Chromosomes, mitosis and stem cells – GCSE Biology exam questions with mark scheme

1Grade 4–54 marks

(a) Describe what happens in a cell before it divides by mitosis. [2]

(b) A student looked at a stained onion root tip. Out of 600 cells, 48 were in mitosis. The whole cell cycle takes 18 hours.

The time spent in mitosis can be estimated using:

time in mitosis = (number of cells in mitosis ÷ total number of cells) × length of cell cycle

Calculate the time spent in mitosis in minutes. [2]

(a) any two from: the cell grows; the number of sub-cellular structures (e.g. ribosomes, mitochondria) increases; the DNA replicates / chromosomes are copied (to form two copies of each chromosome)(1)
(a) second correct point from the list above(1)
(b) 48 ÷ 600 × 18 (= 1.44 hours) or 48 ÷ 600 × 1080 (minutes)(1)
(b) 86.4 minutes (accept 86; correct answer with no working scores 2 marks)(1)
Answer: (a) any two from: cell grows; sub-cellular structures increase in number; DNA replicates (b) 86.4 min
Examiner tip: The question asks for minutes — leaving the answer as 1.44 hours loses the final mark.

2Grade 1–33 marks

(a) Where are stem cells found in an adult human? [1]

(b) Give one way stem cells might be used to treat a medical condition. [1]

(a) bone marrow(1)
(b) any one from: (make insulin-producing cells to) treat diabetes; (make nerve cells to) treat paralysis / spinal injury; replace a named type of damaged or faulty cell (e.g. blood cells, heart muscle cells); allow another named condition treated by replacing cells (e.g. leukaemia via bone marrow transplant, Parkinson's disease); ignore 'cure diseases' / 'replace cells' with no condition or cell type(1)
(c) meristem (tissue); allow root tip(s) / shoot tip(s) / growing tips(1)
Answer: (a) bone marrow (b) e.g. making insulin-producing cells to treat diabetes, or nerve cells to treat paralysis (c) meristems (root and shoot tips)
Examiner tip: 'Replace cells' needs a named condition or cell type to be safe — vague answers like 'cure diseases' are not credited.

3Grade 1–3Foundation3 marks

(a) Put these structures in order of size, starting with the smallest.

cell chromosome gene nucleus [1]

(b) What substance are chromosomes made of? [1]

In the nucleus of a body cell, chromosomes are normally found in ________ . [1]

(a) gene → chromosome → nucleus → cell (all four in the correct order for the mark)(1)
(b) DNA; allow deoxyribonucleic acid; ignore genes / genetic material alone(1)
(c) pairs; allow twos / groups of two(1)
Answer: (a) gene, chromosome, nucleus, cell (b) DNA (c) pairs
Examiner tip: A gene is only a short section of a chromosome, so it is the smallest — students often put chromosome first.

4Grade 4–55 marks

The graph shows how the mass of DNA in one cell changed as the cell went through the cell cycle twice.

(a) Use the graph to find how long one complete cell cycle takes. [1]

(b) Describe what happens to the DNA between 8 hours and 14 hours. [1]

(d) Explain why the cell must double its DNA before it divides. [2]

(a) 20 hours; allow 20 h with no unit if clearly hours from the graph(1)
(b) DNA is replicated / copied; allow (mass of) DNA doubles / chromosomes are copied; ignore DNA increases alone(1)
(c) (cell) division; allow mitosis / cytoplasm (and cell membrane) divides; do not accept meiosis(1)
(d) so each new / daughter cell receives a full / complete set of chromosomes / DNA; allow so each cell gets one copy of every chromosome / the same number of chromosomes as the original cell(1)
(d) so the two cells are genetically identical (to each other and to the parent cell); ignore 'the same' unqualified(1)
Answer: (a) 20 hours (b) it is copied / replicated (mass doubles from 6 to 12) (c) cell division / mitosis (d) each new cell gets a complete set of chromosomes, so the cells are genetically identical
Examiner tip: Measure between the same point on two cycles, e.g. the start of each rise (8 h to 28 h) or from 0 h to the drop at 20 h — 14 hours (the end of the first rise) is a common wrong answer.

5Grade 1–3Foundation5 marks

(a) Which two of these processes involve cell division by mitosis?

Tick (✓) two boxes.

A cut in the skin healing	☐
Egg cells being made in an ovary	☐
A baby's bones growing longer	☐
A sperm cell fertilising an egg cell	☐

[2]

(b) A body cell from a dog contains 78 chromosomes.

