Interpret a stated stem-cell renewal and specialisation model
One possible renewal pattern keeps one stem cell and adds one non-dividing specialised descendant per event. Four events give one stem cell plus four descendants, five cells total.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall chromosomes, genes, chromosome sets, selective gene expression and the distinction between a nucleus and a whole cell.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which assumption defines this original counting model?
What you will learn
- Separate retained stem cells from specialised descendants
- Count a single continuing branch
Original teaching model. Counts and records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Specified event | One continuing stem cell produces two daughters |
| Later states | One remains stem; one specialises |
| After four events | 1 stem + 4 specialised = 5 cells |
| Limit | Only the continuing branch divides in this model |
Separate retained stem cells from specialised descendants
A stem-cell population can be maintained while some descendants become specialised. In one possible original model, each division of the continuing stem cell produces two daughters: one retains the stem-cell state and the other later specialises. This is a supplied pattern used for counting and explanation, not a rule that every stem-cell division must always have exactly these outcomes.
The distinction between genetic information and cell state remains important. Normal accurate mitosis gives matching chromosome information to the daughters. Their later states can differ because they use that information differently, not because one necessarily loses the genes that the other retains. Specialisation follows the division in the stated sequence.
Count a single continuing branch
Begin with one stem cell and no specialised descendants. After four stated divisions of that continuing stem branch, one stem cell remains and four specialised descendants have been produced, giving five cells in total if none are lost. The specialised descendants are stipulated not to divide. Only one parent divides at each step, so the population does not undergo four whole-population doubling rounds.
Other possible stem-cell lineages can retain both daughters in an unspecialised state or produce specialised descendants through different sequences. Their outcomes need their own assumptions and records. Use the given branch structure before doing arithmetic; the presence of the word mitosis does not justify replacing a particular lineage with a universal exponential formula.
Sources for this lesson
Pause and explain
After four stated branch divisions from one stem cell, with one retained and one non-dividing specialised daughter each time, what total remains?
Worked example
One continuing stem branch produces one specialised descendant per division for four divisions. What are the final counts?
Show the worked solution
- Retain one continuing stem cell.
- Add four non-dividing specialised descendants.
Answer One stem cell and four specialised descendants remain: five cells total, with no stated losses.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain how the supplied lineage can maintain a stem-cell population while producing specialised descendants.
Give me a hint
Describe the two later daughter states.
Compare my reasoning
- Retain one daughter in the stem-cell state.
- Allow the other to specialise in the stipulated model.
One daughter retains the stem-cell state while the other later specialises, preserving the continuing branch and supplying descendants.
Look for these in your work
- Use the stated outcome
- Avoid calling it universal
Independently count the final stem, specialised and total cells after four branch divisions with no loss.
Give me a hint
Add one specialised descendant per event.
Compare my reasoning
- Keep one stem cell after each event.
- Count four specialised descendants and add the stem cell.
The final counts are one stem cell, four specialised descendants and five cells total.
Look for these in your work
- Separate the two cell states
- State the no-loss condition
A solution gives sixteen cells after four branch events. Identify its incorrect assumption.
Give me a hint
Ask which parents are actually dividing.
Compare my reasoning
- Recognise that sixteen comes from full-population doubling.
- Compare with only one continuing dividing stem branch.
It incorrectly assumes every cell divides in every step. The supplied model has only one continuing dividing branch and therefore totals five cells.
Look for these in your work
- Identify the substituted model
- Repair the final total
Common mistakes
- One continuing branch is not whole-population doubling.
- The one-retained and one-specialising outcome is stipulated, not universal.
What can you explain now?
After four stated branch divisions from one stem cell, with one retained and one non-dividing specialised daughter each time, what total remains?
Compare with the explanation
Five cells
One stem cell plus four specialised descendants.
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Make it stick
Without your notes, separate retained stem cells from specialised descendants and count a single continuing branch. Distinguish nuclear division, chromosome copies and completed daughter cells before opening the solutions.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.