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

This study guide covers IB Diploma Programme Biology, sections A2.1 (Origins of cells) and A2.3 (Viruses), as set out in the IB Biology guide, first assessment 2025. Both sections are additional higher level, so everything here is HL only: there is no standard level content in either.

Use it with the revision notes and the practice questions. The wider theme is in Theme A: Unity and diversity. Course hub: IB DP Biology. Checklist: printable checklist. To find weak spots first, try the free 10-minute diagnostics.

What this unit covers

Section	What you must be able to do	Tier
A2.1.1	Describe early Earth conditions and the spontaneous formation of carbon compounds	HL only
A2.1.2	Discuss living versus non-living; explain why viruses count as non-living	HL only
A2.1.3	Outline why the origin of cells is hard to explain and hard to test	HL only
A2.1.4	Evaluate the Miller-Urey experiment	HL only
A2.1.5	Explain vesicle formation from fatty acids and why compartments matter	HL only
A2.1.6	Explain RNA as the presumed first genetic material	HL only
A2.1.7	Give evidence for a last universal common ancestor	HL only
A2.1.8	Outline approaches for dating the first cells and LUCA	HL only
A2.1.9	Give evidence for LUCA near hydrothermal vents	HL only
A2.3.1	State the features shared by all viruses	HL only
A2.3.2	Describe the diversity of virus structure	HL only
A2.3.3	Describe the lytic cycle using bacteriophage lambda	HL only
A2.3.4	Describe the lysogenic cycle using bacteriophage lambda	HL only
A2.3.5	Evaluate evidence for several origins of viruses	HL only
A2.3.6	Explain rapid virus evolution using influenza and HIV	HL only

A2.1.1 Early Earth and pre-biotic carbon compounds

Early Earth had almost no free oxygen, so there was no ozone layer and more ultraviolet light reached the surface. The atmosphere held more carbon dioxide and methane than today, giving higher temperatures.

These conditions may have driven chemical reactions that formed a variety of carbon compounds spontaneously. Such reactions do not happen today. This stage is called pre-biotic because it came before cells.

A2.1.2 The cell as the smallest self-sustaining living unit

A cell is the smallest unit that can sustain life on its own. To decide whether something is living, use the characteristics of life: metabolism, response to stimuli, homeostasis, growth, reproduction, nutrition and excretion, all done with its own machinery.

Viruses are classed as non-living. They have no cytoplasm, no ribosomes and few or no enzymes. They cannot respire, grow or make proteins alone, and they reproduce only by taking over a host cell. Outside a host they do nothing.

A2.1.3 The challenge of explaining the origin of cells

Cells are complex, and today they arise only by division of pre-existing cells. An explanation of the first cell must account for four requirements:

• catalysis, so reactions occur fast enough to matter
• self-replication of molecules, so information can be copied
• self-assembly of larger structures from smaller parts
• the emergence of compartmentalisation, so chemistry is separated from the surroundings

Nature of science. Scientific claims must be testable. Here the problem is that the exact conditions of pre-biotic Earth cannot be recreated, and the first protocells did not fossilise. Researchers test plausible steps one at a time.

A2.1.4 Evidence for pre-biotic carbon compounds: Miller-Urey

In 1952 Stanley Miller and Harold Urey sealed water, methane, ammonia and hydrogen in glass apparatus. Water was boiled, the vapour passed a chamber with electrical sparks standing in for lightning, then was cooled and condensed so products collected in a trap. After days, analysis showed amino acids.

Strengths. It showed that organic molecules found in living things can form from simple inorganic ones with an energy source. Later work on stored samples found more amino acid types than first reported. Amino acids have also been found in meteorites, supporting the idea that they form without life.

Limitations. The gas mixture was strongly reducing, and many geologists now think early Earth’s atmosphere was less so. The experiment made amino acids but no polymers, no membranes and no self-replicating molecule. It cannot show that this is what happened.

Worked example: interpreting data

A student repeats a Miller-Urey type experiment. Amino acids in the trap are measured.

Day	Sparks on (µmol)	Sparks off control (µmol)
3	14	0.1
7	41	0.4

1. Rate between days 3 and 7 with sparks: (41 − 14) ÷ (7 − 3) = 27 ÷ 4 = 6.75 µmol per day.
2. Percentage increase from day 3 to day 7: (41 − 14) ÷ 14 × 100 = 193%.
3. The control makes about 1% of the yield on day 7 (0.4 ÷ 41 × 100 = 0.98%), so the energy input is needed.
4. Evaluation: the data support spontaneous formation of amino acids, but not that early Earth’s atmosphere had this composition.

A2.1.5 Vesicles and membrane-bound compartments

Fatty acids are amphipathic: a hydrophilic head and a hydrophobic hydrocarbon tail. In water they can coalesce spontaneously into spherical bilayers, called vesicles, with tails inward and heads facing water on both sides.

A compartment matters because it lets the inside chemistry become different from the outside. Molecules can be kept at high concentration so reactions speed up and products are not lost by diffusion. A compartment also lets natural selection act, because each vesicle holds its own set of molecules.

A2.1.6 RNA as the presumed first genetic material

RNA can be copied by base pairing and also has some catalytic activity, so in the earliest cells a single molecule type could have carried the genes and done the enzymes’ work. This avoids the problem of which came first, since DNA needs proteins to be copied and proteins need DNA to be made. RNA catalysts are called ribozymes.

Evidence that this was real: ribozymes in the ribosome still catalyse peptide bond formation during protein synthesis. DNA, which is more stable, probably took over as the store, and proteins became the main catalysts.

