Identifying phases at Higher Level: The same skill as SL. At HL the 1B items often chain three parts on one micrograph: order the cells, identify one, name a missing phase; mitotic index then follows on 4.4.17.
Practise this as you read
- List labelled cells in order of the phases.
- State a phase not shown in a micrograph.
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The big idea: To name the phase of a cell in a diagram or micrograph, look at where the chromosomes are and whether there is a nucleus.
The points to remember
- Interphase: a round nucleus; no separate chromosomes visible.
- Prophase: thick, visible chromosomes; the nucleus is breaking down.
- Metaphase: chromosomes in a line across the middle.
- Anaphase: two groups moving apart, often V-shaped.
- Telophase: two groups at the ends; new nuclei forming.
Remember it as: Look where the chromosomes are.
Real example: why most cells are in interphase. In a root tip most cells are in interphase, because interphase takes far longer than mitosis. So in any field of view, cells caught in metaphase or anaphase are much rarer than cells in interphase.
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Making and reading a root-tip squash: Root tips are used because their cells divide often. The tip is softened, stained so the chromosomes show up dark, and squashed flat so the cells form a single layer.
| You see | Phase |
|---|---|
| Nucleus, no separate chromosomes | Interphase |
| Thick chromosomes, nuclear membrane fading | Prophase |
| Chromosomes lined up across the middle | Metaphase |
| Two groups pulled apart towards the ends | Anaphase |
| Two groups at the ends, new nuclei, cell dividing | Telophase |
Ordering cells: Asked to put labelled cells in order? Start with interphase if it is there, then prophase, metaphase, anaphase, telophase. Older (2016) SL papers also ask about the proportion of cells that are dividing: a high proportion means the tissue is growing fast.
How it is asked: Almost always with a micrograph or diagram: identify the phase of a labelled cell, put labelled cells in order, name a phase that is missing, or predict what happens next.
Cells P, Q, R and S in a root tip are in prophase (R), anaphase (P), interphase (S) and metaphase (Q). List the letters in the order the cell passes through them.
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