VirusesEdexcel A-Level Biology B: Revision notes

A virus is a tiny infectious particle, much smaller than a bacterium. It has a core of nucleic acid (DNA or RNA) surrounded by a protein coat called a capsid. Some viruses have an outer envelope made from host membrane. They have no cell surface membrane, cytoplasm, organelles or ribosomes.

Viruses are classified by their structure (the shape of the capsid, and whether there is an envelope) and the type of nucleic acid they contain.

• Lambda phage: a bacteriophage with a DNA genome, an icosahedral head and a tail. It infects bacteria such as E. coli.
• Tobacco mosaic virus (TMV): a rod-shaped (helical) capsid around RNA, with no envelope. It infects plants.
• Ebola virus: a filamentous, enveloped virus with RNA. It infects humans through contact with body fluids.
• HIV: a spherical, enveloped RNA retrovirus. It carries the enzyme reverse transcriptase, which makes DNA from the viral RNA. The DNA is inserted into the genome of the helper T cells it infects.

In the lytic cycle:

1. The virus attaches to a specific receptor on the host cell.
2. Its nucleic acid enters the cell (a phage injects its DNA).
3. The host's enzymes and ribosomes replicate the viral nucleic acid and make viral proteins.
4. New virus particles are assembled.
5. The host cell lyses, releasing many new virus particles.

In latency the viral nucleic acid becomes part of the host genome (lambda phage DNA in the bacterial chromosome; HIV DNA as a provirus). It is copied every time the host cell divides, but no new virus particles are made. A trigger, such as stress or ultraviolet light, can switch it into the lytic cycle.

Viruses are not living cells. They have no cell structure and no metabolism, and they cannot reproduce independently, only inside a host cell. Because they use the host's enzymes and ribosomes, antibiotics that target bacterial cell walls or 70S ribosomes have no effect on them.

Antiviral drugs must therefore inhibit virus replication. Examples are reverse transcriptase inhibitors for HIV (such as zidovudine), which stop viral DNA being made. Drugs have to avoid harming host cells, so antivirals are harder to develop than antibiotics.

Viruses are difficult to treat once infection has occurred, so disease control focuses on preventing spread. In the 2014 outbreak in West Africa, Ebola spread by contact with the body fluids of infected people, including the dead. Control measures were:

• Isolation of patients
• Contact tracing and monitoring
• Protective clothing for health workers
• Safe burials, with community education

These reduced contact with infectious body fluids. No licensed vaccine or antiviral existed at the time.

During an epidemic with a high death rate, an experimental drug may be offered before clinical trials are complete. To evaluate:

• For: patients may die anyway, so potential benefit may outweigh risk; it may help to control the epidemic.
• Against: safety and effectiveness are unknown; harm is possible; informed consent is hard in an emergency; supplies are limited, so who receives it is a fairness issue; a placebo group may be unethical, but without controls evidence is weak.

A balanced answer reaches a justified judgement, for example that giving the drug is acceptable with consent, under monitoring, as part of a trial.