Viral life cycle, replication and control

In a nutshell

Viruses are acellular, non-living particles. They have nucleic acids (DNA or RNA) as their genetic material, but they can only reproduce inside the cells of other organisms. These cells are known as host cells. Viruses can sometimes be treated with antivirals. However, these are not always available. This opens the ethical debate of using untested drugs during epidemics.

Viral classification

Viruses are classified by their genetic material and how they replicate. Viruses can have DNA or RNA genomes.

Nucleic acid

Description

Examples

DNA

The DNA acts directly as a template for the synthesis of new viral DNA and mRNA needed to synthesise viral proteins.

Smallpox virus and adenoviruses (these cause colds).

RNA

RNA viruses do not produce DNA. There are two main types of RNA viruses. The first are single stranded RNA (ssRNA) viruses. Positive sense ssRNA viruses can be directly translated at the ribosomes whereas negative sense ssRNA viruses can not be directly translated.

Note: RNA viruses are much more likely to mutate than DNA viruses.

Tobacco mosaic virus, polio and hepatitis C are positive sense ssRNA viruses.

Measles, influenza and Ebola are negative sense ssRNA viruses.

Retroviruses

A type of RNA virus you need to know about are retroviruses. These have a capsid and a lipid envelope and are able to synthesise an enzyme called reverse transcriptase that makes viral DNA from the viral RNA. They also produce an enzyme called integrase that integrates the viral DNA into the host’s DNA. It is then used as a template to create new viral particles.

Example

Human immunodeficiency virus (HIV).

Lytic cycle

The lytic cycle is a cycle of viral replication. which are then assembled within the host cell to produce new viral particles. These viruses are said to be virulent (meaning it can cause disease) and it will go on to infect other host cells.

Biology; Cells, viruses and reproduction of living things; KS5 Year 12; Viral life cycle, replication and control

1.	The virus enters the host cell.
2.	The genetic material is replicated independently of the host DNA.
3.	The host will produce viral components.
4.	The viral components are assembled within the host cell to produce new viral particles.
5.	The cell will lyse and release the viruses that it contained.

Latency

Some viruses can become latent. This means that the virus is still present in the host cell but it does not hijack the host cell machinery to produce new viral particles. The virus is said to be dormant. There are two types of viral latency.

Episomal latency	The viral nucleic acid is inactive and free in the host cell cytoplasm.
Proviral latency	The viral nucleic acid is integrated into the host cell’s genetic material.

Note: Latent viruses can reactivate and follow the lytic cycle.

Example

Chicken pox is caused by the varicella zoster virus and it usually infects children. The child will recover from the infection but some of the viral nucleic acid will remain latent in the host cells. Stress or immunosuppression can reactivate the virus and cause shingles. Shingles usually infects older people.

Antivirals

Antivirals are drugs that are used against viruses. They usually work by inhibiting viral replication in host cells. They may achieve this by targeting the receptors the viruses use to attach to the host cells. By blocking or inactivating these receptors, the virus will not be able to bind and infect the host cell.

Antivirals may also target the viral enzymes, such as integrase and reverse transcriptase, that facilitate viral replication. Finally the antivirals may target protease enzymes. By targeting these enzymes, the new viral particles that the host cell synthesises will not be able to leave the host cell.

Controlling viral diseases

The ability to control a viral disease is very important to prevent a large scale outbreak called a pandemic. The table below describes some ways that viral diseases can be controlled.

control measure	Description
Mass vaccination	If there is a vaccine available this will be used to increase the levels of immunity within a community and reduce the spread of disease.
Pathogen identification	It is crucial that the pathogen is identified in order to enforce the correct control measures. This may be achieved through testing schemes.
Isolating infected individuals	Patients with infection should be isolated to prevent the spread of the disease.
Hygiene measures	Individuals can control disease themselves by washing their hands regularly and thoroughly.
Personal protective equipment	Personal protective equipment is very important for healthcare workers. This will limit the spread of infection from patients. Example Face masks, gloves, goggles and protective gowns are all examples of personal protective equipment that were used in the COVID-19 pandemic to reduce the spread of disease.
Contact tracing	Contact tracing is very important to control diseases. People who have been in contact with infected people can be notified which allows them to take the necessary measures. This may include isolating themselves if they develop symptoms of the disease.

Using untested drugs during epidemics

Treating diseases with drugs that have not been fully tested and approved is seen as unethical however, under circumstances like a pandemic where so many lives are at risk, using a drug that is only part way through its testing process may be considered.

Some drugs may have been tested in the lab but not in humans. These would be considered ‘untested’ even if they have been tested thoroughly in the lab. There are ethical arguments for and against the use of these partially tested drugs during large outbreaks of disease.

For	Against
Despite being called ‘untested’, the drug will have undergone thorough testing already. Therefore, there can be some guarantee of safety.	Some people may believe it is unethical to use drugs that have not yet been tested on humans.
The drug or vaccine could save thousands or even more lives in the event of an epidemic or a pandemic. Some would argue that the benefits outweigh the risks.	By not testing on humans first, any unexpected side effects could be widespread and make the situation worse.
	Who decides who is treated? Deciding who is treated is an ethical dilemma.
	To be treated, the individual must give informed consent that they know the risks of taking the drug or receiving the vaccine. The level of education will impact the ability of the individual to agree or disagree.