5.4 Cloning — Edexcel International GCSE Biology 4BI1 Revision Notes & Topical Questions

Section 1 of 8Cloning

Essential knowledge

A clone is genetically identical to its source because it is produced only by mitosis; no meiosis or fertilisation occurs, so there is no mixing of alleles.

Micropropagation

Micropropagation explants (small pieces, often shoot tips) are taken from a parent plant with desirable features. Shoot tips contain meristem cells that divide actively and are often free of viruses.

Aseptic technique is essential because the nutrient medium also supports bacteria and fungi, which would outcompete or kill the explants.

The medium contains sugar for respiration (explants cannot photosynthesise much at first), mineral ions (nitrate for protein, magnesium for chlorophyll) and plant hormones (auxins/cytokinins) to trigger division and control shoot and root growth.

Explants form a callus by mitosis; the callus can be divided repeatedly, so thousands of plantlets can be made from one parent in a short time and at any time of year.

Commercial uses

Commercial uses producing large numbers of identical crop plants (bananas, potatoes), ornamental plants (orchids), and saving rare or endangered plants that set few seeds.

Mammal cloning (nuclear transfer)

(1) a body cell is taken from the adult to be cloned and its diploid nucleus removed
(2) an egg cell is taken from a female of the same species and its nucleus removed (enucleated)
(3) the diploid nucleus is inserted into the enucleated egg
(4) an electric shock stimulates the cell to divide by mitosis
(5) the embryo is implanted in the uterus of a surrogate mother
(6) the offspring born is a clone of the nucleus donor.

Dolly the sheep

Dolly the sheep nucleus from an udder cell of a Finn Dorset ewe, egg from a Scottish Blackface ewe, surrogate Scottish Blackface ewe. Dolly was white-faced like the Finn Dorset, showing her DNA came from the nucleus donor.

Cloned transgenic animals

Cloned transgenic animals a human gene (e.g. for a blood-clotting protein or antibody) is inserted into an animal cell; cloning that cell's nucleus produces many animals that all carry the gene and secrete the human protein in their milk, from which it is purified for medicines.

Advantages of animal cloning

Advantages of animal cloning copies of animals with high value features (high milk yield, transgenic), possible conservation of endangered species, research into development.

Disadvantages

Disadvantages low success rate, expensive, clones may age early or have health problems, reduced genetic variation makes populations vulnerable to disease, ethical concerns about animal welfare and possible human cloning.

Cloned offspring may not be completely identical in appearance to the donor because the environment also affects phenotype, and a small amount of mitochondrial DNA comes from the egg.

Why plants clone easily

Many plant cells keep the ability to differentiate into any cell type throughout life, so a tiny explant grown in vitro can regenerate a whole plant. In animals only embryonic stem cells can do this, so mammals must be cloned by nuclear transfer instead.

Section 2 of 8Process & sequence

- Select a parent plant with desirable characteristics and cut small explants (e.g. shoot tips, a few mm) using a sterile scalpel.
- Sterilise the surface of the explants (e.g. dilute bleach/ethanol) and rinse in sterile water to kill microorganisms.
- Place the explants on sterile nutrient agar (containing glucose/sucrose, mineral ions such as nitrates,
  - and plant hormones) in sterile containers, using aseptic technique.
- Incubate in warm, lit conditions; cells divide by mitosis to form a callus (mass of undifferentiated cells).
- Transfer the callus to medium with a different hormone balance so that it develops shoots and roots (plantlets);
  - the callus can be divided to make more plantlets.
- Transfer the plantlets to compost in a greenhouse to grow into genetically identical adult plants.
Cloning a mammal by nuclear transferwrite these steps in this order; each step is a marking point.
1. Take a body (e.g. udder) cell from adult A and remove its diploid nucleus.
2. Take an egg cell from female B (same species) and remove its nucleus to leave an enucleated egg.
3. Insert the diploid nucleus from A into the enucleated egg.
4. Give a small electric shock to stimulate the cell to divide by mitosis to form an embryo.
5. Implant the embryo into the uterus of surrogate mother C.
6. The offspring born is a clone of A (genetically identical to the nucleus donor).

Section 3 of 8Compare

Comparison of the two cloning methods
Feature	Plant micropropagation	Mammal cloning (nuclear transfer)
Starting material	Explant (e.g. shoot tip) from parent plant	Diploid nucleus from a body cell of the donor adult
Type of cell division	Mitosis	Mitosis (in the developing embryo)
Conditions	Sterile nutrient agar with hormones, controlled light and temperature	Egg cell stimulated with an electric shock; embryo implanted in surrogate
Number produced	Thousands quickly, from one plant	Very few; low success rate
Genetically identical to	Parent plant	The animal that donated the nucleus (not the egg donor or surrogate)
Example	Orchids, bananas, potatoes	Dolly the sheep (1996)

The clone always matches the source of the nucleus / DNA.

