Cloning: micropropagation and cloned mammals – IGCSE Biology exam questions with mark scheme

1Grade 1–3Triple only3 marks

Micropropagation is used to produce large numbers of plants.

(a) What is the name given to the small piece of tissue that is cut from the parent plant? Put a cross in the box next to your answer. [1]

☐ A embryo ☐ B explant ☐ C gamete ☐ D zygote

(b) Micropropagation is described as growing tissue in vitro. State what is meant by in vitro. [1]

(a) B / explant(1)
(b) in glass / in a test tube / in a container (in the laboratory) / outside the living organism; allow in a culture vessel; ignore 'in a lab' alone(1)
(c) mitosis; reject meiosis(1)
Answer: (a) B explant (b) grown in a glass container / outside the organism (c) mitosis
Examiner tip: Do not confuse 'explant' with 'embryo' — no fertilisation happens in micropropagation, so the cells divide by mitosis, not meiosis.

2Grade 4–5Triple only5 marks

The diagram shows the stages used by a nursery to produce new African violet plants by micropropagation.

(a) Name piece X. [1]

(b) The jelly in Stage 3 contains agar and substances Y. Name two substances, other than agar and water, that are added to the jelly. [2]

(c) The tubes, instruments and jelly are sterilised, and piece X is dipped in dilute bleach before Stage 3. Explain why. [2]

(a) explant(1)
(b) any two from: sugar / glucose / sucrose; mineral ions / named ion e.g. nitrate, magnesium; plant hormones / auxin / cytokinin / growth regulators; allow vitamins; ignore 'nutrients' alone; ignore water — one mark each, max 2 (first mark)(1)
(b) second correct substance from the list above(1)
(c) to kill / remove microorganisms / bacteria / fungi / spores; ignore germs(1)
(c) (otherwise) microorganisms would grow on the sugary / nutrient jelly and compete with / infect / kill the explant / rot the tissue(1)
Answer: (a) explant (b) e.g. glucose/sucrose and plant hormones (auxin/cytokinin); also mineral ions (c) to kill microorganisms, which would otherwise grow on the nutrient jelly and infect or outcompete the plant tissue.
Examiner tip: 'Nutrients' alone is vague — name glucose or mineral ions, and remember plant hormones are what make the explant form shoots and roots.

3Grade 6–7Triple only5 marks

A banana grower uses micropropagation.

She starts with 4 explants.
Every 5 weeks, each explant produces 6 shoots.
Each shoot is then cut off and used as a new explant. The old explant is thrown away.

(a) Calculate the number of plantlets she will have after 20 weeks. [2]

(b) When the plantlets are moved from the culture vessels into compost, 25% of them die. Calculate how many plantlets survive. [1]

(c) The grower needs at least one million plantlets for a new plantation. Calculate the minimum number of weeks needed to produce at least one million plantlets, before any are moved to compost. [2]

(a) 20 ÷ 5 = 4 cycles / evidence of $4 \times 6^4$ or $4 \times 6 \times 6 \times 6 \times 6$(1)
(a) 5184(1)
(b) 3888; allow ecf from (a) × 0.75(1)
(c) evidence of trying successive cycles: $4 \times 6^6 = 186\,624$ (too few) and/or $4 \times 6^7 = 1\,119\,744$ (enough)(1)
(c) 35 (weeks); allow 7 cycles × 5 weeks(1)
Answer: (a) 5184 (b) 3888 (c) 35 weeks
Examiner tip: Each cycle multiplies the number by 6 — students who add 6 × 4 each time (linear growth) or forget the starting 4 explants lose both marks.

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4Grade 4–5Triple only4 marks

An oil palm company has found one palm tree that produces a very high yield of oil.

The company wants to plant 50 000 new trees with this high yield.

Explain two advantages of using micropropagation rather than planting seeds from this tree. [4]

all the new trees are genetically identical (to the parent) / are clones(1)
so all will have the high oil yield / desirable characteristic; seeds come from sexual reproduction so offspring show variation / may not have high yield(1)
large numbers / commercial quantities can be produced quickly / from a small amount of tissue (of one tree)(1)
can be done all year round / in a small space / does not depend on the tree flowering or seed production / germination; allow plants are disease-free (if grown in sterile conditions)(1)
Answer: Micropropagated trees are clones, so all inherit the high oil yield (seedlings would vary); and tens of thousands can be made quickly from small pieces of one tree, all year round, without waiting for seeds.
Examiner tip: Each advantage needs a linked consequence: 'genetically identical' only scores fully when you add 'so they all have the high yield'.

