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DSE Biology: the cell cycle and mitosis (tutor lesson plan)
Students can chant PMAT but cannot count chromosomes against chromatids. Every number here is derived from 46 and 23, and a 24-hour cycle shows that most of the time is not division at all.
Objectives — Students can: (1) state the order and relative length of the cell cycle stages and calculate that mitosis occupies about 4% of a 24-hour cycle; (2) give the correct chromosome and chromatid counts across interphase, prophase, metaphase, anaphase and telophase, distinguishing 46 from 92; (3) use the checkpoint idea to explain why a replication error does not automatically become cancer.
01 Prerequisites (3-minute check)
If any of these fails, fix it before the lesson starts.
| Skill | Quick check | Pass mark |
|---|---|---|
| DNA and genes | Where is a gene located? | A DNA segment on a chromosome |
| Microscopy | How do you recognise two separated chromosomes? | The centromere has split |
| Percentages | What percentage of 24 hours is one hour? | About 4.2% |
02 Lesson flow (45 minutes)
| Time | Stage | Tutor does | Student does |
|---|---|---|---|
| 0–8 min | Count first | Start from 46 chromosomes in 23 pairs and ask for the chromatid count after replication | Answer 92 |
| 8–20 min | Three-part interphase | G1, S and G2 roles, placed on a 24-hour clock | S phase copies, it does not divide |
| 20–32 min | The four stages | Draw each stage with exactly one observable feature | One feature per stage |
| 32–38 min | Number table | Fill chromosome and chromatid counts for five states | All five correct |
| 38–43 min | Checkpoints | Use checkpoints to link mutation with cancer | Stop, repair or apoptosis |
| 43–45 min | Exit check | Mark on the spot and redraw the missed stage | All three correct |
03 Teaching script (read it out)
Scene 1: Settle 46 against 92 first
A human body cell has 46 chromosomes, that is 23 pairs, one from each parent.
S phase replicates each chromosome into two sister chromatids held at one centromere, so the cell then has 46 chromosomes and 92 chromatids. The chromosome count has not changed; the amount of DNA has doubled.
This is the foundation of the whole chapter. A student who says “92 chromosomes after replication” will miss all four stages later. Fix it with one line: count centromeres and you have counted chromosomes.
After mitosis one parent cell gives two daughter cells, each back to 46 chromosomes with one chromatid per chromosome.
S phase replicates each chromosome into two sister chromatids held at one centromere, so the cell then has 46 chromosomes and 92 chromatids. The chromosome count has not changed; the amount of DNA has doubled.
This is the foundation of the whole chapter. A student who says “92 chromosomes after replication” will miss all four stages later. Fix it with one line: count centromeres and you have counted chromosomes.
After mitosis one parent cell gives two daughter cells, each back to 46 chromosomes with one chromatid per chromosome.
Student must say: after S phase, is the chromosome count 46 or 92, and why?
Scene 2: One observable feature per stage
Prophase: chromatin condenses into visible chromosomes, the nuclear envelope breaks down and the spindle forms.
Metaphase: chromosomes line up on the equator, centromeres attached to spindle fibres, the easiest stage to recognise under the microscope.
Anaphase: centromeres split and sister chromatids are pulled to opposite poles, so the chromosome count temporarily becomes 92.
Telophase: nuclear envelopes re-form, chromosomes decondense, cytoplasm divides and two daughter cells appear.
Teach it by asking one question per stage: what do you actually see changing down the microscope? If the student cannot answer, go back to the drawing, not the paragraph.
Metaphase: chromosomes line up on the equator, centromeres attached to spindle fibres, the easiest stage to recognise under the microscope.
Anaphase: centromeres split and sister chromatids are pulled to opposite poles, so the chromosome count temporarily becomes 92.
Telophase: nuclear envelopes re-form, chromosomes decondense, cytoplasm divides and two daughter cells appear.
Teach it by asking one question per stage: what do you actually see changing down the microscope? If the student cannot answer, go back to the drawing, not the paragraph.
Student must say: why is the chromosome count 92 in anaphase rather than 46?
Scene 3: Checkpoints: not every error becomes cancer
In a 24-hour cycle mitosis takes about 1 hour, roughly 4.2%; the rest is growth, copying and checking. The G1/S checkpoint tests DNA for damage, G2 tests whether replication is complete, and the M checkpoint tests whether every centromere is attached to the spindle.
Fail a checkpoint and the cell pauses to repair; if repair fails it undergoes apoptosis. Only when checkpoint control genes such as TP53 are lost as well does a damaged cell keep dividing, and that is one necessary condition for cancer, not “fast division causes cancer”.
