Intro to Virology Flashcards by Jassie McGovern

All of the following statements regarding viruses are true except:

1) have nucleic acid surrounded by a protein coat
2) cellular infectious agent
3) do not carry out metabolism
4) lack organelles and ribosomes

Statement 2: viruses are Acellular infectious agents

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What is the approx. size of a virus?

20-300nm

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What is a virion?

a complete virus particle

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What components are in all virions?

1) genome

2) protein coat called capsid

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What’s an envelope?

lipid layer that is in addition to capsid

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All enveloped viruses also have another layer called…

integument/ matrix

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What are the different types of genome in viruses?

1) DNA (single-stranded or double-stranded)
2) RNA (single-stranded or double-stranded)
3) single-stranded RNA is further divided into positive sense and negative sense

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What’s the difference between +ssRNA and -ssRNA?

+ssRNA is ready for translation

-ssRNA must be converted to + sense before it can be translated

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Name the DNA viruses (HHAPPPy)

Herpesviridae
Hepadnaviridae
Adenoviridae
Poxviridae
Papovaviridae
Parvoviridae

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Which of the DNA viruses are single-stranded?

Parvoviridae

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What are all the -ssRNA?

Always Bring Polymerase Or Fail Replication

Arenaviridae
Bunyaviridae
Paramyxoviridae
Orthomyxoviridae
Filoviridae
Rhabdoviridae

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Which of the RNA viruses are double-stranded?

Reoviridae

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What are all the +ssRNA?

FCCTPRH

Flaviviridae
Coronaviridae
Caliciviridae
Togaviridae
Picornaviridae
Retroviridae
Hepeviridae

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Where do most RNA viruses replicate?

cytoplasm

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Can cells replicate RNA?

NO, humans do not have RNA-dependent RNA polymerase

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How does the replication of +ssRNA differ from -ssRNA?

+ssRNA –> genome is directly translated by host ribosomes into proteins and one of the proteins will be a RNA-dependent RNA polymerase that can replicate the viral genome
-ssRNA –> they must bring their own polymerase to convert neg to pos so that the host ribosomes can recognize and begin translation

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Naked capsids are environmentally _____ while envelopes are environmentally ____

1) stable

2) labile

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How are naked capsids released to environment?

cell lysis

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How are envelopes released to environment?

budding or cell lysis

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Which can survive in the GI tract, naked capsid or envelope?

naked capsid (stable against acid and proteases)

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Which can be in dry environment and which must be in wet environment (between naked capsid and envelope)?

naked capsid –> dry

envelope –> wet

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How do naked capsids spread?

they spread easily via fomaties, hand to hand contact, by dust or by small droplets

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Can naked capsid dry out and still remain its infectivity? What about envelope?

yes, naked capsid will remain its infectivity upon drying. evelopes will not

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How do enveloped viruses spread?

large droplets, secretions, organ transplants and blood transfusions

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True or False. In order to spread, naked capsids will have to kill the host cell.

True. they spread via cell lysis

What are the steps to virus life cycle?

1) attachment 2) entry 3) mRNA production 4) protein and genome synthesis 5) assembly 6) exit/ egress

How do enveloped viruses enter the host cell?

1) direct fusion | 2) hi-jacking host receptor-mediated endocytosis

Describe the direct fusion method of entering host cell

the envelope fuses with host membrane and the viral particle is released into cytoplasm

Describe the hi-jacking method of entering host cell

1) plasma membrane invaginates and surrounds the viral particle 2) pinches off forming an endocytic vesicle 3) 2 membranes involved: viral envelope and host membrane 4) the 2 membranes fuse and the viral particle is released into cytoplasm

How do non-enveloped viruses enter host cells?

via receptor-mediated pathway: to exit the vesicle, they simply modify or lyse the vesicle membrane

Describe how ssDNA gets its genome transcribed.

