Lecture 4: Viruses

Question 1

What are the characteristics of a virus?

Answer 1

Size: 20 – 300 nm
Shape: icosahedral or helical
Method of replication: obligate intracellular parasites; uses host cellular machinery to replicate

Question 2

What is the function of the capsid within a virus?

Answer 2

A protein coat that encloses the genome and the core proteins

Question 3

What is the function of the envelope within a virus?

Answer 3

Lipid bilayer that surrounds the capsid of some viruses that contains glycoproteins that form projections

Question 4

What is the function of the genome within a virus?

Answer 4

Contains either DNA or RNA; can be single stranded or double stranded; circular or linear; single molecule or fragmented; positive or negative polarity.

Question 5

How is a virus transmitted to the human host?

Answer 5

Either across the species barrier or within the species (horizontal or vertical)

Question 6

Describe horizontal transmission

Answer 6

Viruses are transmitted among individuals of the same generation via: airborne, faecal-oral, sexual, vector-borne and blood

Question 7

Describe vertical transmission

Answer 7

Mother-to-child transmission that can occur in utero (during pregnancy), intra partum (at childbirth), and breast feeding.

Question 8

How does a virus replicate?

Answer 8

1. Attachment of virus to host cell involving interaction between the viral glycoproteins and the cell receptor(s)
2. Penetration of the virus into the host cell
3. Enzymatic removal of viral protein coat and liberation of the viral nucleic acid
4. Production of viral specific mRNA/protein by host cell ribosomes
5. Maturation of the new viruses
6. Release of new virus particles

Question 9

How does viral replication differ between RNA and DNA viruses?

Answer 9

Replication of DNA viruses requires host DNA-dependent RNA polymerase to transcribe viral DNA into mRNA and then into proteins. Replication of RNA viruses use viral RNA as a direct RNA template which will be integrated into host translation mechanisms to form viral proteins.

Question 10

How does viral replication differ between retroviruses, + and – RNA viruses?

Answer 10

Retroviruses uses reverse transcription to form cDNA which can be integrated into host DNA.

– RNA viruses uses viral RNA-dependent RNA polymerase.

+ RNA viruses use viral RNA as a direct RNA template.

Question 11

How does HIV replicate?

Answer 11

HIV viruses are single-stranded RNA retrovirus molecules. Retroviruses uses reverse transcription to form cDNA which can be integrated into host DNA. Host DNA-dependent RNA polymerase is then required to transcribe viral DNA into mRNA and then into viral proteins.

Question 12

What is a pathogen?

Answer 12

Disease-causing agent

Question 13

How does a virus cause disease?

Answer 13

1. Direct cytopathic effects caused by the virus replicating in the host cell
2. Indirect immune-mediated cell death.

Question 14

What effects the pathogenesis of viral infections?

Answer 14

1. Whether virus can overcome the natural barriers against infection (eg. skin and mucosal surfaces)
2. Whether primary replication occurs at the initial site of infection
3. Whether virus moves to secondary replication sites

Question 15

Describe the term invasiveness

Answer 15

The capacity of a virus to enter a tissue or an organ

Question 16

Describe the term virulence

Answer 16

The relative ability of a pathogen to cause disease

Question 17

Describe the term tropism

Answer 17

The ability of different viruses to infect different cell types which is dependent on receptors in the host cell, host enzymes for viral maturation, temp. and pH.

Question 18

How can we diagnose a viral infection?

Answer 18

1. Direct detection of whole virus or of parts of the virus

2. Indirect detection of human antibodies against the virus

Question 19

What are the outcomes of a viral infection?

Answer 19

1. Complete resolution and recovery; clearance of the virus from the host
2. Chronic infection; persistence of the virus in the host.

Question 20

What causes chronic viral infection?

Answer 20

• Continuous viral replication
• Latency and reactivation
• Invasion of the genome followed by vertical transmission

Question 21

How can we treat a viral infection?

Answer 21

Antivirals

Can be attachment inhibitors, fusion inhibitors, nucleoside and non-nucleoside reverse transcriptase inhibitors, integrase inhibitors or protease inhibitors.

Question 22

How can we prevent a viral infection?

Answer 22

• Active immunization (vaccines)
• Masks
• Social distancing
• Isolation and quarantine of infected individuals
• Hand washing