IB DP Biology: Origins of cells and viruses (HL) (DP Biology) – Revision Notes

These revision notes condense IB Diploma Programme Biology sections A2.1 (Origins of cells) and A2.3 (Viruses) from the IB Biology guide, first assessment 2025. Both sections are HL only. For full explanations and worked examples, read the study guide, then test yourself with the practice questions.

Related pages: Theme A: Unity and diversity revision notes, the IB DP Biology hub, the printable checklist and the free 10-minute diagnostics.

A2.1 Origins of cells: the chain of ideas

Read this as a sequence. Each step needs the one before.

1. A2.1.1 Early Earth: no free oxygen, so no ozone and more ultraviolet light; more carbon dioxide and methane; higher temperatures. Carbon compounds formed spontaneously by processes that do not now occur.
2. A2.1.4 Evidence: Miller-Urey made amino acids from water, methane, ammonia and hydrogen with sparks.
3. A2.1.5 Fatty acids coalesce into spherical bilayers (vesicles), giving a compartment.
4. A2.1.6 RNA both stores information and catalyses reactions.
5. A2.1.2 and A2.1.3 Cells are the smallest self-sustaining units; their origin needs catalysis, self-replication, self-assembly and compartmentalisation.
6. A2.1.7 to A2.1.9 LUCA, its date, and its likely home near hydrothermal vents.

Key terms

Term	Meaning
Pre-biotic	Before cells existed
Protocell	A hypothesised early membrane-bound structure with simple chemistry
Vesicle	Spherical bilayer, hydrophobic tails inward
Ribozyme	RNA molecule with catalytic activity
LUCA	Last universal common ancestor: the most recent organism from which all life descends
Molecular clock	Dating by counting sequence differences between species
Capsid	Protein coat of a virus
Envelope	Host membrane layer around some viruses
Prophage	Phage DNA integrated into the host chromosome
Obligate parasite	Cannot reproduce without a host

Evaluating Miller-Urey

Supports the idea	Limits the idea
Organic molecules formed from simple inorganic ones	The gas mixture is thought by many to be too reducing
An energy source drove the reactions	Only small molecules formed: no polymer, membrane or replicator
Amino acids also occur in meteorites	Shows what is possible, not what happened

Always give both columns when the command word is evaluate, then reach a judgement.

Living versus non-living

A cell shows the characteristics of life using its own machinery: metabolism, response, homeostasis, growth, reproduction, nutrition and excretion. Viruses are non-living because they have no cytoplasm, no ribosomes, few or no enzymes, and no metabolism of their own. They reproduce only inside a host.

RNA first

• RNA is replicated by complementary base pairing.
• RNA can fold and catalyse, so it could act as gene and enzyme.
• Evidence: ribozymes in the ribosome still catalyse peptide bond formation.
• Later: DNA took over information storage, proteins took over catalysis.

LUCA: evidence and dating

Evidence for LUCA	Link to vents	Dating methods
Universal genetic code	Fossilised life in ancient vent precipitates	Radiometric dating of rocks
Genes shared by all organisms	Conserved sequences from genomic analysis	Microbial fossils in old rock
Other lineages probably became extinct	Anaerobic, hydrogen-using organism inferred	Molecular clocks

Remember the nature of science point: the first protocells did not fossilise and pre-biotic conditions cannot be recreated, so hypotheses here are hard to test.

A2.3 Viruses

Common features (A2.3.1)

Small fixed size, DNA or RNA genetic material, protein capsid, no cytoplasm, few or no enzymes.

Diversity (A2.3.2)

Virus	Genetic material	Enveloped?
Bacteriophage lambda	double-stranded DNA	no
Coronaviruses	single-stranded RNA	yes
HIV	single-stranded RNA	yes

Lytic versus lysogenic (A2.3.3 and A2.3.4), bacteriophage lambda

Stage	Lytic cycle	Lysogenic cycle
Start	Attachment, DNA injected	Attachment, DNA injected
Viral DNA	Copied at once	Integrated as a prophage
Host cell	Makes new phages, then bursts	Survives and divides
Outcome	New phages released by lysis	Prophage copied to daughter cells
Switch	None	Stress can trigger induction, then lytic cycle

Method for a lytic cycle answer: attach, inject, synthesise, assemble, lyse. Name the order every time.

Origins and evolution (A2.3.5 and A2.3.6)

• Diversity suggests several origins.
• Shared extreme obligate parasitism means common features may be convergent evolution.
• The genetic code is shared with living organisms.
• Fast evolution: short generations, large numbers, high mutation rate, no proofreading in many RNA viruses.
• Influenza: gradual change by point mutations, sudden change by mixing segments; vaccines reformulated.
• HIV: error-prone reverse transcriptase; resistance arises; combination drug treatment.

