Cloning — reproductive vs therapeutic, Dolly the sheep process

Question

Explain the process used to create Dolly the sheep. Distinguish between reproductive cloning and therapeutic cloning, and state why Dolly was considered genetically identical to only one of the three sheep involved.

Cloning Process Flowchart

flowchart TD
    A["Donor Sheep (Finn Dorset) — mammary cell taken"] --> B["Nucleus extracted from mammary cell (2n)"]
    C["Enucleated egg cell from Scottish Blackface ewe"] --> D["Nucleus removed from egg"]
    B --> E["Donor nucleus inserted into enucleated egg"]
    D --> E
    E --> F["Electric stimulation — triggers cell division"]
    F --> G["Embryo develops in vitro"]
    G --> H["Embryo implanted into surrogate Blackface ewe"]
    H --> I["Dolly born — genetically identical to Finn Dorset donor"]
    style A fill:#f9f,stroke:#333,stroke-width:2px
    style I fill:#bbf,stroke:#333,stroke-width:2px

Solution — Step by Step

Dolly was created by Somatic Cell Nuclear Transfer (SCNT). Ian Wilmut and Keith Campbell at the Roslin Institute (1996) took a mammary gland cell from a Finn Dorset ewe (Sheep 1). This cell was diploid (2n) and fully differentiated — it was not a stem cell or gamete.

An egg cell was taken from a Scottish Blackface ewe (Sheep 2). Its nucleus was removed (enucleation) — leaving behind the cytoplasm with all its mitochondria and cellular machinery. The donor nucleus from the mammary cell was then inserted into this enucleated egg. An electric pulse fused the two and stimulated division.

The reconstructed cell began dividing like a normal embryo. After reaching the blastocyst stage in vitro, it was implanted into the uterus of a third Scottish Blackface ewe (Sheep 3 — the surrogate mother). After a normal gestation period, Dolly was born on 5th July 1996.

Dolly’s nuclear DNA came entirely from Sheep 1 (Finn Dorset) — so her genes, appearance, and traits matched Sheep 1. She was white-faced like the Finn Dorset, not black-faced like the other two. However, her mitochondrial DNA came from Sheep 2 (the egg donor). So technically, Dolly was not a 100% genetic clone — but for all practical purposes, she was genetically identical to Sheep 1.

Why This Works

The experiment proved a revolutionary idea: a differentiated somatic cell can be reprogrammed to behave like a totipotent cell (capable of forming a complete organism). Before Dolly, scientists believed differentiation was irreversible — that once a cell became a mammary cell, it could never “go back.” Dolly showed that the egg cytoplasm contains factors that can reset the nuclear programme.

Three sheep, three roles — this is a classic NEET question: Sheep 1 = nuclear donor (Finn Dorset), Sheep 2 = enucleated egg donor (Scottish Blackface), Sheep 3 = surrogate mother (Scottish Blackface). Dolly resembles Sheep 1.

Reproductive vs Therapeutic Cloning

Feature	Reproductive Cloning	Therapeutic Cloning
Goal	Produce a new organism	Produce embryonic stem cells
Embryo implanted?	Yes, into surrogate	No — harvested at blastocyst stage
End product	Live offspring (clone)	Stem cells for treatment
Example	Dolly the sheep	Stem cell therapy for Parkinson’s
Ethical concern	”Playing God”, low success rate	Destruction of embryo

Common Mistake

Students write that Dolly was genetically identical to the surrogate mother (Sheep 3). This is wrong. The surrogate only provided the uterus — she contributed zero genetic material. Dolly’s nuclear DNA = Sheep 1, mitochondrial DNA = Sheep 2. If the question asks “which sheep is Dolly a clone of?”, the answer is the mammary cell donor (Sheep 1).