IB Biology · Theme A: Unity and diversity · HL only
A2.3 Viruses
A one-page summary of A2.3 Viruses, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
How can viruses exist with so few genes?
In what ways do viruses vary?
What A2.3 covers
What every virus has — and how they vary
- A2.3.1What every virus shares
- A2.3.2Viruses are highly diverse
- A2.3.2Shape, and a molecular lock and key
Two ways to replicate
- A2.3.3The lytic cycle of phage lambda
- A2.3.4The lysogenic cycle: a quieter route
Where viruses come from, and where they're going
- A2.3.5Several origins from other organisms
- A2.3.5 continuedTwo competing hypotheses
- A2.3.6Why some viruses evolve fast
- A2.3.6Why HIV needs several drugs at once
A2.3 Viruses: summary
What every virus has
- Small fixed size; DNA or RNA; a protein capsid.
- No cytoplasm; few or no enzymes.
Diversity
- DNA or RNA, never both; enveloped or not; helical, icosahedral, or head-tail.
- Host specificity: attachment proteins must fit host receptors.
Lytic cycle
- Attachment → injection → replication → assembly → lysis.
- Ends with the host cell bursting, releasing new phages.
Lysogenic cycle
- Genome integrates as a prophage, copied passively with the host.
- Stress can trigger excision and a switch to lytic.
Origins
- Several origins: diversity, convergent evolution and a shared genetic code are the evidence.
- Progressive (escape) vs regressive (reduction) hypotheses.
Rapid evolution
- Drift = gradual point mutations; shift = abrupt segment reassortment.
- Combination therapy resists chance mutation better than a single drug.
Key terms
- Capsid
- The protein coat, built from repeating subunits, that encloses a virus's genome.
- Envelope
- An extra lipid layer, derived from host membrane, surrounding the capsid in some viruses.
- Bacteriophage
- A virus that infects bacteria, often with a head-and-tail structure.
- Lytic cycle
- The pathway in which a virus immediately replicates inside the host and lyses it.
- Lysogenic cycle
- The pathway in which a viral genome integrates as a prophage, replicated passively.
- Prophage
- A viral genome integrated into, and replicated as part of, the host chromosome.
- Antigenic drift
- The gradual accumulation of point mutations in viral surface proteins over time.
- Antigenic shift
- An abrupt change in viral surface proteins from segment reassortment between two strains.
Sample exam questions
Three of the 22 multiple-choice questions for A2.3. Try each one before opening the answer.
Question 1. Which feature is common to all viruses?
- A plasma membrane made of the virus's own phospholipids
- A protein capsid enclosing the genetic material
- Cytoplasm containing ribosomes
- A cell wall made of peptidoglycan
Show the answer
Answer: B. Every virus has a capsid, a coat built from protein subunits, that surrounds and protects its nucleic acid.
Question 2. Which statement best explains why viruses are described as non-cellular?
- They are simply too small to see with a light microscope
- They contain no genetic material at all
- They are made only of carbohydrate
- They have no cytoplasm, own membrane or organelles
Show the answer
Answer: D. A cell has cytoplasm bounded by its own membrane and carries out metabolism; a virus has none of these, so it is not a cell.
Question 3. One hypothesis is that some viruses evolved from the cells they infect. Which observation best supports this idea?
- Viruses are much smaller than the cells they infect
- Viruses cannot reproduce outside a host cell
- All viruses contain either DNA or RNA as a genome
- Some viral genes closely resemble host cell genes
Show the answer
Answer: D. The progressive (escape) hypothesis proposes that viruses arose from host genetic elements that escaped from cells, so viral genes that closely match host genes are evidence for it. The regressive hypothesis is a different proposal: that viruses arose from whole parasitic cells which lost genes by reductive evolution.
Linking questions
Questions that connect A2.3 to other parts of the course, the kind that come up in Paper 2.
- How does a virus that uses RNA as its genetic material differ from the DNA found in cellular organisms? (see A1.2)
- In what ways do viruses challenge the idea that all living organisms are made of cells? (see A2.2)
- How does the body's immune system respond to and defend against viral infection? (see C3.2)
- How does natural selection acting on mutations account for the rapid evolution of viruses such as influenza and HIV? (see A4.1, D4.1)
Practise A2.3
Study notes, every question and full markschemes for A2.3 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.