Viruses


Viruses are the smallest of all known microorganisms. They are not cells and are not considered to be alive. They are purely arrangements of protein and genetic material.

Types of Viruses

Specification Reference

"Understand that the classification of viruses is based on structure and nucleic acid types as illustrated by ? (lambda) phage (DNA), tobacco mosaic virus and Ebola (RNA) and human immunodeficiency virus (RNA retrovirus)."
  • All viruses have a protein coat known as a capsid
  • This is made up of simple repeating units known as capsomeres
  • This makes virus construction quick and simple
  • Some viruses also have an envelope allowing them to evade recognition
The following explains the different types of viruses:

DNA Virus
  • Genetic material is DNA
  • Acts directly as a template for new DNA and mRNA to make viral proteins
  • Geometrical in shape and infect bacteria
  • Examples include the smallpox virus
RNA Virus
  • Some contain positive ssRNA which act directly as mRNA at the ribosomes (tobacco mosaic virus)
  • Some contain negative ssRNA which must be transcribed before it can acts as mRNA at the ribosomes (Ebola)
RNA Retrovirus
  • Special virus with a single strand of RNA
  • This codes for the protein reverse transcriptase
  • This creates DNA molecules of the viral genome
  • These are incorporated into the hosts DNA which now produces the virus
  • The cell now acts as a vial factory producing viruses which will exit the cell through exocytosis (HIV)

Lysogenic & Lytic Pathways

Specification Reference

"Know the lytic cycle of a virus and latency."
Once a virus enters a host cell there are two different routes to infection:

Lysogenic Pathway
  1. These viruses are non-virulent
  2. They insert their DNA into the cell as a provirus
  3. This DNA is then incorporated into the hosts DNA
  4. The viral genome has suppressor genes making it impossible for the material to be transcribed
  5. The virus is latent
Lytic Pathway
  1. DNA is inserted
  2. The DNA is replicated immediately to produce new viruses
  3. These new mature viruses begin to fill the host cell
  4. This cell eventually bursts
  5. The new viruses are free to invade other cells and the viruses is virulent
Cells can be turned from the lysogenic state when the cell is damaged, reducing the amount of repressor protein. This allows the viral genes to be transcribed and new mature viruses will be produced.

Positive ssRNA Virus Replication

The single strand of RNA is sense so can be transcribed directly at the ribosomes. The proteins made usually include RNA polymerase.

Negative ssRNA Virus Replication

The single strand of RNA is antisense so must be first transcribed using RNA replicase. The virus will then use this RNA to make additional copies of itself.

RNA Retrovirus Replication

These viruses have RNA which cannot be used as mRNA. Instead the RNA codes for the protein reverse transcriptase which converts the RNA into DNA. This DNA is then incorporated into the host cells DNA. The cells DNA will now code for new viral material and new viruses will exit the cell through exocytosis. The cell is acting as a viral factory.

Antiviral Treatment

Specification Reference

"Know that viruses are not living cells and so antivirals must work by inhibiting virus replication."
Viruses are not living cells because they lack metabolism. A virus just acts as an infective agent using a host cell to replicate its genome and produce new viruses. Therefore, to stop viruses treatment must stop them during replication. Some antivirals achieve this by doing the following:
  • target virus receptors
  • target viral enzymes
  • inhibit protease enzymes preventing bussing from the membrane of host cells.

Preventing the Spread of Viruses

Specification Reference

"Know that as viruses can be difficult to treat once infection has occurred, the focus of disease control should be on preventing the spread, as exemplified by the 2014 Ebola outbreak in West Africa."
Viruses can spread quickly, especially in areas with poor hygiene. This was the case in 2014 when the Ebola viruses spread throughout West Africa claiming a total of 4,877 deaths. Despite vaccinations being an effective way to treat some diseases for viruses such as Ebola there is no effective solution. Instead disease control and prevention is key.

  • Early identification and response to an outbreak can significantly reduce the spread of the virus, especially in the short-term when no effective vaccination exits.
Methods of Controlling Viral Outbreaks
  • Rapid Identification - allows for a quicker and more effective response.
  • Nursing in Isolation - prevents further spread of the virus although a access to such facilities are seldom found in LIC's.
  • Sterilising Equipment - used needles and other equipment can quickly spread disease from person to person.
  • Other Methods - include wearing protective clothing and identifying contacts.

Ethical Implications

Specification Reference

"Be able to evaluate the ethical implications of using untested drugs during epidemics."
During epidemics there is pressure to provide effective treatment quickly. This leads to the use of drugs even if they have not completed trials and this can result in a number of ethical issues, including the following:
  • Severity of the disease
  • Availability of other treatment
  • Effectiveness of normal disease control
  • Informed consent
  • Freedom of choice
Many people fell that despite the potential benefits of using untested drugs, they should not be used. Their reasons include the following:
  • Unexpected side effects
  • Deciding who gets the treatment and who doesn't
  • People may feel like "Guinea pigs"
  • Patients may not have the clarity of thought to give informed consent
  • The drugs may be blamed for an inevitable death