3.1 viruses and detection Flashcards
what is a virus? What are its 4 fundamental characteristics?
An infectious, obligate intracellular parasite comprising genetic material (DNA or RNA) surrounded by a protein coat and/or an envelope derived from a host cell membrane
– Infectious – spreads from host to host
– Obligate intracellular– restricted to replicating within a cell
– Genetic material - Nucleic acid – DNA or RNA genome
– Protein coat – Viral protein coat protects the nucleic acid genome
5 properties common to all viruses
- Viruses replicate inside host cells (make many copies), they do not grow and divide like bacteria, archaea, or eukaryotic cells
- All viruses must make mRNA to be translated by host ribosomes
- Viruses are absolutely dependent on the host cell’s ribosomes to synthesize viral proteins
- Viruses do not carry ribosomes!!!
- Viruses are absolutely dependent on the host cell to provide energy (e.g., ATP) for viral replication
what is the difference between a virus and a virion?
A virus is an organism of two phases
Virus: within the infected cell
Virion: not a living organism
Virus: is a broad general term for any aspect of the infectious agent and includes:
- Infectious or inactivated virus particle
- Viral nucleic acid and protein in the infected cell
Virion: is the physical particle in the extra-cellular phase which can spread to new host cells
4 basic components of a virus
I. Nucleic acid genome (DNA/RNA)
II. Proteins- structural or non- structural
III. Lipids: envelope
IV. Carbohydrates: associated with protein or lipid e.g. glycoprotein
Which of the following statements correctly defines a virus?
a) An obligate intracellular parasite
b) An intracellular co-symbiotic parasite
c) An obligate extracellular parasite
d) A non-obligate intracellular parasite
e) None of the above
a) An obligate intracellular parasite
Why are all viruses considered obligate intracellular parasites?
a. All viruses lack genomes
b. All viruses can synthesize their own ATP
c. All viruses lack ribosomes
d. Viruses produce so many copies of themselves
c. All viruses lack ribosomes
4 ways to classify viruses
I. Nature and sequence of nucleic acid
* DNA or RNA
* Double Stranded or Single Stranded nucleic acid
* Single Stranded nucleic acid is (+) or (–) sense
* Segmented or non-segmented genomes
II. Symmetry of protein shell (capsid)
- Helical, icosahedral or complex
III. Presence or absence of a lipid membrane (envelope)
IV. Diameter or shape of the virion
baltimore classification scheme
Class I: Double stranded DNA – e.g. Adenoviridae, Herpesviridae
Class II: Single stranded DNA – Parvoviridae, Circoviridae
Class III: Double stranded RNA – Birnaviridae, Reoviridae
Class IV: Positive sense single strand RNA – Caliciviridae, Coronaviridae
Class V: Negative sense single strand RNA – Filoviridae, Rhabdoviridae
Class VI: Retroviruses - Retroviridae (reverse transcribing RNA viruses)
Class VII: Double stranded DNA viruses that replicate through a single stranded RNA intermediate - Hepadnaviridae (human pathogen)
different types of symmetry of the viral protein shell
Helical viruses: have their capsid shaped into a filamentous, or rod-shaped structure with central cavity that encloses its RNA
Icosahedral viruses: consist of identical subunits (capsomers) that make up equilateral triangles that are, in turn, arranged in a symmetrical fashion
what is the outermost surface of a naked virus?
outermost surface is the capsid
what is a viral envelope? What is its purpose?
Have an additional lipid bilayer surrounding the capsid. The lipid bilayer carries viral glycoproteins essential for viral infection
practical differences between enveloped and naked viruses in terms of control
The presence of envelope is important for viral disease transmission and control programs
*Enveloped viruses: are rapidly inactivated by solvents (e.g., alcohol and ether), detergents (e.g., hand soap) and bile salts (most enteric animal viruses are not enveloped)
*Naked viruses: are not inactivated by common detergents and are generally much more stable in the environment
what tool best visualizes virion morphology?
electron microscope
what is the virus replication cycle for an enveloped virus?
- recognition of target cell
- attachment
- penetration
- uncoating
- mRNA synthesis
- protein synthesis
- genome replication
- envelopment
- budding and release
consequences of virus-host interactions
-resistant host - no relationship
-asymptomatic or subclinical infections
>recovery or persistent infection, immunity
-acute viral infection, disease
>death, recovery, or persistent infection
-chronic, disease
>recurrent clinical disease, or persistent infection
