandemic
- Epidemics result from antigenic changes in the SAME virus
- Pandemics result from the emergence of a new influenza virus *Only occurs in influenza A*
Describe Antigenic drift

Effects of antigenic drift
Major changes in the viral antigens circulating the population
What are zoonotic viruses
- viruses that also infect animals
- such as influenza A where birds and swine act as reservoirs
What is the key factor regarding antigenic shifts in influenza A
Zoonotic aspect of Influenza A
Factors regarding antigenic shifts in influenza A
- Zoonotic
- Individual cells may multiply infected with viruses of different strains in birds, swine or humans
- Segmented nature of the influenza genome
- The end result is that random recombination of RNA segments from different strains of virus (including from different species)
Bacteria cell type
Prokaryotic
Virus cell type?
non-classified
Bacteria size of cell
typically 0.2-2.0 μm
Viruses size of cell
18-300 nm
Bacteria nucleic acid
Single circular chromosome (DNA)
Bacteria metabolism
- Heterophilic
- Aerobic
- Anaerobic
Bacteria reproduction
Most capable of independent reproduction
Bacteria method of cultivation
Visible growth on agar surface for most
Virus nucleic acid
- RNA or DNA
- Single or double-stranded
Virus metabolism
Dependent on host machinery
Virus reproduction
Dependent on host machinery
Virus method of cultivation
- No visible growth on agar
- requires host cells for propagation