(i) How many chromosomes are in each cell produced when this cell divides by mitosis? [1]

(ii) How many pairs of chromosomes does a dog body cell contain? [1]

Mitosis produces two cells that are genetically ________ . [1]

(a) a cut in the skin healing (ticked)(1)
(a) a baby's bones growing longer (ticked)
If more than two boxes are ticked, deduct 1 mark for each extra tick(1)
(b)(i) 78(1)
(b)(ii) 39(1)
(c) identical; allow the same; do not accept similar(1)
Answer: (a) skin healing; bones growing (b)(i) 78 (ii) 39 (c) identical
Examiner tip: Mitosis keeps the chromosome number the same — halving 78 to 39 in (b)(i) confuses mitosis with meiosis.

6Grade 6–76 marks

A student looked at a stained root tip with a microscope. She counted the cells in three zones of the root, and how many of those cells were dividing by mitosis.

Zone	Distance from root tip in mm	Number of cells counted	Number of cells dividing	Percentage of cells dividing
A	0–2	312	47	X
B	2–4	290	18	6.2
C	4–6	305	3	1.0

(a) Calculate value X. Give your answer to 1 decimal place. [2]

(b) Describe and explain the pattern in the percentage of cells dividing. [3]

(a) 47 ÷ 312 × 100(1)
(a) 15.1 (%); correct answer with no working scores 2 marks; 15.06 / 15 scores 1 mark only(1)
(b) the percentage (of cells dividing) decreases as the distance from the tip increases; allow use of data, e.g. 15.1% in zone A but only 1.0% in zone C(1)
(b) (zone A / near the tip is) meristem tissue / contains undifferentiated cells / stem cells (which divide by mitosis); allow tip is where growth occurs(1)
(b) further from the tip the cells have differentiated / become specialised (so most no longer divide); ignore 'cells are older' alone(1)
(c) results are more representative / more reliable; allow reduces the effect of chance / random variation / an anomalous area; ignore 'more accurate' / 'fair test'(1)
Answer: (a) 15.1 (b) fewer cells divide further from the tip; the tip is a meristem with undifferentiated cells that divide; further back cells have differentiated so no longer divide (c) a larger sample is more representative
Examiner tip: The 'explain' marks need biology, not just the trend: say the tip is a meristem of undifferentiated cells and that specialised cells stop dividing.

7Grade 6–76 marks

In type 1 diabetes, the cells in the pancreas that make insulin are destroyed.

Scientists are researching a treatment that uses therapeutic cloning:

An embryo is made that has the same genes as the patient.
Stem cells are taken from the embryo.
The stem cells are made to differentiate into insulin-making cells.
These cells are put into the patient's pancreas.

Evaluate the use of therapeutic cloning to treat type 1 diabetes. [6]

Advantage: embryonic stem cells can differentiate into (almost) any cell type, so can form insulin-making cells(1)
Advantage: the cells have the patient's own genes, so they will not be rejected by the immune system(1)
Advantage: no need for immunosuppressant drugs (and their side effects, e.g. more infections)(1)
Advantage: no need to wait for a (matching) donor(1)
Advantage: could be a long-term cure – patient may no longer need insulin injections / blood glucose controlled naturally(1)
Disadvantage: risk of transferring a viral infection (in the cultured cells) to the patient(1)
Disadvantage: ethical / religious objections – an embryo (a potential human life) is created and destroyed(1)
Disadvantage: the treatment is still experimental / not proven safe; cells could divide uncontrollably / form tumours (allow)(1)
Disadvantage: expensive / time-consuming to produce the cells(1)
Disadvantage: in type 1 diabetes the immune system destroyed the original cells, so the new cells might be attacked again (credit as insight)(1)
Conclusion: a justified judgement, e.g. a promising treatment if safety is proven, but only if the ethical objections are accepted(1)

Level	Marks	What it looks like
Level 3	5–6	A detailed evaluation that gives both advantages and disadvantages, linking each to the biology (e.g. same genes so no rejection), and reaches a conclusion that is supported by the points made.
Level 2	3–4	Some advantages and disadvantages given, with at least one point explained; or one side covered in detail. Any conclusion is not fully justified.
Level 1	1–2	Simple statements of one or two relevant points, e.g. 'it could cure diabetes' or 'some people think it is wrong', with little explanation.
No marks	0	No relevant content.

Answer: Balanced evaluation: patient's own genes → no rejection, no immunosuppressants, no donor needed, possible cure; against: viral infection risk, ethical/religious objections to destroying embryos, unproven/tumour risk, cost; with a justified conclusion.
Examiner tip: Level 3 needs BOTH sides and a conclusion that says why — a list of advantages only, however detailed, is capped at Level 2.

8Grade 4–56 marks

A botanic garden is trying to save a rare species of orchid. Scientists take a small piece of tissue from the meristem at the tip of one orchid shoot and grow it in a sterile culture.

Every 6 weeks, each piece of tissue grows and is cut into 5 pieces. Each new piece can grow into a new orchid plant.