5Grade 8–9Triple only6 marks

The first cloned camel was born in 2009. Scientists now want to clone a female racing camel that has won many races.

Describe how the scientists could produce a cloned camel from this racing camel. [6]

a body (somatic) cell / mature cell, e.g. skin cell, is taken from the racing camel(1)
the (diploid) nucleus is removed from this body cell(1)
an egg cell is taken from another (female) camel(1)
the nucleus of the egg cell is removed / the egg is enucleated(1)
the diploid nucleus from the racing camel is inserted into / fused with the enucleated egg cell(1)
an electric shock / electrical pulse / chemical stimulates the cell to divide(1)
the cell divides by mitosis to form an embryo(1)
the embryo is implanted into the uterus of a (third) surrogate camel(1)
the surrogate gives birth to a calf that is genetically identical to the racing camel (nucleus donor); ignore references to sperm / fertilisation (none occurs)(1)

Level	Marks	What it looks like
Level 0	0	No rewardable material.
Level 1	1–2	One or two isolated stages are described (e.g. 'the nucleus is removed' or 'it is put into another camel') with little or no correct sequence; key terms such as enucleated or diploid are missing or misused.
Level 2	3–4	Most stages are described in a mostly logical order, but one important stage is missing or vague (e.g. the source of the nucleus is not identified as a body cell, or how the embryo starts dividing is not stated); some correct terminology.
Level 3	5–6	A complete, correctly sequenced description from removing the egg nucleus to the birth of the clone, identifying the diploid nucleus from a body cell of the animal being cloned, stimulation to divide by mitosis and implantation into a surrogate; terminology used accurately throughout.

Answer: Take a body cell from the racing camel and remove its diploid nucleus; remove the nucleus from an egg cell of another camel; insert the body-cell nucleus into the enucleated egg; electric shock stimulates mitosis to form an embryo; implant the embryo into a surrogate mother; the calf born is a clone of the racing camel.
Examiner tip: Say where each cell comes from — top-band answers make clear that the nucleus is from a body cell of the animal being cloned and the egg is enucleated, then name the surrogate.

6Grade 6–7Triple only5 marks

A research team tried to clone a cow by nuclear transfer. The table shows their results.

Stage	Number
enucleated egg cells given a body-cell nucleus	420
cells that developed into embryos	84
embryos that produced a pregnancy	9
live calves born	3

(a) Calculate the percentage of the egg cells given a body-cell nucleus that developed into embryos. [1]

(b) Calculate the percentage of embryos that resulted in a live calf. Give your answer to 2 significant figures. [2]

(a) 20 (%)(1)
(b) 3 ÷ 84 × 100 / 3.571…(1)
(b) 3.6 (%) (2 s.f.); correct answer with no working scores 2(1)
(c) the success rate is very low / only 3 calves from 420 eggs / only 0.71% of eggs give a calf; allow most embryos fail to give a pregnancy(1)
(c) so very many eggs / procedures / surrogate cows are needed for each calf born; allow lots of time / skilled work / equipment is wasted on cells that fail(1)
Answer: (a) 20% (b) 3.6% (c) Only 3 of 420 eggs (0.71%) gave a calf, so many eggs, procedures and surrogate mothers are needed per clone.
Examiner tip: In (b) divide by the number of embryos (84), not the 420 starting eggs — read the question to see which number is the total.

7Grade 6–7Triple only4 marks

Dolly the sheep was produced by cloning.

The nucleus came from an udder cell of a Finn Dorset sheep, which has a white face.
The egg cell came from a Scottish Blackface sheep, which has a black face. Its nucleus was removed.
The embryo was placed in the uterus of a different Scottish Blackface sheep.

