Biology A 2.3 - Viruses

What is a virus

Viruses are non-cellular infectious particles that are capable of causing disease within a host organism

They are obligate intracellular parasites that hijack a cell’s molecular machinery and energy resources

Explain how viruses affect host cells

By commandeering a host cell’s resources, they cause the disruption of normal homeostatic processes

Why are viruses not considered to be living

they cannot independently perform all the functions of life

They lack metabolism, cannot replicate autonomously, are unable to grow and do not maintain homeostasis

What are the structural features common to viruses

Protein coat – Viruses are enclosed within a protein shell called a capsid

Internal components – Viruses do not contain any cytoplasm and have few or no enzymes

Genetic material – Viruses contain a nucleic acid core consisting of either DNA or RNA

Size – Viruses contain few molecules and so have a small, fixed size (they do not grow)

How are viruses diverse

viruses are highly diverse in their shape and structure.

• Genetic material may be RNA or DNA, which can be either single- or double-stranded.

• The viral genome may be comprised of either circular or linear molecules of nucleic acid.

• Viruses exist in a range of sizes.

• The structure of the protein shell can vary (capsids can be helical, polyhedral, spherical or complex)

• Viruses may possess distinct attachment proteins according to the type of host cell they infect

• Some viruses are enveloped ( Covered in a fat (lipid) membrane stolen from the host cell) and others are not enveloped.

What are examples of viruses

Bacteriophage lambda – A non-enveloped virus with a complex structure composed of ds-DNA (linear)

Coronavirus – An enveloped virus containing ss-RNA (linear) within a spherical capsid (cause of Covid-19)

HIV – An enveloped retrovirus consisting of ss-RNA within an icosahedral capsid (infects helper T cells)

Why are viruses unable to undertake independent replication

lack metabolism and they are obligate intracellular parasites

How do viruses replicate?

Viruses replicate by invading a host cell and utilising its cellular machinery to propagate new copies (virions)

What are the basic steps of viral replication are common to all viruses

1. They attach to specific receptor proteins on the host cell (meaning a virus can only infect certain cell types)

2. Their genetic material is inserted into the host cell (mechanism may vary according to capsid structure)

3. Cell machinery is commandeered to replicate genetic material and synthesise viral proteins

4. The component parts are assembled into new viral particles before being released from the host cell

What is the lytic cycle?

The lytic cycle is a viral replication process in which new virions are produced inside a host cell, followed by lysis (rupturing) of the host cell to release the virions, destroying the cell.

How does the lytic cycle work?

Enzymes (such as lysozymes) are synthesised from the viral genetic material and degrade the cell membrane or cell wall

Eventually the host cell lyses and the virions are released (the infected host cell is destroyed as a result of this process)

What is the lysogenic cycle

The lysogenic cycle is a viral replication pathway in which viral genetic material is integrated into the host cell’s genome and remains dormant.

How does the lysogenic cycle work

An enzyme (integrase) is synthesised from the viral genetic material that catalyses the insertion of viral sequences into the host cell genome

A viral gene coding a repressor protein prevents the integrated viral sequences from being expressed while the cell remains in a state of latency

During this time, the infected cell may reproduce, creating new cells with the integrated viral sequences

The viral sequences remain dormant until activated by environmental stressors (such as UV radiation or exposure to certain chemicals)

At this point, the lytic cycle is triggered – potentially resulting in the destruction of an entire cell line

What is the difference between the lysogenic and lytic cycles?

Lysogenic: Viral DNA is integrated into the host genome and remains dormant; the host cell can reproduce normally.

Lytic: Viral DNA is expressed to produce new virions, which are released by lysis of the host cell, destroying it.

Give examples of viruses that can undergo alternating lysogenic and lytic cycles.

• bacteriophage lambda

• HIV