L22: Introduction to Virology

Question 1

Structure of virion

Answer 1

1. ) viral attachment proteins
2. ) envelope (some viruses)
3. ) matrix/tegument (some viruses)
4. ) capsid: helical, icosahedral or complex (larger viruses) forms
5. ) genome

Question 2

Types of viral genomes

Answer 2

1. ) DNA (ds or ss)

2. ) RNA (ds or ss) – if ss, negative or positive sense

Question 3

What is meant by the sense of RNA viral genomes?

Answer 3

• Positive sense = in correct orientation to be bound by host ribosome and to produce peptide
• Negative sense = inert by itself, requires production of + sense RNA using this as template

Question 4

List of DNA virus families that cause human diseases

Answer 4

• Mnemonic: HHAPPPy
• H: herpesviridae (HSV, VZV)
• H: hepadnaviridae (HepB)
• A: adenoviridae (adenovirus)
• P: parvovirdae (parvovirus)*
• P: papovaviridae (HPV)
• P: poxviridae (small pox, monkey pox)
• ssDNA, rest are dsDNA

Question 5

List of negative sense RNA viruses that cause human diseases

Answer 5

• Mnemonic: Always Bring Polymerase Or Fail Replication
• A: arenaviridae (Lassa fever virus)
• B: bunyaviridae (Lacrosse encephalitis virus)
• P: paramyxoviridae (Mumps, Measles viruses)
• O: orthomyxoviridae (Influenza virus)
• F: filovirdae (Ebola virus)
• R: rhabdoviridae (Rabies virus)

Question 6

List of double stranded RNA viruses that cause human diseases

Answer 6

• Reoviridae (reovirus)

Question 7

List of positive sense RNA viruses that cause human diseases

Answer 7

• Mnemonic: I went to a Retro Hippy (heppy) Toga Picnic w/Cali Flavored Corona
• Retroviridae (HIV)
• Hepeviridae (Hep E)
• Togaviridae (rubella)
• Picornaviridae (rhinovirus and polio virus)
• Caliciviridae (Norwalk virus)
• Flaviviridae (West Nile virus)
• Coronaviridae (SARS coronavirus)

Question 8

RNA viruses upon entry into host cells can have their RNA replicated using the cell’s machinery – True / False

Answer 8

• False, all RNA viruses except for pos sense RNA viruses are required to carry RNA-dependent RNA polymerases to host to ensure replication

Question 9

Compare and contrast properties of naked capsid vs enveloped viruses

Answer 9

1. ) Naked capsid: stable to temp, acid, proteases, detergents, drying, spread easily by small droplets, survive adverse conditions of gut, resistant to detergents and poor sewage treatment, released from host cell by lysis
2. ) Enveloped: disrupted by acid, detergents, drying and heat, must stay wet, cannot survive GI, spread by large droplets – including bodily secretions and blood transfusions, released from host cell by lysis or budding

Question 10

Steps of viral life cycle

Answer 10

1. ) Attachment
2. ) Entry
3. ) mRNA production
4. ) Protein and genome synthesis
5. ) Assembly
6. ) Egress

Question 11

Options for viral attachment and entry?

Answer 11

1. ) Penetration via viral attachments and fusion with cell membrane
2. ) Entry via viral attachments and hijacking of vesicular trafficking pathway

Question 12

How does gene transcription/translation occur with ssDNA, dsDNA, dsRNA, -ssRNA and +ssRNA?

Answer 12

1. ) ssDNA: use of cellular DNA repair enzymes to form dsDNA then use of cellular RNA pol to form mRNA
2. ) dsDNA: use of cellular RNA pol to form mRNA
3. ) dsRNA: use of carried RNA-dep-RNA pol to form mRNA
4. ) –ssRNA: use of carried RNA-dep-RNA pol to form mRNA
5. ) +ssRNA: if retrovirus, use reverse transcriptase to integrate into genome or begin translation as is

Question 13

What are the possible options to replicate viral DNA given that replication machinery is not always available in the host cell at all times?

Answer 13

1. ) Make cellular DNA machinery available

2. ) Encode viral proteins to synthesize genome: own polymerases etc. – done by large viruses typically

Question 14

How do +ssRNA viruses replicate if they don’t have their own RNA-dependent RNA pol with them?

Answer 14

• Their +ssRNA serves as mRNA for host machinery and provided it encodes for the polymerase they need, they can make it using the host’s machinery

Question 15

Options for viral egress

Answer 15

1. ) Budding

2. ) Lysis

Question 16

How can viruses expand their genome or gain new genetic mutations?

Answer 16

1. ) Point mutations – rather slow change for it to be clinically significant
2. ) Recombination (only in DNA viruses) – requires two of same viral particles each with homologous, yet varied genomes, two infect a particular cell
3. ) Reassortment in segmented viruses – requires two of the same viruses each with different combinations of segmented genomes

Question 17

Types of viral infections

Answer 17

1. ) Acute
2. ) Chronic: viral load increases then decreases, but not down to zero – typically after 6 months
3. ) Latent: acute infection followed by reactivations later on in life

Question 18

Routes of viral invasion

Answer 18

1. ) oral transmission
2. ) conjunctival
3. ) direct skin contact
4. ) transplacental
5. ) droplet transmission and inhalation
6. ) direct inoculation – injections, trauma, insect bites
7. ) sexual transmission

Question 19

What are plaque assays?

Answer 19

• Culture monolayer of cells in petri dish and place viruses on top
• Look for plaques, which are areas of cytopathic changes where viruses have infected/destroyed cells

Question 20

How would a lab technician detect: a.) infectious virions, b.) viral antigens, c.) viral genomes, d.) antibodies against viruses

Answer 20

• a.) plaque assay – requires viruses that produce cytopathic effects
• b.) ELISA, etc. – requires specific antibodies
• c.) DNA by PCR, RNA by rt-PCR – requires knowledge of sequences
• d.) ELISA, western blot – requires time for immune response by host, must differentiate from past infections