Continuity and Change
Topic D — Continuity and Change
This topic has two engines running side by side: continuity — the machinery that copies and passes on genetic information faithfully (DNA replication, mitosis, meiosis, inheritance) — and change — the sources of variation and the pressures that act on it (mutation, natural selection, climate change). Water potential and homeostasis sit here because both are about systems resisting or responding to change at the cell and organism level. Exam questions love to test whether you can reason from a number (water potential, allele ratio) to a direction (water movement, phenotype) — that's where marks are lost.
Overview
Every subtopic here answers one of two questions: how is information copied accurately? or how does information change, and with what consequence? DNA replication and mitosis are about fidelity — getting an identical copy. Meiosis, mutation, and natural selection are about generating and filtering variation. Water potential and homeostasis are the physiology underneath both stories — cells must survive and regulate before any of the genetics matters.
- Continuity strand: water potential (cell survival), DNA replication, cell/nuclear division, inheritance.
- Change strand: mutation and gene editing, natural selection, climate change, sustainability.
- Protein synthesis and homeostasis link both — genes are only useful once expressed and regulated.
The shape of the chapter
Command term guidance
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Determine | Obtain a numerical answer showing the working (e.g. read water potential off a graph). | AO2 | No working shown = lose the method mark even if final value correct. |
| Explain | Give reasons, not just describe — link cause to mechanism to outcome. | AO2 | Describing 'water moves in' without stating why (ψ gradient) caps the mark. |
| Predict | State outcome using a pattern/data not explicitly given. | AO3 | Must commit to a direction/value, not just 'it will change'. |
| Compare | Give similarities AND differences explicitly. | AO3 | Two separate descriptions with no linking word scores as description, not comparison. |
| Outline | Brief account of main points only. | AO1 | Over-long essay-style answers waste time without extra marks. |
Key point
Overview