Continuity and Change
IB Biology HL — Topic D: Continuity and Change
This topic is where IB Biology gets genuinely mechanistic — you're expected to reason through what happens if X protein can't bind Y sequence, not just recall definitions. The lac operon questions in every exam bank test the same skill: trace the control logic step by step (repressor → operator → RNA polymerase → transcription) rather than pattern-matching to 'induced' or 'repressed'. Treat every scenario as a circuit diagram with a fault in it.
Overview — The Shape of the Topic
What ties these twelve subtopics together
Continuity and change is fundamentally about how information is copied faithfully (DNA replication, protein synthesis) and how it is used flexibly and controlled in response to environment (gene expression, homeostasis, mutation, natural selection). The exam constantly asks you to trace a single change through a control system — block one protein, one binding site, one enzyme — and predict the downstream consequence.
- Molecular strand: DNA replication → protein synthesis → gene expression control (operons, splicing) → mutation/editing.
- Cellular strand: cell and nuclear division, homeostasis, water potential — how cells maintain and transmit stable internal conditions.
- Population strand: reproduction, inheritance, sustainability, climate change, natural selection — how traits and populations change over generations.
The shape of the chapter
Command terms that dominate this topic
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Predict | State the outcome using reasoning from a rule/model — you must justify, not just assert. | AO3 | Bare answer with no mechanism loses the reasoning mark even if the conclusion is correct. |
| Explain | Give reasons/mechanism, usually 'because...' linking cause to effect. | AO2 | Restating the question ('expression increases because more is expressed') scores zero. |
| Compare and contrast | Similarities AND differences, explicitly stated. | AO2 | Listing only differences forfeits the similarity marking point. |
| Deduce | Reach a conclusion from given data using a stated rule (e.g. Hardy–Weinberg, chi-squared). | AO3 | Working must be shown — final number alone earns partial credit only. |
Key point
Overview