Cloning (AQA GCSE Biology): Revision Note

Tissue culture

• Tissue culture is a process in which very small pieces of plants (‘tissue’) are grown (‘cultured’) using nutrient media

• Because they are initially grown in petri dishes on nutrient agar, they are described as grown ‘in vitro’ – outside a living organism

• How to propagate plants in vitro:

  • Cells are scraped from the parent plant (these cells are known as explants)

  • The cells are transferred to a sterile petri dish containing nutrient agar

  • Hormones (e.g. auxins) are added to encourage plants to grow into small masses of tissue (callus tissue)

  • Tissue continues to grow and forms plantlets that can be transferred to individual potting trays and develop into plants

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• Clones are genetically identical individuals

• The cloning of plants has many important commercial uses

• It allows a variety of a plant with desirable characteristics to be produced:

  • cheaply

  • with a greater yield (a large number of plants can be produced)

  • quickly

  • at any time of the year

• It can also ensure diseases prevalent in other areas of the world are not imported and spread by ensuring native varieties of plants are produced in large enough quantities to supply demand in one country without importing plants from abroad

• Tissue culture can also be an important process in preserving rare plant species

Cuttings

• An older and more simple method to clone plants (mainly used by gardeners) is by taking cuttings

• Gardeners take cuttings from good parent plants (e.g. those that are healthiest and best-looking) – a section of the parent plant with a new bud is cut off

• These cuttings are then planted and grow into genetically identical versions of the original plant

• Plants cloned by taking cuttings can be produced cheaply and quickly

Embryo cloning

• It is possible to clone animals using embryo transplants

• For example, if a farmer wants the best cattle, they must first create an offspring from the best bull and best cow, and then clone this offspring to create many genetically identical copies (clones)

• This process is known as embryo cloning and is achieved in the following way:

  • Egg cells from the best cow are artificially fertilised using sperm cells taken from the best bull

  • This forms an embryo

  • The developing animal embryo is then split apart many times before the cells of the embryo become specialised

  • This forms many separate embryos that are genetically identical

  • These cloned embryos are then transplanted into host mothers

  • The calves born from these host mothers are all genetically identical

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Adult cell cloning

• Adult cell cloning is achieved in the following way:

  • The nucleus is removed from an unfertilised egg cell

  • The nucleus from an adult body cell, such as a skin cell, is inserted into the egg cell

  • A very small electric shock stimulates the egg cell to divide (by mitosis) to form an embryo

  • These embryo cells contain the same genetic information as the adult skin cell

  • When the embryo has developed into a ball of cells, it is inserted into the womb of an adult female (known as the surrogate mother) to continue its development until birth

• This process was used to create the first clone (exact genetic copy) of a mammal in 1996

• Scientists in Scotland successfully cloned an adult female sheep

• The clone was called Dolly

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Benefits and risks of cloning

Benefits:

• Cloning could be used to help preserve endangered species of plants and animals

• Cloning makes it possible to quickly and cheaply produce commercial quantities of consistently high quality plants at any time of the year

• Cloning allows farmers to ensure consistently high quality livestock

Risks:

• Cloning can result in a reduced gene pool (similarly to selective breeding). This means there are fewer different alleles in a population, leaving the population with a smaller chance of having resistance to new diseases

• There is some evidence that cloned animals may not be as healthy as normal ones

• Some people worry that humans might be cloned in the future. This is considered deeply unethical by the vast majority of people. In addition, early attempts might be unsuccessful (eg. they could result in children born with severe disabilities)