(a) The scientists start with 1 piece of tissue. Calculate how many pieces of tissue they will have after 24 weeks. [2]

(b) Explain why tissue from a meristem is used. [2]

(d) All the orchids produced are clones. Suggest one disadvantage of this. [1]

(a) 24 ÷ 6 = 4 (times cut) or 5 × 5 × 5 × 5 / 5⁴(1)
(a) 625; correct answer with no working scores 2 marks(1)
(b) (meristem) cells are undifferentiated / are stem cells / can divide (by mitosis)(1)
(b) can differentiate into any type of plant cell / can form roots, stems and leaves (so a whole plant grows); allow can form all the tissues of a plant(1)
(c) large numbers of plants can be produced quickly / cheaply / from a very small amount of tissue; allow few / no plants need to be taken from the wild; allow plants can be returned to the wild(1)
(d) no (genetic) variation, so a new disease / change in the environment could kill all of them; allow all could be susceptible to the same disease; ignore 'they are all the same' unqualified(1)
Answer: (a) 625 (b) meristem cells are undifferentiated / can divide and can differentiate into any plant cell type (c) many plants produced quickly from one small sample (d) no genetic variation, so one disease could wipe them all out
Examiner tip: In (a) the number multiplies by 5 each time — adding 5 + 5 + 5 + 5 = 20 is the most common wrong answer.

9Grade 8–9Higher6 marks

Scientists grew adult bone marrow stem cells in a culture. At the start there were $2.5 \times 10^4$ cells. After 120 hours there were $8.0 \times 10^5$ cells.

Assume all the cells divided at the same rate and no cells died.

(a) Calculate how many times the cells divided by mitosis in the 120 hours. [2]

(b) Calculate the mean length of the cell cycle. [1]

(c) The scientists then added a chemical that made the stem cells differentiate into red blood cells. The number of cells stopped increasing.

Suggest why. [2]

(d) A patient with leukaemia needs new bone marrow. Suggest why a transplant of bone marrow from a donor might fail. [1]

(a) $8.0 \times 10^5 \div 2.5 \times 10^4 = 32$(1)
(a) $32 = 2^5$ so 5 (divisions); allow 1 mark for 2 → 4 → 8 → 16 → 32 shown with 5 steps; correct answer with no working scores 2 marks(1)
(b) 24 hours; allow error carried forward: 120 ÷ their (a)(1)
(c) the cells have become specialised / differentiated(1)
(c) (most) specialised cells lose the ability to divide (by mitosis); allow red blood cells have no nucleus so cannot divide(1)
(d) the donor cells may be rejected (by the patient's immune system); allow the donor cells have different antigens / are recognised as foreign / are attacked by the patient's white blood cells; ignore viral infection (a risk to the patient, not a reason the transplant fails)(1)
Answer: (a) 5 (b) 24 hours (c) the cells became specialised, and specialised cells (e.g. red blood cells with no nucleus) cannot divide (d) the donor cells may be rejected by the patient's immune system
Examiner tip: Each mitosis doubles the cell number, so find the power of 2 (32 = 2⁵) — dividing 32 by 2 to get 16 divisions scores only 1 of the 2 marks in (a).

10Grade 6–75 marks

(a) Stem cells are undifferentiated. What does undifferentiated mean? [1]

(b) Compare the stem cells in a plant meristem with the stem cells in adult human bone marrow. [2]

Explain why scientists think embryonic stem cells may be more useful than adult bone marrow stem cells for treating paralysis. [2]

(a) not specialised / has not developed into a particular type of cell; allow has not developed a specific function; ignore 'can become any cell' alone(1)
(b) meristem (stem) cells can differentiate into any type of plant cell, whereas bone marrow stem cells form only some / a limited range of cell types, e.g. blood cells(1)
(b) a second valid comparison, e.g. both can divide (by mitosis) to produce more cells / both are found in the adult organism; or meristems are found at root and shoot tips whereas these stem cells are in the marrow inside bones; or meristem cells can grow into a whole new plant (clone) but bone marrow cells cannot form a whole human; ignore 'both are undifferentiated' (given in the question)(1)
(c) embryonic stem cells can differentiate into most / many (types of) human cells(1)
(c) including nerve cells, whereas bone marrow stem cells form a limited range of cells / mostly blood cells (so may not form nerve cells); allow so they can replace the damaged nerve cells(1)
Answer: (a) not specialised (b) meristem cells can form any type of plant cell but bone marrow stem cells form only some types, such as blood cells; both divide by mitosis to make more cells (c) embryonic stem cells can become most cell types, including nerve cells, which bone marrow stem cells cannot
Examiner tip: 'Compare' needs a statement about both types — writing only about meristems or only about bone marrow scores no marks, so use a linking word such as 'whereas'.

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