OCR A Level Biology A: Natural and artificial cloning (H420) – Revision Notes

These notes condense section 6.2.1(a) to (d) of the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025. For full explanations and worked examples, read the study guide. When you are ready to test yourself, use the practice questions, the OCR A Level Biology hub, the printable checklist and the free diagnostics.

Definitions to recall

Term	Meaning
Clone	Genetically identical individuals (or cells) produced from one parent by asexual reproduction, using mitosis
Vegetative propagation	A part of a plant (stem, root, bulb and so on) grows into a new plant
Cutting	A piece of stem, leaf or root that grows into a new plant
Totipotent	A cell able to divide and differentiate into any cell type
Explant	The piece of tissue taken from a parent to start a culture
Callus	A mass of undifferentiated plant cells
Micropropagation	Making many plantlets from a small piece of tissue, using sterile culture
Embryo splitting	An early embryo separates into groups of cells that each form an individual
Enucleation	Removal of the nucleus from a cell, here an egg
SCNT	Somatic cell nuclear transfer: a somatic nucleus is put into an enucleated egg
Surrogate	The female that carries the cloned embryo

6.2.1(a) Natural plant clones and cuttings

Structure	What it is	Example
Runner	Horizontal stem above the soil, new plants at nodes	strawberry
Rhizome	Underground horizontal stem with buds	iris
Stem tuber	Swollen underground stem storing starch	potato
Bulb	Underground bud with fleshy leaf bases	daffodil
Corm	Swollen stem base	crocus
Sucker	New shoot from a root or stem base	raspberry

Why they matter: fast, no pollinator, uniform plants. Weakness: low genetic variation.

Method for a stem cutting

1. Clean, sharp blade; healthy non-flowering shoot.
2. Cut just below a node.
3. Remove lower leaves (less transpiration).
4. Optional rooting powder (auxin).
5. Moist compost, humid cover, warm and light.
6. Repeat with several cuttings and compare.

Practical link: dissect plant material to see nodes, buds and vascular tissue before cutting.

6.2.1(b) Micropropagation

Steps in order: select parent, take explant, sterilise, culture on agar medium with sugar, minerals, vitamins and growth regulators, multiply shoots, root them, transfer to compost, harden off.

Hormone ratio	Result
High cytokinin, low auxin	Shoots
High auxin, low cytokinin	Roots
Balanced	Callus

Why aseptic: the nutrient medium supports fast microbial growth.

Multiplication formula: number after n cycles = (shoots per cycle)ⁿ from one explant. Example: 3 shoots per cycle, 4 cycles gives 3⁴ = 81.

Plant cloning: for and against

For	Against
Many identical plants, quickly, all year	Costly, needs skill and sterile equipment
Disease-free stock from meristem tissue	A contaminated vessel wastes the batch
Rare species conserved	Genetically uniform, so one disease can kill all
Uniform crop	Plantlets may not survive transfer to soil

Use in horticulture: uniform ornamental plants. Use in agriculture: disease-free planting material.

6.2.1(c) Natural animal clones

• Identical (monozygotic) twins: an early embryo splits in two. Same genome.
• Phenotype can differ because of environment and mutation.
• Other examples: armadillo, Hydra budding, aphid parthenogenesis.

6.2.1(d) Artificial animal clones

Embryo twinning

IVF, then split the early embryo while cells are totipotent, culture the pieces, implant in surrogates. Offspring identical to each other, not to either parent.

SCNT

Step	Detail
1	Take a somatic cell from the animal to copy
2	Remove the nucleus of an unfertilised egg (enucleation)
3	Insert the somatic nucleus, by fusion with a small electric pulse or by injection
4	Stimulate division and culture to an early embryo
5	Implant in a surrogate

Who donates what:

Contributor	What they give
Somatic cell donor	Nuclear DNA, so the clone matches this animal
Egg donor	Cytoplasm and mitochondrial DNA
Surrogate	Womb environment only

Evaluating animal cloning

Area	For	Against
Agriculture	Copy high-yield animals, uniform herd	Less diversity; costly
Medicine	Animals producing human proteins; disease models	Ethical concerns; low success
Conservation	Adds individuals to tiny populations	Habitat loss unsolved
Longevity	Some studies show healthy ageing of clones	Dolly died young of lung disease and arthritis, and the evidence is limited

Typical efficiency is low. For example, 4 births from 160 reconstructed eggs is 2.5%.

Worked reminders

Percentage rooted

A trial uses 45 cuttings in each group. 36 root with powder and 27 without.