Parvovirus (the only ssDNA) | - uses host DNA repair enzymes --> dsDNA

Describe how +ssRNA get their genome transcribed.

these can directly be translated into proteins by host ribosomes

What is the exception to +ssRNA?

retrovirus | this needs to be reverse transcribed into DNA by a viral reverse transcriptase first

Describe how -ssRNA is transcribed

they must bring their own viral RNA polymerase to convert genome to +ssRNA first

Describe how dsDNA get their genome transcribed

host DNA-dependent RNA polymerase

What is the biggest challenge for genome replication in DNA viruses?

cellular DNA replication machinery is not available all the time

How do DNA viruses mitigate the challenge?

1) force the machinery to become available by stimulating cell growth and DNA synthesis 2) encode viral proteins to synthesize genome (only big viruses can do this bc they are big enough to carry all the genome)

What is the biggest challenge for genome replication in RNA viruses?

no cellular RNA-dependent RNA polymerase

How do RNA viruses mitigate the challenge?

+ssRNA --> make host machinery translate a RNA-dependent RNA polymerase -ssRNA --> bring their own polymerase to convert neg sense to pos sense, then their genome encodes for more pol

What is the general pathway to viral assembly and egress?

1) individual viral proteins form into capsid subunits 2) the subunits combine to form complete capsid 3) viral genome and other essential virion components are selectively packaged into capsid 4) envelope acquired using host membrane (for enveloped viruses) 5) virus exits cell

What are the two ways virions exit into extracellular space?

1) budding | 2) lysis

What are some ways viruses can change their antigenicity?

1) point mutations 2) recombination 3) reassortment

How do point mutations occur for viruses?

RNA polymerases lack proofreading capabilities thus error rates are high for RNA viruses.

Briefly describe how recombination can lead to change in antigenicities

DNA viruses utilize cell's recombination machinery (ex. HSV1 and HSV2) - breaking and forming covalent bonds

Briefly describe how reassortment can lead to change in antigenicities

classic example is influenza: these viruses have segmented genomes and they simply just mix with each other. do not require breaking or forming of covalent bonds

What are the three types of viral infection?

1) acute 2) chronic 3) latent

Describe actue viral infections and give an example of a virus that does this

viral load increases dramatically upon infection then it is cleared by host immune response. Ex. common cold

Describe chronic viral infections and give an example of a virus that does this

viral load increases dramatically upon infection then it is mostly cleared by host but it is incomplete thus the virus lingers. Ex. Hep.

Describe latent viral infections and give an example of a virus that does this

viral load increases dramatically upon infection then it is cleared by immune response, however, the virus lays dormant until it is reactivated by a triggering event. Ex. HSV1

What are some ways viruses can invade the human body (7)?

1) conjunctiva 2) oral transmission 3) droplet transmission (inhalation) 4) direct inoculation (bug bite) 5) direct skin contact 6) sexual transmission 7) trans-placental

Which diagnostic tool is used to detect infectious viruses?

plaque assay

What are advantages to performing a plaque assay?

positivity shows active viral infection

What are limitations to plaque assay?

restricted to viruses that replicate in tissue culture and produce cytopathic effect

What viral diagnostic tool is used to detect virion particles?

electron microscopy

What are advantages to using electron microscope?

helpful in identification of emerging viruses

What are limitations to using electron microscope?

relatively expensive and technically challengin

What viral diagnostic tool is used to detect viral proteins and glycoproteins?

antigen detection ie. ELISA

What are advantages to using antigen detection?

sensitive and quick

What are limitations in using antigen detection?

requires specific Ab

What viral diagnostic tool is used to detect DNA genomes?

PCR

What are advantages to using PCR?

highly sensitive

What are limitations in using PCR?

DNA sequence info must be available

What viral diagnostic tool is used to detect RNA genomes?

Reverse PCR

What are advantages in using reverse PCR?

highly sensitive

What are limitations in using reverse PCR?

RNA sequence info must be available

What viral diagnostic tool is used to detect anti-viral Abs?

serology ie. Western blot

What are advantages in using serology?

sensitive and quick

What are limitations in using serology?

time must be allowed for initiation of immune response. care must be taken to differentiate present from past infections