2.2

• How are viruses classified?

  According to structure & type of nucleic acid
• Name 3 types of virus and give examples

  • DNA virus e.g. lambda (ƛ) phage
  • RNA virus e.g. tobacco mosaic virus & Ebola
  • RNA retrovirus e.g. human immunodeficiency virus (HIV)
• Describe the structure of a DNA virus

  • Nucleic acid: DNA (which can act directly as a template for mRNA transcription/ DNA replication)
  • Geometrical shape
• Describe the features of an RNA retrovirus

  • Single strand of RNA
  • Reverse transcriptase enzyme produces cDNA from RNA template
  • Double-stranded viral DNA integrates into host cell's genome
• How do antivirals work?

  • Viruses have no metabolism = difficult to treat infection after initial contact. Antivirals inhibit viral replication by:
  • Targeting viral antigens to prevent entry into host cell
  • Targeting viral enzymes to prevent DNA replication & virion assembly
• Outline common methods of controlling viral outbreaks

  • Rapid identification & testing the individual's contacts for disease
  • Reduce person-to-person contact with virus:

  1. Quarantine
  2. Sterilisation of equipment
  3. Protective clothing
• Suggest arguments against the use of untrialled drugs during epidemics

  • Unanticipated severe side effects
  • Difficult to decide which individuals to treat first
  • Drug may be falsely attributed as the cause of death
  • Severely ill patients cannot give informed consent
• Compare and contrast the tobacco mosaic virus and Ebola:

  • 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.
• Why are viruses classified as non-living?

  They are acellular: no cytoplasm, no metabolism and cannot self-replicate
• What is the focus of disease control for viral infections?

  Preventing spread (especially when there is no effective vaccine)
• Suggest the ethical issues surrounding the use of untrialled drugs during epidemics

  • Severity of outbreak
  • How effective other available treatments are
  • Difficulty obtaining informed consent
• What is the Ebola virus?

  An RNA virus which causes major internal bleeding and is spread through contact with bodily fluids of an infected person
• What is the (lambda) phage virus?

  A type of DNA virus which infects the bacterium E. coli.
• What is the tobacco mosaic virus?

  A type of single-stranded RNA virus which infects plant cells and causes discolouration of the leaves in a mosia-like pattern and hinders their growth
• What is the human immunodeficiency virus (HIV)?

  An RNA retrovirus which is transmitted through certain body fluids like blood or semen and attacks the immune system which can lead to the development of AIDS
• Give the definition of lytic cycle 


  The replication of a virus within a host cell which ultimately leads to the rapture and death of the host cell
• What is another word for rupture?

  Lysis
• Give the definition of latency

  The amount of time between exposure to a pathogen and the presentation of symptoms caused by the pathogen
• What are the two types of viral replication cycles?

  • Lytic cycle
  • Lysogenic cycle
• Outline the process of the Lytic cycle:


  1. Attaches to host cell
  2. Virus can inject either its DNA or RNA into the cell
  3. Cell takes the genetic material from the virus and starts following the instructions. Instructions = tell cell to make copies of the virus
  4. Makes so many copies of the virus that it can cause the cell membrane of the host to rupture
  5. New viral copies get out of the cell and infect other cells
• Outline the process of the Lysogenic cycle:

  1. Genetic material is injected but the genetic material stays hidden in the host’s genetic material
  2. When host makes new cells, it replicates its own genetic material and the viral genetic material -> then those daughter cells make new cells = viral genetic material replicated again
  3. Problem if it gets triggered to go into the Lytic cycle from that point (trigger can vary)