Biology • Year 12 • Module 5 • Lesson 7
Mitosis — Maintaining Genetic Stability in Somatic Cells
Apply the stages of mitosis to real growth data, a wound-healing scenario, a flawed student diagram, and a cell-cycle data table.
1. Sequence the steps — one round of mitotic division
The seven events below have been listed out of order. In the right-hand column, number them 1 to 7 in the order they occur during one complete round of cell division. 7 marks (1 per correctly placed event)
| Letter | Event (shuffled) | Order (1–7) |
|---|---|---|
| 1.1 | Sister chromatids are pulled to opposite poles of the cell. | |
| 1.2 | Chromosomes condense and the nuclear envelope begins to break down. | |
| 1.3 | The cytoplasm is divided, producing two separate daughter cells. | |
| 1.4 | DNA replication produces two identical sister chromatids per chromosome. | |
| 1.5 | New nuclear envelopes re-form around each set of chromosomes. | |
| 1.6 | Replicated chromosomes line up along the cell equator. | |
| 1.7 | Each daughter cell ends with the same chromosome number as the parent somatic cell. |
2. Data response — wound healing in skin (mitosis in action)
A team monitored a 1.0 cm² shallow wound on the forearms of healthy volunteers. They counted the number of dividing (mitotic) basal-layer cells per mm² of wound edge at four time points after the injury, and measured the wound area still open. 7 marks
| Time after wounding (hours) | Mitotic cells / mm² at wound edge | Wound area still open (% of original) |
|---|---|---|
| 0 | 3 | 100% |
| 24 | 32 | 92% |
| 72 | 78 | 54% |
| 168 (1 week) | 22 | 8% |
Hypothetical data, after typical re-epithelialisation profiles reported in human skin-healing studies.
2.1 Describe the relationship between mitotic-cell count at the wound edge and the percentage of the wound still open over the 168-hour period. 2 marks
2.2 Using lesson content, explain why mitosis (not meiosis) is the type of cell division observed in the basal layer during this repair process. 3 marks
2.3 The mitotic count drops sharply between 72 and 168 hours, even though the wound is not fully closed. Predict why the rate of mitosis falls, using the lesson's "growth and repair" framing. 2 marks
3. Graph response — mitotic index varies between tissues
The mitotic index is the proportion of cells in a tissue that are visibly undergoing mitosis at a given moment. The graph below shows typical mitotic-index values (per 1000 cells) measured in five human tissues. 7 marks
Typical adult mitotic-index values per 1000 cells, drawn from histology references (illustrative).
3.1 Identify the tissue with the highest mitotic index and state its approximate value. 1 mark
3.2 Bone marrow, gut lining and skin all have high mitotic indices. Using lesson content, explain why these tissues need a high rate of mitosis. 3 marks
3.3 Cardiac muscle has a mitotic index close to zero. Predict one clinical consequence of this fact after a heart attack damages cardiac tissue, and link it back to the role of mitosis. 3 marks
4. Diagram critique — what's wrong with this student's mitosis diagram?
A Year 12 student has drawn the diagram below to summarise mitosis. There are three biological errors. Identify each error and write the correction. 6 marks (2 per error: 1 identify, 1 correct)
4.1 Error 1: What is wrong?
Correction:
4.2 Error 2: What is wrong?
Correction:
4.3 Error 3: What is wrong?
Correction:
Q1 — Correct order
1.4 → 1.2 → 1.6 → 1.1 → 1.5 → 1.3 → 1.7. That is: 1 DNA replication produces two identical sister chromatids per chromosome → 2 chromosomes condense and the nuclear envelope begins to break down (prophase) → 3 replicated chromosomes line up along the cell equator (metaphase) → 4 sister chromatids are pulled to opposite poles (anaphase) → 5 new nuclear envelopes re-form around each set of chromosomes (telophase) → 6 the cytoplasm is divided, producing two separate daughter cells (cytokinesis) → 7 each daughter cell ends with the same chromosome number as the parent somatic cell.
Q2.1 — Trend (2 marks)
The number of mitotic cells at the wound edge rises sharply over the first 72 hours (from 3 to 78 per mm²) while the open wound area falls from 100% to 54% [1]. Mitotic counts then fall back towards baseline (to 22 per mm² at 168 h) as the wound nears closure (only 8% still open) — the two variables show a strong inverse relationship across the period, with mitosis driving the closure [1].
Q2.2 — Why mitosis, not meiosis (3 marks)
The basal layer of the skin is made up of somatic cells, and the cells produced during repair must function as more skin cells — therefore they need the same chromosome number and the same hereditary information as the parent cells [1]. Mitosis is the cell division that maintains chromosome number and produces genetically identical daughter cells suitable for replacing damaged tissue [1]. Meiosis would halve chromosome number and produce gametes, which would not function as replacement skin cells and would be unable to maintain the tissue [1].
Q2.3 — Why mitosis slows by 168 h (2 marks)
The lesson frames mitosis as a process used for growth and repair, not as something running constantly at maximum rate. Once most of the lost epidermis has been replaced (only 8% open at 168 h), the demand for new cells drops [1]. Body tissues regulate mitosis so that division slows back to baseline when the tissue is restored — otherwise repair would produce excess tissue (or, in disease states, tumour-like growth) [1].
Q3.1 — Highest mitotic index (1 mark)
Bone marrow, with a mitotic index of approximately 60 per 1000 cells.
Q3.2 — Why these tissues need high mitosis (3 marks)
Bone marrow continuously produces new blood cells (red cells live ~120 days; many white cells far less), so it must constantly run mitosis to replace them [1]. Gut lining cells are lost into the lumen every few days through normal wear, so the lining is regenerated by mitosis in the crypt cells [1]. Skin epidermis loses cells from the surface daily and replaces them from basal-layer mitosis — all three tissues need a high rate of mitosis because their function depends on rapid repair / replacement of somatic cells while maintaining chromosome number [1].
Q3.3 — Clinical consequence in cardiac muscle (3 marks)
Cardiac muscle has very limited mitotic capacity (mitotic index near zero), so when cardiac cells are killed during a heart attack, the body cannot easily replace them through mitosis [1]. The dead area is instead repaired by scar tissue (fibrous tissue), which does not contract like cardiac muscle [1]. This permanently reduces the heart's pumping ability — exactly the link the lesson makes about cells (like neurons and cardiac muscle) with limited mitotic capacity not being easily replaced after damage [1].
Q4 — Diagram critique (6 marks)
4.1 Error 1: Prophase is drawn with a single, unreplicated chromosome — there are no sister chromatids visible. Correction: DNA replication must occur before mitosis, so each chromosome entering prophase should already be shown as two identical sister chromatids joined at a centromere. [1 + 1]
4.2 Error 2: Anaphase is drawn separating whole chromosomes rather than sister chromatids. Correction: in anaphase, the sister chromatids of each replicated chromosome are pulled to opposite poles (not whole un-replicated chromosomes); this is the step that lets each daughter cell receive one complete set. [1 + 1]
4.3 Error 3: The daughter cells are drawn with half the chromosome number of the parent, and the caption says mitosis produces haploid daughter cells. Correction: mitosis maintains chromosome number — daughter cells have the same chromosome number as the parent somatic cell. Halving chromosome number is a property of meiosis (gamete formation), not mitosis. [1 + 1]