Animal Cloning

Animal cloning is far harder than plant cloning because animal cells lose their totipotency early in development. OCR A-Level Biology A specification 6.2.1 (c) requires you to distinguish natural from artificial cloning, to explain the technique of somatic cell nuclear transfer (SCNT), and to evaluate the ethical, economic and scientific arguments surrounding animal cloning — centred on the famous case of Dolly the sheep.

Key Definitions:

• Clone — an organism genetically identical to another.
• Natural cloning — asexual reproduction producing genetically identical offspring (e.g. monozygotic twinning).
• Embryo splitting (artificial twinning) — physically dividing an early embryo into two or more embryos.
• Somatic cell nuclear transfer (SCNT) — transferring the nucleus of a body cell into an enucleated egg.
• Totipotent — able to form any cell type, including extra-embryonic tissues.
• Pluripotent — able to form any cell type of the body but not extra-embryonic tissues.

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Natural Cloning in Animals

Asexual reproduction is common in invertebrates (sponges, corals, flatworms, Hydra, aphids, some lizards, some sharks). Among vertebrates it is rare, but natural twinning does produce clones:

• Monozygotic (identical) twins result from a single fertilised egg splitting into two embryos within the first two weeks. Both have identical genotypes.
• Parthenogenesis — in some animals (aphids, Komodo dragons, hammerhead sharks) eggs develop without fertilisation, producing clones of the mother.

Identical twins are the classic natural human clones. Studies of twins separated at birth have been a mainstay of research into the relative contributions of nature and nurture.

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Artificial Embryo Twinning (Embryo Splitting)

This is the simplest form of artificial animal cloning, mimicking natural twinning.

1. An egg is fertilised in vitro (IVF).
2. The embryo is allowed to divide to the 2-, 4- or 8-cell stage.
3. The cells (blastomeres) are separated and each is allowed to develop as a separate embryo.
4. Each embryo is implanted into a surrogate mother.

The resulting offspring are genetically identical to each other (but not to either parent — they arose from sexual fertilisation). This technique has been used commercially in cattle since the 1980s to produce multiple copies of valuable embryos.

Its main limitation is that the total number of clones is small (typically 2–4) because early embryonic cells lose totipotency quickly.

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Somatic Cell Nuclear Transfer (SCNT)

SCNT is the most powerful animal cloning technique, producing clones of adult animals with known phenotypes. It was the method used to produce Dolly the sheep in 1996.

The Technique

1. A somatic cell (e.g. from the udder, skin, fibroblast) is taken from the donor — the animal to be cloned.
2. The donor cell is starved of nutrients, causing it to enter G₀ (quiescent phase), which allows its nucleus to be "reset".
3. An unfertilised egg is taken from another sheep and its nucleus is removed (enucleated) with a micropipette.
4. The donor cell is fused with the enucleated egg using an electric pulse.
5. The reconstructed cell is stimulated (electrically and/or chemically) to divide, behaving like a fertilised egg.
6. The resulting embryo is cultured in vitro for a few days.
7. The embryo is implanted into a surrogate mother.
8. If development continues, the surrogate gives birth to a clone of the original donor.

The offspring's nuclear DNA matches the donor, but its mitochondrial DNA matches the egg donor — a tiny but real distinction.

Dolly the Sheep

Dolly was born on 5 July 1996 at the Roslin Institute in Edinburgh, created by Ian Wilmut, Keith Campbell and colleagues. She was cloned from a mammary gland cell of a 6-year-old Finn Dorset ewe. The egg came from a Scottish Blackface; the surrogate mother was another Blackface. Dolly was unambiguously Finn Dorset — proving the nuclear DNA was in control.

Dolly's significance:

• First mammal cloned from an adult somatic cell, overturning the long-held belief that adult cells were irreversibly differentiated.
• Demonstrated that the cytoplasm of an egg can reprogramme a somatic nucleus.
• Opened the door to therapeutic cloning and, eventually, induced pluripotent stem cells (iPSCs).