• With powder: 36 ÷ 45 × 100 = 80%.
• Without: 27 ÷ 45 × 100 = 60%.
• Difference: 20 percentage points. Relative increase: (36 − 27) ÷ 27 × 100 = 33.3%.

Say which comparison you are giving, percentage points or percentage increase.

Culture vessels

A lab sets up 800 culture vessels and 15% are lost to contamination.

• Lost: 0.15 × 800 = 120 vessels.
• Remaining: 800 − 120 = 680 vessels (800 × 0.85).

Reading a results table

Three calves formed by splitting one early embryo later give milk yields of 6.8, 7.4 and 7.1 thousand litres per year.

• Mean: (6.8 + 7.4 + 7.1) ÷ 3 = 7.1 thousand litres.
• The range is 0.6 thousand litres.

The calves share a genotype, so the spread is due to the environment (feeding, health, housing). Calves from a sexual cross would also differ genetically.

Method in steps: evaluating a cloning use

1. Name the use (for example, disease-free potato planting stock, or cloned dairy cattle).
2. Give a benefit with a reason.
3. Give a drawback with a reason (cost, uniformity, welfare, success rate).
4. Make a judgement that depends on the context, and say why.

Must-know distinctions

• Embryo twinning vs SCNT: twinning copies an embryo from a sexual cross, so the adult features are unknown. SCNT copies an existing adult.
• Natural vs artificial: natural clones form without human intervention; artificial ones need laboratory methods.
• Cuttings vs micropropagation: cuttings are cheap, low-tech and give few plants. Micropropagation gives many plants but needs sterile conditions.
• Genotype vs phenotype in clones: the same genotype, but phenotype can differ with environment.

Command words and what they ask for

Command word	What to do for this topic
State	Give a short fact, such as “mitosis”
Describe	Give the steps in order, with the key conditions
Explain	Give a reason for each step, such as why leaves are removed
Compare	Give similarities and differences, with the same feature for each method
Evaluate	Weigh benefits against risks and reach a judgement
Suggest	Apply what you know to an unfamiliar example

Linking ideas across the specification

• Totipotency and differentiation link to stem cells and gene expression in cell biology.
• Mitosis explains why clones share a genome, while meiosis and fertilisation explain why sexual offspring differ.
• Reduced diversity links to selection and the risks of small populations.
• Aseptic technique and sterile media link to the practical skills on the course.
• Ethical arguments link to the how-science-works idea of weighing benefits and risks.

Quick self-test

1. Name two plant structures used for natural vegetative propagation.
2. Why is a cutting cut just below a node?
3. Why are lower leaves removed from a cutting?
4. What are totipotent cells?
5. Which hormone ratio encourages shoot formation in tissue culture?
6. One explant yields 4 shoots per cycle. How many after 3 cycles?
7. Give two disadvantages of micropropagation.
8. How do identical twins form?
9. In SCNT, whose nuclear DNA does the clone carry?
10. Why is the clone not identical in every way to the nucleus donor?
11. 120 reconstructed eggs give 3 live births. Calculate the percentage.
12. Give one argument for and one against animal cloning in agriculture.

Answers

1. Any two of: runner, rhizome, tuber, bulb, corm, sucker.
2. The node has meristematic cells and tissue that can form roots.
3. To reduce water loss by transpiration (and rotting).
4. Cells that can divide and differentiate into any cell type.
5. A high cytokinin to auxin ratio.
6. 4³ = 64.
7. Any two of: cost, skilled staff, contamination risk, low genetic diversity, losses on transfer.
8. An early embryo splits into two groups of cells, which each develop into an individual.
9. The somatic cell (nucleus) donor.
10. Mitochondrial DNA comes from the egg, and the environment (including the womb) differs.
11. 3 ÷ 120 × 100 = 2.5%.
12. For: copy high-yielding animals. Against: reduced genetic diversity, so disease risk, or welfare concerns and low success rates.

Where marks are usually lost

• Saying the SCNT clone matches the egg donor or surrogate.
• Describing micropropagation without sterilisation or growth regulators.
• Leaving out why: “remove leaves” needs the reason, which is less transpiration.
• Calling clones “identical” without noting that the environment affects phenotype.
• Writing only advantages when asked to evaluate. Answers usually gain credit for a reasoned judgement.
• Confusing embryo splitting (natural twins) with artificial embryo twinning.
• Describing “cloning” without saying that mitosis is involved and meiosis is not.
• Not using the data given when asked to interpret a table of results. Quote values and compare them.

Official syllabus

OCR Advanced GCE in Biology A (H420), Version 4.1 (April 2026), first assessment 2025, published by OCR. Section 6.2.1(a) to (d) is covered here.