Require a full causal sentence here; “cells divide so they become cancer” is not accepted.
Fail a checkpoint and the cell pauses to repair; if repair fails it undergoes apoptosis. Only when checkpoint control genes such as TP53 are lost as well does a damaged cell keep dividing, and that is one necessary condition for cancer, not “fast division causes cancer”.
Require a full causal sentence here; “cells divide so they become cancer” is not accepted.
Student must say: which comes first, checkpoint failure or DNA mutation, and why does the order matter?
04 Question types and worked steps
| Type | Example | Steps | Answer |
|---|---|---|---|
| Number judgement | Give chromosome and chromatid counts after S phase, in anaphase and at the end of telophase | Anchor on centromere count equals chromosome count, tabulate each state, never guess | After S: 46/92. Anaphase: 92/0. End of telophase: 46/0 |
| Stage matching | Match the four stages to nuclear envelope breakdown, equatorial alignment, centromere splitting, envelope re-formation | Lock one stage with one observable feature, then eliminate | Prophase breakdown, metaphase equator, anaphase split, telophase re-form |
| Image interpretation | A cell with chromosomes lined up centrally: which stage? | Check single-row alignment on the equator and whether centromeres have split | Metaphase, or anaphase if already pulled apart |
| Application | Why does a drug that blocks spindle formation slow tumour growth? | Spindle function, then failure to align and separate, then M checkpoint arrest, then stalled division | Chromosomes cannot segregate, the checkpoint halts division and growth slows |
Sanity check (teach this): Numbers checked by code: 46 chromosomes is 23 pairs; after S phase there are 92 chromatids but still 46 chromosomes; mitosis gives two daughter cells of 46 each; one hour of a 24-hour cycle is 4.17%. Every figure here is asserted, so do not improvise new numbers in class.
05 Independent practice (3 questions)
| Question | Answer | What to watch |
|---|---|---|
| A parent cell has 46 chromosomes. After one mitosis, how many chromosomes in each daughter cell? | 46 each, identical to the parent, because replication is followed by equal segregation. | Whether the student sees copy-then-share rather than halving |
| Fill the table: chromosome and chromatid counts in prophase, metaphase, anaphase and end of telophase. | Prophase 46/92, metaphase 46/92, anaphase 92/0, end of telophase 46/0. | Anaphase at 92, and whether split chromatids are recognised as chromosomes |
| Using checkpoints, explain why sunburnt skin cells do not usually become cancer. | Damage triggers the G1/S or G2 checkpoint, the cell pauses to repair and otherwise undergoes apoptosis, so most damaged cells never divide again. | Whether pause, repair and apoptosis are all named |
06 Common mistakes and what to say
| Mistake | Student writes | Say this |
|---|---|---|
| 92 chromosomes after replication | “After S phase there are 92 chromosomes.” | “Count centromeres and you have counted chromosomes. Replication doubles chromatids, not chromosomes.” |
| Confusing anaphase with telophase | “Once chromosomes reach the poles it is telophase.” | “Reaching the poles is still anaphase; telophase starts when the envelope re-forms and the cytoplasm divides.” |
| Equating the cycle with mitosis | “The cell cycle is those four stages.” | “Division is about 4%. The other 96% is interphase, and without it there is nothing to divide.” |
| Fast division causes cancer | “Cancer happens because cells divide too fast.” | “Cancer follows checkpoint failure. Name the checkpoint that failed before you talk about speed.” |
07 Homework (paper, section and question numbers)
- Redo the independent practice on this page — Questions 1 to 3: redraw the five-state number table from memory and use pause, repair, apoptosis in the application answer.
- Preview the next chapter: inheritance and DNA — Find the inheritance chapter on the subject index, read its first section and draft an answer to “the biggest difference between mitosis and meiosis”.
08 Exit check (last 5 minutes)
| Question | Answer | Decision |
|---|---|---|
| Chromosome count after S phase? | 46, with 92 chromatids | Answering 92 means chromosome and chromatid are still confused; reteach act one |
| In which stage do centromeres split? | Anaphase | Answering metaphase means only the “middle” feature was memorised |
| Mitosis as a percentage of a 24-hour cycle? | About 4.2% | Answering “half” means the cycle is still treated as division |
09 Tutor FAQ
Must students memorise the English stage names?
Yes. DSE papers use English, and PMAT is the skeleton of every answer, but each name must carry one observable feature.
Simplest way to separate chromosome from chromatid?
Count centromeres: two sister chromatids sharing one centromere still count as one chromosome.
Why use a 24-hour example?
It turns “interphase is most of the cycle” into a percentage students can calculate, so the cycle stops being a synonym for division.
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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.
