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Cloning mammals: Dolly the sheep

Use of biological resources · Cloning · note 2 of 3

Spec 5.19B
Biology only (what this means)Biology only: only in International GCSE Biology. Double Award students can skip it. What the labels mean
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Cloning mammals: Dolly the sheepSpec 5.19B

In short

Mammals such as Dolly the sheep are cloned by putting a diploid nucleus from a mature body cell into an enucleated egg cell, which has had its own nucleus removed. An electric shock fuses them and starts division by mitosis. The embryo is implanted into a surrogate mother, and the offspring is genetically identical to the nucleus donor.

A clone is a genetically identical copy. Mammals can be cloned by putting a diploid nucleus from a mature body cell into an enucleated egg cell (an egg cell with its nucleus removed). This was how Dolly the sheep was produced.

  1. A diploid nucleus is taken from a mature body cell of the animal to be cloned. For Dolly this was a cell from the udder of an adult sheep.
  2. The nucleus is removed from an unfertilised egg cell, so the egg cell is enucleated.
  3. The diploid nucleus from the mature cell is introduced into the enucleated egg cell. An electric shock is used to fuse them and to stimulate the egg cell to start dividing.
  4. The egg cell divides by mitosis to form an embryo.
  5. The embryo is implanted into the uterus of a surrogate mother, where it develops. The lamb born is genetically identical to the sheep that gave the nucleus.
Common mistake:

The egg cell has its own nucleus removed, and the nucleus from the body cell is diploid. Do not say the egg cell is fertilised by a sperm.

Cloning Dolly the sheep: a diploid nucleus from an udder body cell of sheep A is put into an enucleated egg cell from sheep B, an electric shock makes it divide by mitosis to form an embryo, the embryo is implanted into a surrogate mother (sheep C), and the lamb born is genetically identical to sheep A. (opens full size in a new tab)
Dolly was a clone of the sheep that gave the diploid nucleus.

Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026

Frequently asked questions

What is micropropagation and why is it used?

Micropropagation is growing new plants from small pieces of tissue called explants on sterile nutrient agar containing plant growth hormones. It is used because commercial quantities of genetically identical plants are produced quickly from a small amount of tissue, so plants with desirable characteristics are copied exactly, all year round and in a small space.

How was Dolly the sheep cloned?

Dolly was cloned by taking a diploid nucleus from a mature udder cell of an adult sheep and putting it into an enucleated egg cell. An electric shock fused them and started the egg dividing by mitosis. The embryo was implanted into a surrogate mother, and Dolly was genetically identical to the nucleus donor.

How are cloned transgenic animals used to produce human proteins?

A human gene for a useful protein is inserted into an animal cell, such as from a sheep or goat, and the cell is cloned by nuclear transfer. The transgenic animal makes the human protein in its milk, where it is collected and purified. Cloning gives many identical animals that all make the protein.