Virus Structure & Living Debate

In brief:Viruses are microscopic parasitic agents that straddle the boundary between living and non-living. They are not classified in any domain of life.

Viruses are microscopic agents that straddle the boundary between living and non-living. They are not classified in any domain of life.

Virus structure:
• Nucleic acid core: either DNA OR RNA (never both), can be single or double-stranded
• Capsid: protein coat surrounding the nucleic acid. Protects genetic material and helps attach to host cells
• Envelope (some viruses): outer layer made from host cell phospholipids and viral proteins. E.g. influenza, HIV
• Sizes: typically 20–300 nm (much smaller than bacteria)

Virus shapes:
• Helical/rod: e.g. tobacco mosaic virus
• Icosahedral (spherical): e.g. adenovirus
• Complex: e.g. bacteriophage (head + tail + tail fibres)

The Living/Non-Living Debate:

Virus structure — Structure of a virus (HIV) - capsid, envelope and genetic material
Wikimedia Commons (CC)

comparison

Are Viruses Alive?

Living Traits

Have genetic material (DNA/RNA)
Can replicate (inside host)
Pass genetic info to offspring
Can mutate and evolve

Non-Living Traits

Not made of cells
Only one type of nucleic acid
No organelles or ribosomes
Cannot metabolise independently
Cannot replicate outside a host

Key Points

1Viruses are not made of cells and do not contain organelles like ribosomes.
2Structure: nucleic acid (single/double stranded DNA OR RNA) surrounded by a protein coat (capsid).
3Some viruses have an envelope outside the capsid, formed from host cell phospholipids and proteins.
4Virus shapes: rod/helical, spherical, complex (e.g. bacteriophage).
5Living traits: have genetic material (DNA/RNA), can replicate in host, pass genetic info to next generation.
6Non-living traits: non-cellular, only one nucleic acid type, no organelles, cannot metabolise independently, cannot replicate outside host.
7When outside a host cell, a virus is dormant and called a 'virion'.
8Viruses are obligate parasites - they can only replicate inside living cells.

Learning Outcomes

Discuss the difficulty of defining viruses, their economic and medical importance
Model how viruses replicate within cells

Viral Replication

In brief:Viruses hijack host cell machinery to replicate. The process follows six key steps. Viruses cannot replicate independently.

Viruses cannot replicate independently - they must hijack a living host cell's machinery. The replication cycle follows six key steps:

1. Attachment: Virus surface proteins bind to specific receptors on the host cell (lock and key fit). This is why viruses only infect certain cell types.
2. Penetration: Viral genetic material (DNA or RNA) is injected into the host cell. The capsid usually stays outside.
3. Replication: The host cell's machinery makes copies of the viral DNA/RNA.
4. Synthesis: Viral genes instruct the host cell's ribosomes to make viral proteins (capsid proteins, enzymes).
5. Assembly: New virions are assembled from the replicated genetic material and newly made proteins.
6. Release: The host cell bursts (lyses), releasing hundreds of new virions that infect neighbouring cells.

This process can take hours to days depending on the virus. Each released virion can infect a new cell, leading to exponential spread.

process

The 6 Steps of Viral Replication

1⃣ Attachment
Binds to receptor

→

2⃣ Penetration
DNA/RNA injected

→

3⃣ Replication
Copies viral genes

→

4⃣ Synthesis
Makes viral proteins

→

5⃣ Assembly
Builds new virions

→

6⃣ Release
Cell bursts (lysis)

diagram

Visual Summary of the 6 Steps

Key Points

11. Attachment: virus attaches to specific receptors on the host cell surface.
22. Penetration: viral genetic material (DNA or RNA) is injected into the host cell. Capsid stays outside.
33. Replication: copies of viral DNA/RNA are made using host cell materials.
44. Synthesis: viral genes instruct host cell ribosomes to make viral proteins. Virus hijacks host metabolism.
55. Assembly: new virions are assembled from viral genetic material and viral proteins.
66. Release: host cell may burst (lyse) to release new virions which infect other cells.
7Each virus only infects specific cell types by matching surface proteins to host receptors.
8The replication process can take days to weeks depending on the virus.

Learning Outcomes

Model how viruses replicate within cells
Explore factors that contribute to the emergence of infectious diseases in plants and animals

Economic & Medical Importance

In brief:Viruses have enormous economic and medical significance - from pandemics to vaccines, crop disease to gene therapy.

Key Points

1COVID-19: huge global economic impact - job losses, supply chain disruption, healthcare strain.
2Plant viruses: reduce crop yield - e.g. tobacco mosaic virus turns chloroplasts yellow, reduces photosynthesis.
3Animal viruses: foot and mouth disease in cattle causes deaths, milk production loss, trade restrictions.
4Vaccines: use weakened/inactive virus (or viral proteins/RNA) to stimulate immunity without causing disease.
5Antibiotics do NOT work against viruses - only vaccines and antiviral drugs help.
6Gene therapy: modified viruses used as vectors to carry therapeutic genes into human cells (e.g. cancer treatment).
7Bacteriophages: being researched as alternatives to antibiotics - they target and kill specific bacteria.

Learning Outcomes

Discuss the difficulty of defining viruses, their economic and medical importance
Discuss the importance of a knowledge of emerging diseases in society