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

This guide covers 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. It sits in Module 6, Genetics, evolution and ecosystems. It teaches natural and artificial cloning in plants and animals, with the arguments for and against. Sections 6.2.1(e) to (i), on microorganisms and immobilised enzymes in biotechnology, are outside this unit.

Use it with the revision notes and the practice questions. The OCR A Level Biology hub and the printable checklist list the rest of the course, and the free diagnostics show where to start. Development of Practical Skills supports the practical ideas.

What this unit covers

Spec reference	What you must be able to do
6.2.1(a)(i)	Natural plant clones and clones for horticulture (vegetative propagation)
6.2.1(a)(ii)	How to take plant cuttings
6.2.1(b)(i)	Artificial plant clones by micropropagation and tissue culture
6.2.1(b)(ii)	Arguments for and against plant cloning; uses in horticulture and agriculture
6.2.1(c)	Natural animal clones, including twins formed by embryo splitting
6.2.1(d)(i)	Artificial embryo twinning; enucleation and somatic cell nuclear transfer (SCNT)
6.2.1(d)(ii)	Arguments for and against animal cloning, including agriculture, medicine and longevity

The specification links this unit to practical activity group 2 (dissection of plant material to produce cuttings) and to how science works: applications and implications (HSW9), ethics (HSW10) and society’s use of science (HSW12).

Clones: the key idea

A clone is a group of genetically identical individuals, or cells, that come from one parent by asexual reproduction. Asexual reproduction uses mitosis only, with no meiosis and no fertilisation, so offspring carry the parent’s alleles. Differences between clones come from the environment, apart from rare mutations. Cloning is “natural” when the organism does it unaided and “artificial” when people intervene.

6.2.1(a)(i) Natural clones in plants

Many plants reproduce by vegetative propagation. A part of the parent grows into a new, independent plant. The organs involved are modified shoots or leaves:

Structure	How a new plant forms	Example
Runner (stolon)	A horizontal stem grows over the surface and puts down roots and shoots at nodes	strawberry, spider plant
Rhizome	An underground horizontal stem carries buds, which grow into new shoots	iris, couch grass
Stem tuber	A swollen underground stem stores starch and has buds (“eyes”) that sprout	potato
Bulb	An underground bud with fleshy leaf bases stores food; side bulbs grow into new plants	daffodil, onion
Corm	A swollen stem base stores food; new corms form on top of the old one	crocus
Sucker	A new shoot grows from a root or from the base of the stem	raspberry

Vegetative propagation is rapid and needs no pollinator. The weakness is low genetic variation: one disease can affect every clone.

Horticulture uses these natural methods on purpose. A grower divides rhizomes, separates bulbs and plants runners to get uniform plants with known flower colour or fruit quality, which seed-grown plants might not give.

6.2.1(a)(ii) Taking a cutting

A cutting is a piece of stem, leaf or root that grows into a new plant. Stem cuttings from a plant such as Pelargonium are a simple cloning technique.

1. Choose a healthy, non-flowering shoot and cut it with a clean, sharp blade (to avoid crushing tissue and letting infection in).
2. Cut just below a node. The node region has meristematic cells and tissue that can form new roots.
3. Remove the lower leaves, to cut water loss by transpiration and stop rotting.
4. Optionally dip the cut end in rooting powder, which contains an auxin that promotes root growth.
5. Push the cutting into moist, well-drained compost. Keep it humid, to reduce water loss until roots form.
6. Keep it warm and in the light.

For the practical link, you may dissect plant material to find nodes, vascular tissue and buds. Record a labelled drawing.

Worked example 1: evaluating a rooting treatment

A student takes 30 cuttings of one plant species for each group. After four weeks, 24 cuttings treated with rooting powder have rooted and 17 untreated cuttings have rooted.

• Percentage rooted with powder: 24 ÷ 30 × 100 = 80%.
• Percentage rooted without: 17 ÷ 30 × 100 = 56.7%.
• Difference: 80 − 56.7 = 23.3 percentage points. Rooting powder gave 41.2% more rooted cuttings than no powder, because (24 − 17) ÷ 17 × 100 = 41.2%.

To evaluate, note that one species and one trial is limited. Cuttings should match in length and node position, share light and temperature, and the trial should be repeated.

6.2.1(b)(i) Micropropagation and tissue culture

Micropropagation produces large numbers of identical plantlets from a very small piece of tissue, under sterile conditions. Tissue culture is the growing of cells or tissues in an artificial medium. Micropropagation relies on the fact that many plant cells are totipotent. A mature plant cell can be stimulated to divide and differentiate into any cell type, given the right signals.

1. Select a parent with the desired features and take an explant, such as a shoot tip (apical meristem).
2. Sterilise the explant, for example with dilute bleach or ethanol, to remove bacteria and fungi. Use aseptic technique throughout.
3. Place it on a sterile nutrient agar medium containing sugar (an energy and carbon source), mineral ions, vitamins and plant growth regulators.
4. Control the hormone balance. Cytokinin encourages shoot formation and auxin encourages root formation. A high cytokinin-to-auxin ratio gives shoots and a high auxin-to-cytokinin ratio gives roots. Cells may first form a mass of undifferentiated cells called a callus.
5. Multiply. Shoots are cut up and transferred to fresh medium (sub-culturing). Each cycle multiplies the number.
6. Root the shoots on a medium that favours roots, then transfer them to compost and acclimatise them gradually to normal humidity (“hardening off”).

Sterility is essential because the sugar-rich medium suits microbes, which would outgrow the plantlets.

