Virus structure
CIE A-Level BiologyΒ· Unit 1: Cell StructureΒ· 10 min read
1. General Viral Structure and Key Componentsβ βββββ± 3 min
Virion
A complete, infectious viral particle that exists outside of a host cell. All virions have two core components: genetic material and a protein coat called a capsid.
Capsid
The protein coat that surrounds and protects the viral genetic material, built from repeating protein subunits called capsomeres. Capsids can be helical, icosahedral, or complex in shape.
All virions contain genetic material, which can be either single-stranded or double-stranded DNA or RNA. A defining feature of viruses is that they never contain both DNA and RNA, unlike all living cells.
Name and state the function of four key structural components of a typical bacteriophage.
- 1
- Head (capsid):
- 2
Function: Protects the viral genetic material and holds it in place.
- 3
- Viral DNA:
- 4
Function: Carries all genetic instructions required to produce new viral proteins and assemble new virions inside the host bacterium.
- 5
- Tail fibers:
- 6
Function: Bind to specific receptor molecules on the surface of the host bacterial cell to enable attachment.
- 7
- Contractile sheath:
- 8
Function: Contracts after attachment to inject the viral DNA into the host bacterial cytoplasm.
Exam tip:
CIE frequently asks to label diagrams of bacteriophages, so memorise the position and function of each core structure.
2. Enveloped vs Non-Enveloped Virusesβ β ββββ± 3 min
Beyond the core capsid and genetic material, some viruses have an additional outer layer called an envelope. This structural difference impacts how viruses enter host cells and interact with the immune system.
Viral Envelope
A lipid bilayer membrane derived from the host cell (from the cell membrane, nuclear membrane, or endoplasmic reticulum) that surrounds the capsid. Envelopes are embedded with viral glycoprotein spikes that aid host cell attachment.
State one similarity and two differences between non-enveloped tobacco mosaic virus (TMV) and enveloped influenza virus.
- 1
Similarity: Both viruses have a protein capsid surrounding their genetic material, and both have RNA as their genetic material.
- 2
Difference 1: TMV is non-enveloped, so its outermost layer is the protein capsid. Influenza has an outer lipid envelope surrounding its capsid, with embedded glycoprotein spikes.
- 3
Difference 2: TMV infects plant cells and spreads through physical damage to plant cell walls, while influenza enters host animal cells by fusing its envelope with the host cell membrane.
3. Comparing Viruses to Cellular Lifeβ β ββββ± 4 min
A key examinable point is that viruses are acellular, meaning they do not have a cell structure. This sets them apart from all prokaryotic and eukaryotic organisms.
Viruses lack cytoplasm, ribosomes, and all other organelles
Viruses cannot carry out independent metabolic reactions (e.g. respiration)
Viruses cannot replicate on their own, they require host cell machinery
Viruses have only one type of nucleic acid (DNA or RNA), while all cells have both
Give three structural differences between a bacterium (prokaryotic cell) and a virus.
- 1
- Bacteria are cellular (have a cell structure), while viruses are acellular with no cell structure.
- 2
- Bacteria contain both DNA and RNA, while viruses contain only DNA or RNA, never both.
- 3
- Bacteria have their own ribosomes for protein synthesis, while viruses do not have ribosomes or any other organelles.
Test your understanding of core differences:
Which feature is shared by all viruses and all prokaryotic cells?
A. Ribosomes
B. Nucleic acid
C. Lipid envelope
D. Cell wall
Reveal answer
B βAll viruses contain nucleic acid (DNA or RNA) and all prokaryotic cells contain DNA and RNA, so this is the shared feature. Viruses do not have ribosomes or cell walls, and only some viruses have envelopes.
4. Common Pitfalls
Wrong move:
Stating that viruses contain both DNA and RNA
Why:
All living cells have both DNA and RNA, but viruses only carry one type of nucleic acid in their virion
Correct move:
Always state that viruses have either DNA or RNA, never both
Wrong move:
Confusing the capsid and the viral envelope
Why:
Many students mix up which layer is always present vs which is optional
Correct move:
Remember: capsid = protein coat that is always present; envelope = lipid outer layer only found in some viruses
Wrong move:
Calling viruses 'living cells'
Why:
CIE 9700 follows the standard classification that viruses are not living or cellular
Correct move:
Refer to viruses as acellular infectious particles, not cells
Wrong move:
Claiming the entire bacteriophage virion enters the host bacterium
Why:
Most students generalise entry from animal viruses to bacteriophages incorrectly
Correct move:
Recall that only the bacteriophage DNA enters the host; the capsid remains outside the bacterial cell
5. Quick Reference Cheatsheet
Component | Description | Core Function |
|---|---|---|
Virion | Complete infectious viral particle | Transmit virus between host cells |
Capsid | Protein coat of capsomeres | Protect viral genetic material |
Envelope | Host-derived lipid bilayer | Aid host cell attachment and entry |
Genetic material | Either DNA or RNA | Store instructions for new virions |
Bacteriophage tail fibers | Protein appendages | Bind to host bacterial receptors |
6. Frequently Asked
Are viruses considered living cells for CIE 9700?
No. Viruses are acellular (have no cell structure) and cannot carry out independent metabolic processes or replicate without a host, so they are not classified as living cells.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 1
Structure of bacteriophage multiple choice
- 2021 Β· 2
Compare virus and bacterial cell structure
- 2023 Β· 1
Enveloped virus features multiple choice
Going deeper
What's Next
Understanding virus structure is a foundational concept for multiple later topics in CIE A-Level Biology. This sub-topic reinforces the core distinction between cellular and acellular life, a key theme of Unit 1 cell structure. You will build on this knowledge when studying how viruses cause disease, how the immune system recognises viral pathogens, and how vaccines target viral surface proteins in later units of the syllabus.