Must-know distinctions

• LUCA is not the first life. It is the most recent organism that all living things descend from. Other lineages probably existed and went extinct, likely through competition with LUCA and its descendants.
• Gene versus enzyme in RNA world. One molecule type did both jobs. Later they separated: DNA stored information and proteins catalysed.
• Capsid versus envelope. The capsid is protein and every virus has one. The envelope is host membrane and only some viruses have it.
• Influenza change. Small steady change comes from point mutations. Sudden large change comes from mixing of genome segments between strains.
• Pre-biotic chemistry versus cells. Chemistry produced molecules. Compartments, replication and catalysis together are needed before something counts as a cell.
• Evidence versus explanation. A universal genetic code is evidence for common ancestry. It does not say where the ancestor lived; that comes from the vent fossils and conserved sequences.

Small worked reminders

Rate of amino acid formation. 14 µmol on day 3 and 41 µmol on day 7: (41 − 14) ÷ (7 − 3) = 6.75 µmol per day.

Time scale. If life appears 1060 million years after a 4540 million year old Earth formed, that is 1060 ÷ 4540 × 100 = 23% of the way through. Life has existed for the remaining 77%.

Lambda burst size. 3.6 × 10⁹ phages from 1.5 × 10⁷ infected cells gives 240 per cell.

Mutations per genome. Genome length × error rate. A value below 1 means most copies are identical to the parent, but huge numbers of copies still produce many variants.

Command words for this unit

• Describe: give the stages in order, for example the five phases of the lytic cycle.
• Explain: give the reason as well, for example why a compartment lets internal chemistry differ.
• Discuss: give more than one view, for example why viruses are classed as non-living even though they have genes.
• Evaluate: weigh strengths and limitations, then judge. Use this for Miller-Urey and for evidence on viral origins.
• Outline: a brief account, for example the approaches used to date LUCA.

Method boxes

Actual size from a micrograph. Convert the scale bar to the same units. Magnification = bar length ÷ real length. Real size = image size ÷ magnification. Show the units.

Burst size. Burst size = phages released ÷ infected cells.

Percentage change. (new − old) ÷ old × 100.

Quick self-test

1. State two features that all viruses share.
2. A micrograph shows a virus 60 mm wide at ×300 000. Calculate its actual diameter in nm.
3. A virus is 150 nm across. How wide is its image at ×200 000, in mm?
4. Name the structure that lets fatty acids create an inside different from the outside.
5. What is a ribozyme, and where is one still used today?
6. A culture has 8.0 × 10⁵ infected cells. After lysis it holds 2.0 × 10⁸ phages. Calculate the burst size.
7. In an experiment, amino acids rise from 20 µmol on day 2 to 62 µmol on day 8. Calculate the mean rate and the percentage increase.
8. A genome of 11 000 nucleotides has an error rate of 2.0 × 10⁻⁵ per nucleotide per copy. Calculate mutations per genome copy.
9. Give two reasons why a lambda prophage is passed on without killing its host.
10. State one reason the Miller-Urey result does not prove how life began.

Answers

1. Any two: nucleic acid as genetic material, a protein capsid, small fixed size, no cytoplasm, few or no enzymes.
2. 60 mm = 60 000 000 nm; ÷ 300 000 = 200 nm.
3. 150 nm × 200 000 = 30 000 000 nm = 30 mm.
4. A vesicle (spherical bilayer).
5. An RNA molecule with catalytic activity; in the ribosome, where it catalyses peptide bond formation.
6. 2.0 × 10⁸ ÷ 8.0 × 10⁵ = 250.
7. Rate = 42 ÷ 6 = 7.0 µmol per day; increase = 42 ÷ 20 × 100 = 210%.
8. 11 000 × 2.0 × 10⁻⁵ = 0.22.
9. It is integrated into the host chromosome and a repressor keeps lytic genes off, so it is copied with the host DNA at division.
10. It shows what can happen, not what did; the gas mixture may not match early Earth, and no polymers or membranes were made.

Where marks are usually lost

• Calling viruses living, or saying they respire. They are non-living.
• Writing that Miller and Urey created life instead of amino acids.
• Giving only strengths or only limitations when asked to evaluate.
• Stating that early Earth had oxygen, or leaving out the effect of no ozone layer.
• Mixing lytic and lysogenic steps, for example saying the host bursts in lysogeny.
• Forgetting that the capsid stays outside after a phage injects its DNA.
• Skipping unit conversion between mm and nm in magnification questions.
• Naming HIV as DNA-based, when it has an RNA genome and uses reverse transcriptase.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Sections A2.1 and A2.3 (additional higher level).