Worked example 2: multiplication rate

One explant gives 6 shoots per four-week culture cycle, and each shoot is subcultured to give 6 more.

• After 4 cycles (16 weeks): 6⁴ = 1296 shoots.
• If 5% of the shoots are lost to contamination, 1296 × 0.95 = 1231.2, so about 1230 plantlets remain.

6.2.1(b)(ii) Plant cloning: arguments for and against

For	Against
Many identical plants quickly from one parent	Initial cost: equipment, sterile rooms and trained staff
Works all year, in a small space	One contamination can ruin a batch
Meristem tissue is often free of viruses, so plantlets are disease-free	Clones share the same alleles, so one disease or pest can destroy the entire crop
Preserves rare or endangered species and plants that seed poorly	Reduced genetic diversity limits adaptation and future breeding
Uniform crops, easier to harvest and sell	Plantlets may fail when moved to soil
Helps plants that seed poorly or flower slowly	Occasional unexpected variation (somaclonal variation)

Evaluation: horticulture gains uniform, healthy ornamental stock, and agriculture gains disease-free planting material for crops that propagate vegetatively. Cost and loss of diversity are the main drawbacks, so weigh benefit against risk for a named use.

6.2.1(c) Natural clones in animals

The specification’s example is identical (monozygotic) twins. After a zygote has divided by mitosis to form a small ball of cells, the early embryo splits into two separate groups, each developing into a separate individual. This is embryo splitting. The twins share a genome because they come from one fertilised egg.

Other natural animal clones include the identical offspring of the nine-banded armadillo, budding in Hydra and parthenogenesis in aphids.

Identical twins are not exactly the same. Mutation and different environments (diet, disease, lifestyle) create differences in phenotype, so genotype does not alone determine phenotype.

6.2.1(d)(i) Artificial cloning in animals

Artificial embryo twinning

1. Eggs and sperm from selected parents are used in vitro fertilisation.
2. The embryo divides to a very early stage (a few cells), when the cells are totipotent.
3. The embryo is split into two or more groups of cells, which are cultured separately in vitro.
4. The embryos are implanted in surrogate mothers, where they develop.

The offspring are identical to each other but, as they come from a sexual cross, not to either parent. This method cannot copy an adult animal that has already proved its worth.

Enucleation and somatic cell nuclear transfer (SCNT)

SCNT can clone an existing, adult animal.

1. A somatic cell (any body cell, not a gamete) is taken from the animal to be cloned. Its diploid nucleus holds the full genome.
2. An unfertilised egg is taken from a donor female and enucleated: its own nucleus is removed.
3. The somatic cell nucleus is put into the enucleated egg, by fusing the whole cell to the egg with a small electric pulse or by injecting the nucleus.
4. The egg is stimulated to divide, for example with an electrical or chemical stimulus. It develops in culture to an early embryo.
5. The embryo is implanted into a surrogate mother and develops to birth.

The clone’s nuclear DNA is identical to the nucleus donor’s. Mitochondrial DNA comes from the egg cytoplasm, so the clone is not identical in every respect. The surrogate contributes no DNA but provides the womb environment.

Worked example 3: SCNT efficiency

In a trial, 150 reconstructed eggs gave 60 embryos, 12 pregnancies and 5 births. 3 of the 5 were alive after a year.

• Embryos per reconstructed egg: 60 ÷ 150 × 100 = 40%.
• Births per reconstructed egg: 5 ÷ 150 × 100 = 3.3%.
• Survival of births to one year: 3 ÷ 5 × 100 = 60%.
• Overall, 3 ÷ 150 × 100 = 2% produced a surviving animal.

Low efficiency makes the technique costly and raises welfare concerns.

6.2.1(d)(ii) Animal cloning: arguments for and against

Area	Possible benefits	Concerns
Agriculture	Copy animals with high milk yield, meat quality or disease resistance; more uniform herds	Reduced genetic diversity; disease can affect all clones; very high cost
Medicine	Cloned animals producing human proteins in their milk; animals as models of disease; possible source of tissue	Ethical issues over the use of animals; low success rates
Conservation	Add individuals to a tiny population; a black-footed ferret was cloned in 2020 from cells frozen in 1988, restoring lost diversity	Does not solve habitat loss
Welfare		Many failed pregnancies and abnormal births; ethical objections to treating animals as products

Longevity of cloned animals

Dolly, the first mammal cloned from an adult cell (born 1996), was euthanised at about six and a half after lung disease and arthritis, and her short telomeres led some to ask whether clones age early. A later study of cloned sheep aged 7 to 9, four from Dolly’s cell line, found no clear premature ageing, only mild arthritis in some. The evidence is limited, so say that claims about longevity are not settled.

Worked example 4: genotype and environment

Cattle clones from two embryo-split pairs have milk yields (litres per year): pair 1 6950 and 7310; pair 2 7880 and 7540.

• Pair means: 7130 and 7710 litres.
• Differences within pairs: 360 and 340 litres, which is 5.0% and 4.4% of the pair means.

Clones within a pair share a genotype, so their differences are due to environment. The gap between pair means hints that genotype matters, but two pairs is too few to be sure.

Common errors

• Saying a SCNT clone is identical to the egg donor or the surrogate. The nuclear DNA matches the nucleus donor.
• Calling embryo splitting “cloning an adult”. It copies an embryo.
• Forgetting aseptic technique or the hormones in micropropagation.
• Writing that clones are “exactly identical”. The environment still alters phenotype.
• Giving one-sided answers to an evaluation. Include a benefit, a risk and a judgement.

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.