Advantages and disadvantages of cloning
Advantages	Disadvantages
Large numbers of offspring with known, desirable characteristics	No genetic variation: all clones equally vulnerable to the same disease or environmental change
Plants: fast, can be done all year, in small space	Micropropagation needs sterile lab conditions, skilled staff, expensive
Plants that are hard to grow from seed (e.g. rare orchids) can be produced	Contamination by fungi/bacteria can destroy a whole batch
Animals: copies of transgenic animals making human proteins (e.g. in milk)	Animal cloning has a low success rate; many embryos fail
Could help conserve endangered species	Cloned animals may have health problems / shorter lives; ethical objections

Section 4 of 8Worked examples

1080 plantlets

A nursery takes 5 explants. Every 4 weeks each piece of callus is divided into 6 pieces. How many plantlets could be produced after 12 weeks (assuming no losses)?

Number of rounds n = 12 ÷ 4 = 3. N = N₀ × kⁿ = 5 × 6³ = 5 × 216 = 1080 plantlets.

10.5%

In the Dolly experiment, 277 eggs received a donor nucleus; 29 developed into embryos and 1 lamb was born. Calculate (a) the percentage of eggs that became embryos and (b) the percentage of embryos that produced a live lamb.

(a) 29 ÷ 277 × 100 = 10.469... ≈ 10.5%. (b) 1 ÷ 29 × 100 = 3.448... ≈ 3.4%. Both show a low success rate.

328 plantlets

A batch of 400 plantlets was started; 18% were lost to fungal contamination. How many plantlets survived?

Lost = 0.18 × 400 = 72. Survived = 400 − 72 = 328 plantlets (82%).

Section 5 of 8Keep going

Cloning Edexcel O Level Biology: what to do next
These Cloning Edexcel O Level Biology notes covered the key ideas, worked examples and exam technique for this topic. Check your Cloning Edexcel O Level Biology understanding with the practice questions below, then reread any Cloning Edexcel O Level Biology point you were unsure about before moving on.
Cloning Edexcel O Level Biology: revision checklist
- Revise the main Cloning Edexcel O Level Biology definitions until you can write them from memory.
- Practise Cloning Edexcel O Level Biology exam questions under timed conditions.
- Link each Cloning Edexcel O Level Biology idea to a diagram or a worked example.
- List the common Cloning Edexcel O Level Biology mistakes and how to avoid them.
- Explain one Cloning Edexcel O Level Biology idea out loud as if teaching a friend.
- Summarise Cloning Edexcel O Level Biology in five key points on a revision card.
- Check which command words appear most often in Cloning Edexcel O Level Biology questions.
- Come back to Cloning Edexcel O Level Biology after a week for spaced practice.
- Practice: Cloning Edexcel O Level Biology practice questions
- Previous lesson: 5.3 Genetic Modification (Genetic Engineering) — O Level Notes
- Course: Edexcel O Level Biology — all units
- Official syllabus: Pearson Edexcel International GCSEs

Section 6 of 8Key terms

Section 7 of 8Command terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

Give an account of what happens / the pattern in data, quoting values; no reasons needed.

Explain

Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

Suggest

Use knowledge in a new context; more than one answer may be accepted.

Compare

Similarities and differences; each point must refer to both items.

Calculate

Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Section 8 of 8Exam technique

Keywords the mark scheme looks for in this lesson

Clone · Asexual reproduction · Mitosis · Micropropagation · Explant · Tissue culture · Aseptic technique · Callus · Plant hormones (in cloning) · Nucleus transfer · Enucleated egg cell · Surrogate mother · Diploid · Transgenic animal

Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1

Use specification keywords exactly (e.g. enzyme–substrate complex, active site, complementary).

2Link ideas

Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.

3Use the information in the question

“discuss/evaluate” answers must use the data provided and match the number of marks.

4

Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.

5Avoid vague words

“it”, “stuff”, “goes up”, “energy is made”. Name the substance and the process.

6

Extended (6-mark) responses are marked on content AND a logical, linked line of reasoning.

Words to avoid

1

the clone is identical to the surrogate motherthe clone is genetically identical to the animal that donated the nucleus

2

the sperm nucleus is put into the egga diploid nucleus from a body cell is inserted into an enucleated egg cell

3

the cells divide by meiosisthe cells divide by mitosis, producing genetically identical cells

4

the explants are put in foodexplants are placed on sterile agar containing nutrients (sugar, mineral ions) and plant hormones

5

it is kept cleanaseptic technique is used to prevent contamination by bacteria and fungi

6

clones are exactly the sameclones are genetically identical, though the environment can cause differences in phenotype