5d Cloning¶

Part of 5 Use of Biological Resources.

Cloning makes genetically identical copies. In plants it is done by micropropagation, which grows many plants from small pieces of tissue. In mammals it is done by transferring a diploid nucleus from a body cell into an egg cell that has had its own nucleus removed.

What You Need to Learn¶

Further detail: Pearson Edexcel International GCSE Biology specification.

How plants are cloned by micropropagation and cuttings, how a mammal is cloned, and the advantages and disadvantages of cloning. The whole topic is Biology-only content.

Biology-Only Content¶

This content is required for Biology-only students and is not required for Combined Science students.

Micropropagation¶

Micropropagation, also called tissue culture, produces many genetically identical plant clones from small samples of tissue from a parent plant:

1. Small tissue samples, called explants, are taken from the parent plant.
2. The explants are placed in a sterile growth medium containing nutrients and hormones, which stimulate cell division and growth.
3. The explants develop into plantlets. These are clones of the parent, because the cells divided by mitosis.
4. The plantlets are moved into compost and grown on.

The conditions are sterile because the nutrient medium would also feed bacteria and fungi, which would compete with the explants.

A simpler and older way to clone a plant is by cuttings. A section of stem is cut from the parent and rooted in compost, sometimes with rooting hormone, and the new plant is a clone of the parent.

Micropropagation is valuable because:

• it produces large numbers of plants with a wanted feature, such as disease resistance, high yield or a particular flower colour, quickly;
• the plants are genetically identical, so the quality of the crop is predictable;
• it can preserve rare or endangered species, by making many copies from a very small amount of tissue.

Cloning Mammals¶

The first mammal cloned from an adult cell was Dolly the sheep, in 1996. The method is somatic cell nuclear transfer:

1. The nucleus is removed from an unfertilised egg cell, leaving an enucleated egg cell.
2. A nucleus is taken from an adult body cell of the animal to be cloned. It is diploid, so it contains the full set of chromosomes.
3. The diploid nucleus is inserted into the enucleated egg cell.
4. An electric shock stimulates the egg cell to start dividing, and it forms an embryo.
5. The embryo is implanted into the womb of a surrogate female.
6. The offspring is a clone of the animal that donated the body-cell nucleus.

Uses and Limits of Cloning¶

Cloned transgenic animals. A transgenic animal contains a gene from another species. If it is cloned, many identical animals can produce a useful human protein, such as a hormone, in their milk, and the protein can be collected and purified.

Common Confusions¶

• Cloning vs sexual reproduction: Sexual reproduction produces genetically varied offspring. Cloning produces genetically identical copies.
• Micropropagation vs cuttings: Micropropagation uses tissue culture in the laboratory, with hormones and sterile conditions. A cutting is a simpler traditional technique.
• Clone vs surrogate: The surrogate only carries and nourishes the embryo. The clone is of the animal that donated the nucleus.
• Cloning vs genetic modification: Cloning copies a whole genome unchanged. Genetic modification changes the genome by inserting a gene.

Check Yourself¶

1. Define a clone.
2. Describe how explants are used to produce many plants identical to a parent.
3. Explain why the conditions used in micropropagation must be sterile.
4. A lamb is produced from an egg from sheep A, a body-cell nucleus from sheep B and a womb from sheep C. State which sheep the lamb is a clone of, and explain why.
5. Give one advantage and one disadvantage of cloning a farm animal with a high milk yield.

Key Terms¶

• Cloning: the production of genetically identical copies of cells or organisms.
• Micropropagation: a laboratory method of cloning plants using small tissue explants grown in sterile nutrient medium.
• Tissue culture: growing plant cells or tissues in vitro on a nutrient medium to produce clones.
• Explant: a small plant tissue sample for micropropagation.
• In vitro: outside the living organism in controlled artificial conditions.
• Enucleated egg cell: an egg cell from which the nucleus has been removed.
• Diploid nucleus: a nucleus containing two full sets of chromosomes.
• Somatic cell nuclear transfer: the cloning technique in which a diploid body-cell nucleus is inserted into an enucleated egg to create an embryo.
• Surrogate mother: the female into whom a cloned embryo is implanted for development.

Connected Pages¶

• 3b Inheritance
• 5c Genetic Modification
• 5b Selective Breeding
• IGCSE Biology Key Terms Glossary