Describe virus structure without calling viruses cells
Viruses are non-cellular structures containing DNA or RNA inside a protein capsid; some also have a phospholipid envelope.
AS Level · Cambridge International Biology 9700, 2028–2030. Interpret only the structures supported by the provided image or description. Written practice is self-checked; it does not demonstrate practical microscopy competence.
Before you begin
- Recognise that organisms are made of cells and that a light microscope produces a magnified image.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What forms the capsid of a virus?
What you will learn
- State the components of a virus and the envelope found in some viruses
- Distinguish a non-cellular virus from a prokaryotic or eukaryotic cell
A nucleic acid core and protein capsid
All viruses are non-cellular structures with a nucleic acid core made of either DNA or RNA and a capsid made of protein. The capsid surrounds and protects the nucleic acid. A virus is not a cell and does not have the full organisation of a prokaryotic or eukaryotic cell.
Some viruses also have an outer envelope made of phospholipids. The envelope is not present in every virus. Do not describe every virus as having DNA, a nucleus, cytoplasm or ribosomes.
Classify from the stated components
A virus is identified by its nucleic acid and protein capsid, not by cell organelles. A virus contains either DNA or RNA as its genetic material; questions may specify which.
The presence or absence of an envelope does not change the fact that a virus is non-cellular. A virus relies on a host cell for replication and protein synthesis; avoid claiming that a virus can carry out independent cellular metabolism.
Pause and explain
Which statement describes all viruses in this syllabus?
Worked example
A particle contains RNA enclosed by a protein coat and has an outer phospholipid layer. Describe its structure.
Show the worked solution
- RNA is the nucleic acid core.
- The protein coat is the capsid.
- The outer layer is an envelope.
Answer It is a non-cellular virus with an RNA core and a protein capsid. The outer phospholipid layer is an envelope; some viruses have one.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the basic structure shared by all viruses.
Give me a hint
Include the genetic material and capsid.
Compare my reasoning
- A virus is non-cellular.
- Its core contains DNA or RNA surrounded by a protein capsid.
All viruses are non-cellular structures with a nucleic acid core of either DNA or RNA and a protein capsid.
Look for these in your work
- Non-cellular stated
- Core and capsid included
A virus has no phospholipid envelope. Does that rule out its identity as a virus? Explain.
Give me a hint
Check whether the envelope is universal.
Compare my reasoning
- Only some viruses have an outer envelope.
- The core and capsid are shared features.
No. Some viruses have an outer phospholipid envelope, but not all do. A non-cellular particle with a DNA or RNA core and a protein capsid is consistent with the shared virus structure.
Look for these in your work
- Envelope variation stated
- Core and capsid referenced
A student describes a virus as a tiny cell with a nucleus and ribosomes. Correct the description.
Give me a hint
Use the non-cellular features.
Compare my reasoning
- Viruses are non-cellular.
- They have a DNA or RNA core and a protein capsid, but no nucleus or ribosomes.
A virus is not a tiny cell. It is a non-cellular structure containing either DNA or RNA within a protein capsid. Some viruses have a phospholipid envelope, but viruses do not have a nucleus or ribosomes.
Look for these in your work
- Non-cellular status
- Correct structures
Common mistakes
- Calling the capsid a cell wall
- Claiming every virus contains DNA and RNA together
- Treating the envelope as a feature of all viruses
What can you explain now?
What forms the capsid of a virus?
Compare with the explanation
Protein
The capsid is made of protein.
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Make it stick
Which virus structures are universal, and which feature is present only in some viruses?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.