Genetic Engineering

Cloning means producing genetically identical copies of an organism. Cloning of plants by tissue culture / micropropagation was covered in topic 17. Cloning of animals is much harder because animal cells, unlike plant cells, lose their ability to develop into a whole organism very early in life. Only stem cells in the embryo can do this.

There are two main techniques for cloning animals.

Adult cell cloning (somatic cell nuclear transfer)

This is the technique that produced Dolly the sheep in 1996, the world's first mammal cloned from an adult cell. The five steps:

1. Take an unfertilised egg cell from a female of the species. Using a fine needle, remove its nucleus. The egg is now enucleated (it has no DNA of its own).
2. Take a body cell (any normal somatic cell, often a skin or udder cell) from the adult animal you want to clone. This cell contains the full diploid set of chromosomes.
3. Insert the diploid nucleus from the body cell into the enucleated egg. The egg now contains DNA identical to the adult animal.
4. Give the cell a mild electric shock to fool it into thinking it has been fertilised. The cell starts to divide by mitosis, forming an embryo.
5. Once the embryo has reached a small ball of cells, implant it into the uterus of a surrogate mother. The embryo develops normally and is eventually born.

The newborn is a genetic clone of the animal that supplied the body cell, with the same DNA in every cell. The surrogate mother contributes nothing genetically; she just carries the pregnancy.

Dolly the sheep

In 1996, scientists at the Roslin Institute in Scotland used this technique on sheep:

• DNA donor: a 6-year-old Finn Dorset ewe (white-faced)
• Egg donor: a Scottish Blackface ewe
• Surrogate mother: another Scottish Blackface ewe

Dolly was born with a white face like the DNA donor, not black like the surrogate, proving she was a true clone of the DNA donor and not a normal offspring of the surrogate. She lived to age 6 and had several lambs of her own. Dolly's success started the whole modern field of mammalian cloning.

Embryo cloning (embryo splitting)

A simpler technique called embryo cloning has been used in livestock breeding for decades:

1. Two parent animals with the desired traits are mated normally, producing a fertilised egg (zygote).
2. The zygote divides into an early embryo (a ball of identical stem cells).
3. The embryo is divided into smaller groups of cells, each of which is genetically identical.
4. Each piece is implanted into a separate surrogate mother.