A2.1.7 Evidence that LUCA existed

LUCA is the most recent organism from which all living things descend. Evidence:

• The genetic code is universal. Almost all organisms use the same codons for the same amino acids.
• Shared genes. Many genes, for example those for ribosome components and for DNA and RNA polymerases, are found in all three domains.

Other forms of life may well have evolved but became extinct, probably through competition from LUCA and its descendants. LUCA was therefore not the only early organism, only the one whose descendants survive.

A2.1.8 Dating the first cells and LUCA

Approaches to dating include:

• radiometric dating of rocks containing fossils or chemical traces, using the decay of radioactive isotopes
• fossil evidence such as layered microbial structures preserved in ancient rock
• molecular clocks, which compare gene sequences between species and use the number of differences to estimate the time since a common ancestor

Each has uncertainty, because old rocks are rare and single-cell fossils are hard to interpret. Appreciate the immense time span: Earth is about 4.5 billion years old, and life has existed for most of it.

Worked example: time on a clock face

Suppose Earth formed 4540 million years ago and the oldest accepted evidence of life is 3480 million years ago.

1. Time between: 4540 − 3480 = 1060 million years.
2. Scale Earth’s history to 24 hours: 1060 ÷ 4540 × 24 = 5.60 hours, which is about 05:36 after Earth forms.
3. Life has therefore been present for 3480 ÷ 4540 × 100 = 77% of Earth’s history.

A2.1.9 LUCA and hydrothermal vents

Hydrothermal vents are cracks on the seafloor where heated, mineral-rich water emerges. They offer chemical energy and protection from ultraviolet light. Two lines of evidence link LUCA to them:

• Fossilised evidence of life in ancient seafloor vent precipitates, such as tiny tubes and filaments. The oldest finds are debated.
• Conserved sequences. Genomic comparison of many species has identified gene families that were probably present in LUCA, about 350 in one analysis. They suggest an anaerobic organism that used hydrogen and carbon dioxide, consistent with a vent environment.

A2.3.1 Features common to viruses

Only a short list of features is found in every virus:

• a small and constant size
• DNA or RNA carrying the genetic information
• a protein capsid
• no cytoplasm
• very few enzymes, or none

Viruses are measured in nanometres and do not grow in size.

Worked example: calculating a virus size

A micrograph has a scale bar of 20 mm representing 50 nm. A capsid measures 38 mm across.

1. Magnification = 20 mm ÷ 50 nm = 20 000 000 nm ÷ 50 nm = ×400 000.
2. Actual size = image size ÷ magnification = 38 000 000 nm ÷ 400 000 = 95 nm.

A2.3.2 Structural diversity among viruses

Viruses differ widely in shape and structure. Their genetic material can be DNA or RNA, and either can be single-stranded or double-stranded. Some are enveloped by a layer of host cell membrane; others are not.

Virus	Genetic material	Envelope	Notes
Bacteriophage lambda	double-stranded DNA	no	head and tail; infects bacteria
Coronaviruses	single-stranded RNA	yes	spike proteins project from envelope
HIV	single-stranded RNA	yes	carries reverse transcriptase

A2.3.3 The lytic cycle

Viruses depend on the host cell for energy, nutrition and protein synthesis. In the lytic cycle of bacteriophage lambda:

1. Attachment. The phage tail binds to a specific receptor on the bacterial surface.
2. Entry. The phage injects its DNA; the capsid stays outside.
3. Synthesis. Host machinery copies phage DNA and makes capsid proteins.
4. Assembly. New capsids assemble and package DNA.
5. Lysis. The cell wall breaks down, the cell bursts and new phages are released.

Worked example: burst size

A culture contains 1.5 × 10⁷ infected bacteria per mL. After lysis the phage count is 3.6 × 10⁹ per mL. Burst size = 3.6 × 10⁹ ÷ 1.5 × 10⁷ = 240 phages per cell.

A2.3.4 The lysogenic cycle

After injection, lambda DNA can instead integrate into the host chromosome as a prophage. A repressor protein keeps lytic genes switched off. The prophage is copied each time the host divides, passing to daughter cells without harm. Stress such as DNA damage can trigger induction: the prophage leaves the chromosome and the lytic cycle begins.

A2.3.5 Evidence for multiple viral origins

The diversity of viruses suggests more than one origin. Possible origins include escaped fragments of host genetic material or reduced parasitic cells. All viruses live as extreme obligate parasites, so features they have in common, such as a capsid and small genome, may be convergent evolution. Viruses also read the same genetic code as cellular organisms, which is consistent with viruses having arisen from or alongside cellular life.

A2.3.6 Rapid evolution in viruses

Reasons for very rapid evolution: short generation times, huge numbers of offspring, and, in many RNA viruses, replication enzymes that lack proofreading, so mutation rate is high.

• Influenza. Point mutations in surface proteins cause gradual change; the genome is in separate segments, so mixing of segments between strains can cause sudden change. Vaccines have to be reformulated regularly.
• HIV. Reverse transcriptase makes errors when copying RNA to DNA. Drug resistance arises in a patient, so combinations of drugs are used.

Worked example: mutations per genome

HIV genome length is 9700 nucleotides. With an error rate of 3.0 × 10⁻⁵ per nucleotide per copy, mutations per genome = 9700 × 3.0 × 10⁻⁵ = 0.29. About one new genome in four differs from its parent.

Common errors

• Saying Miller and Urey made life. They made amino acids.
• Saying RNA is more stable than DNA. It is not; RNA is the more reactive.
• Placing the prophage free in the cytoplasm. It is integrated in the host chromosome.

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).