(a) Predict the face colour of Dolly. Explain your answer. [3]

(b) The udder cell contained 54 chromosomes. State how many chromosomes were in a normal body cell of Dolly. [1]

(a) white (face)(1)
(a) the nucleus / genes / DNA / chromosomes came from the Finn Dorset (udder cell); allow she is genetically identical to the Finn Dorset(1)
(a) the egg cell's nucleus was removed (so no Blackface genes) / the surrogate only provides nutrients / a place to develop / does not pass on genes; allow either idea for the mark; ignore references to mitochondrial DNA(1)
(b) 54(1)
Answer: (a) White face: all her nuclear genes came from the Finn Dorset udder-cell nucleus; the egg's own nucleus had been removed and the surrogate contributed no genes. (b) 54
Examiner tip: The surrogate mother gives birth but passes on no genes — saying Dolly 'looks like her mother' loses the mark.

8Grade 6–7Triple only5 marks

Scientists have produced a transgenic goat that has the human gene for antithrombin. Antithrombin is a human protein that stops blood from clotting. The goat secretes the protein in its milk.

(a) Explain why the term transgenic is used to describe this goat. [1]

(b) The scientists then use cloning to produce a herd of goats from this one transgenic goat. Explain why cloning is used rather than breeding the goat with other goats. [2]

(c) Each goat produces 1.5 g of antithrombin per litre of milk and 800 litres of milk per year. Calculate the mass of antithrombin, in kg, produced by a herd of 25 cloned goats in one year. [2]

(a) it contains genetic material / a gene from a different species (human)(1)
(b) clones are genetically identical so all will have the human (antithrombin) gene / all produce antithrombin(1)
(b) sexual reproduction / breeding: only (about) half the offspring may inherit the gene / offspring show variation; allow cloning is quicker than making more transgenic goats from scratch(1)
(c) 1.5 × 800 × 25 / 30 000 g / 1200 g per goat(1)
(c) 30 (kg)(1)
Answer: (a) It has a gene from another species (human). (b) Every clone has the antithrombin gene, whereas breeding passes it to only some offspring. (c) 30 kg
Examiner tip: In (c) convert grams to kilograms at the end — 30 000 is in grams, so the answer is 30 kg.

9Grade 8–9Triple only6 marks

The diagram shows the stages used to clone a sheep.

(a) Describe what happens at stage B. [1]

(b) Explain why the nucleus used in stage A is taken from a body cell and not from a sperm cell. [2]

Cell	Number of chromosomes
egg cell from sheep Q before stage B
one cell of the embryo after stage D

(d) Which sheep, P, Q or R, will the lamb be genetically identical to? [1]

(a) the nucleus is removed (from the egg cell) / the egg is enucleated(1)
(b) a body cell nucleus is diploid / has two sets of chromosomes / a full set of genes; sperm (nucleus) is haploid / has half the number of chromosomes(1)
(b) the embryo / lamb needs the full (diploid) set / the lamb will have all the genes of (one) adult so it is a clone; allow sperm genes would combine / are not identical to one adult(1)
(c) 27(1)
(c) 54(1)
(d) P(1)
Answer: (a) The egg's nucleus is removed. (b) A body-cell nucleus is diploid (full set of genes); a sperm is haploid. (c) 27; 54 (d) P
Examiner tip: An egg cell is a gamete, so it is haploid (27) — and stage D is mitosis, which keeps the diploid number (54).

10Grade 6–7Triple only5 marks

A student wants to produce several genetically identical cauliflower plants in the school laboratory using tissue culture.

She has a cauliflower head, a sterile scalpel, sterile tubes containing nutrient agar jelly with plant hormones, dilute bleach solution and sterile distilled water.

Describe a method she could use. [5]

cut small pieces / explants (of about 2–5 mm) from the florets / surface of the cauliflower head with the scalpel(1)
dip / soak the explants in dilute bleach (for several minutes) to sterilise the surface, then rinse in sterile (distilled) water(1)
use aseptic technique, e.g. work near a Bunsen flame / wash hands / disinfect bench / sterilise forceps by flaming / open tube lids only briefly(1)
place each explant on (the surface of) the nutrient agar in a tube and seal / replace the lid(1)
keep in warm (about 20–25 °C), lit conditions for several weeks; then, when shoots and roots form, transfer plantlets to compost; allow either for the mark(1)
Answer: Cut tiny explants from the florets, sterilise in dilute bleach and rinse in sterile water; using aseptic technique, place each on the nutrient-hormone agar and seal; keep warm and lit for several weeks; transfer plantlets with shoots and roots to compost.
Examiner tip: Plants need light to photosynthesise once shoots grow — saying 'keep in a dark incubator' loses the conditions mark, and forgetting to rinse off the bleach can kill the explant.

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