Classify viruses by their genomes
Classify viruses by DNA or RNA and whether the nucleic acid is single or double stranded.
Section 18.1 · Classification
A Level; Cambridge International Biology 9700, 2028–2030. Written responses are self-checked; this does not certify supervised fieldwork or practical competence.
Before you begin
- Classification compares shared characteristics; populations occupy environments and contain variation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What two genome properties are used in the specified virus classification?
What you will learn
- Use nucleic-acid type to classify a virus
- Distinguish single-stranded from double-stranded viral genomes
Build the explanation
For this syllabus outcome, classify a virus by genome nucleic acid (DNA or RNA) and whether that genome is single stranded or double stranded. These two properties distinguish categories such as single-stranded RNA and double-stranded DNA viruses.
Use evidence within scope
Virus classification here is limited to genome type and strand number. Viruses are not cells and are not placed in the three cellular domains; do not classify them by a host or disease when the question asks for genome type.
Pause and explain
A record says “RNA virus” but gives no strand information. What classification detail is missing?
Worked example
A virus contains single-stranded RNA. State its classification by the specified features.
Show the worked solution
- Identify the nucleic-acid type.
- Identify the number of strands.
Answer An RNA virus with a single-stranded genome.
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Test this topic →From guided practice to a new situation
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.
A genome is double-stranded DNA. Classify the virus by the specified features.
Give me a hint
Identify the required evidence and sequence the reasoning.
Compare my reasoning
- Name DNA.
- State that it is double stranded.
A double-stranded DNA virus.
Look for these in your work
- Relevant evidence selected
- Reasoning linked to conclusion
Why does a virus with RNA not belong to the domain Archaea?
Give me a hint
Use the supplied example or data to justify a conclusion.
Compare my reasoning
- The domains classify cellular organisms.
- Viruses are not cells.
The domains classify cellular life, while viruses are not cells; genome type alone does not place a virus in Archaea.
Look for these in your work
- Concept applied
- Limit or assumption stated
A record says “RNA virus” but gives no strand information. What classification detail is missing?
Give me a hint
Apply the concept in a new context and qualify its limits.
Compare my reasoning
- RNA type is known.
- The strand number remains unspecified.
Whether the RNA is single stranded or double stranded is missing.
Look for these in your work
- New context handled
- Conclusion appropriately bounded
Common mistakes
- Calling a virus a prokaryote
- Confusing an RNA virus with a single-stranded DNA virus
What can you explain now?
What two genome properties are used in the specified virus classification?
Compare with the explanation
DNA or RNA, and single or double stranded
Both the nucleic-acid type and strand number are required.
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Make it stick
Which genome details distinguish the virus categories required here?
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