5d Cloning¶

Part of 5 Use of Biological Resources.

Cloning produces genetically identical copies. In this course the two main contexts are micropropagation in plants and cloning mammals by transferring a diploid nucleus into an enucleated egg cell.

Learning Objectives¶

Biology-Only Content¶

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

Micropropagation¶

Micropropagation (tissue culture) is the process of producing many genetically identical plant clones from small tissue samples of a parent plant.

How it works: 1. Small samples (explants) of tissue are taken from the parent plant. 2. The explants are placed in a sterile growth medium containing nutrients and hormones to stimulate cell division and growth. 3. The explants develop into plantlets, which are genetically identical clones of the parent. 4. The plantlets are transferred into compost and grown on.

An older and simpler method of plant cloning is cuttings: a section of stem is cut from the parent plant and rooted in compost (sometimes with rooting hormone applied). The resulting plant is a genetic clone of the parent.

Why micropropagation is valuable: - Produces large commercial quantities of plants with desirable characteristics (e.g. disease resistance, high yield, particular flower colour) quickly. - All plants produced are genetically identical, so the quality of the crop is predictable. - Can preserve rare or endangered plant species by producing many copies from a small amount of tissue.

Cloning Mammals¶

The first successfully cloned mammal was Dolly the sheep (1996). The process used was somatic cell nuclear transfer:

1. The nucleus is removed from an unfertilised egg cell (producing an enucleated egg cell).
2. The nucleus is removed from an adult body cell of the animal to be cloned (this nucleus is diploid — it contains the full set of chromosomes).
3. The diploid nucleus from the body cell is inserted into the enucleated egg cell.
4. An electric shock stimulates the egg cell to begin dividing, forming an embryo.
5. The embryo is implanted into the womb of a surrogate female.
6. The resulting offspring is a genetic clone of the animal that donated the body cell nucleus.

Uses and Limits of Cloning¶

Cloned transgenic animals: Transgenic animals contain genes from a different species inserted into their genome. If such an animal is then cloned, large numbers of identical animals can produce a useful human protein (such as antibiotics or hormones) in their milk, which can then be collected and purified.

Evaluating cloning:

Common Confusions¶

• Cloning vs sexual reproduction: Sexual reproduction produces genetically varied offspring; cloning produces genetically identical copies.
• Micropropagation vs cuttings: Micropropagation uses laboratory tissue culture with hormones and sterile conditions. Cuttings are a simpler traditional technique.

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 sample of plant tissue used as the starting material for micropropagation.
• In vitro: outside the living organism; in an artificial, controlled environment.
• Enucleated egg cell: an egg cell from which the nucleus has been removed.
• Diploid nucleus: a nucleus containing two full sets of chromosomes (46 in humans).
• 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