Topic 2.2 Viruses

Question 1

How are viruses classified?

Answer 1

According to structure and type of nucleic acid

Question 2

Name three types of virus and give example

Answer 2

• DNA virus e.g. lambda (λ) phage
• RNA virus e.g. tobacco mosaic virus (TMV) and ebola
• RNA retrovirus e.g. human immunodeficiency virus (HIV)

Question 3

Describe the structure of a DNA virus

Answer 3

Nucleic acid: DNA (which can act directly as a template for mRNA transcription/ DNA repliction).

Geometrical shape.

Question 4

Compare and contrast the tobacco mosaic virus and Ebola

Answer 4

Both are RNA viruses.

Tobacco mosaic virus contains ssRNA, which can be directly translated into proteins by ribosomes.

Ebola contains negative ssRNA, which needs to be transcribed to produce mRNA before translation

Question 5

Describe the features of a retrovirus

Answer 5

Single strand of RNA.

Reverse transcriptase enzyme produces cDNA from RNA tmeplate.

Double-stranded viral DNA integrates into host cells genome.

Question 6

Describe the lysogenic pathway

Answer 6

1. Non-virulent viruses inject DNA into host cell DNA as provirus. Viral DNA replicates when host cell divides
2. Virus produces repressor proteins to inhibit transcription
3. Latent virus enters lytic pathway when host cell is damaged or immune system weakens

Question 7

Describe the lytic cycle

Answer 7

1. Virilent viruses inject nucleic acid into host cell cytoplasm. Viral genetic information replicates immediatly, independent of host cell DNA
2. Many virions (viral particles) assemble, causing cell lysis

Question 8

Why are viruses classified as non living?

Answer 8

They are acellular: no cytoplasm, no metabolism and cannot self-replicate

Question 9

How do antivirals work?

Answer 9

Viruses have no metabolism = difficult to treat infection after initial contact. Antivirals inhibit replication by:
* Targeting viral antigens to prevent entry into host cell
* Targeting viral enzymes to prevent DNA replication and virion assembly

Question 10

What is the focus of disease control for viral infections?

Answer 10

Preventing spread (especially when there is no effective vaccine)

Question 11

Outline what happened during the 2014 Ebola outbreak

Answer 11

Virus spread rapidly through areas of West Africa with poor hygiene. 4877 deaths. No effective vaccine, so disease control measures aimed to reduce spread.

Question 12

Outline common methods of controlling viral outbreaks

Answer 12

• Rapid identification and testing the individuals contacts for disease
• Reduce person to person contact with virus:
  ~ Quarantine
  ~ Sterilisation of equipment
  ~ Protective clothing

Question 13

Suggest ethical issues surrounding the use of untrialled drugs during epidemics

Answer 13

• Severity of outbreak
• How effective other available treatments are
• Difficulty obtaining informed consent

Question 14

Suggest arguments against the use of untrialled drugs during epidemics

Answer 14

• Unanticipated severe side effects
• Difficult to decide which individuals to treat first
• Drug may be falsly attributed as the cause of death
• Severly ill patients cannot give informed consent