Cloning (AQA GCSE Biology): Revision Note

Tissue culture

• Clones are genetically identical individuals

• Tissue culture is a process by which plants can be cloned

• During tissue culture:

  • a small group of cells is scraped from the parent plant

  • the cells are allowed to grow into new individual plants on nutrient agar

  • growth occurs in sterile conditions to prevent bacterial or fungal growth from outcompeting the plant cells

• Hormones (e.g. auxins) are added to the nutrient medium to encourage cell growth and division

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• Tissue culture is a commercially valuable technique because it allows plant with desirable characteristics to be produced:

  • cheaply

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

  • quickly

  • at any time of the year

  • in a disease-free way

    • it avoids importing plants that may carry disease

• Tissue culture can be a way of preserving rare plant species

  • E.g. a single rare individual can be cloned and many new plants cultivated in a safe environment before being planted out in the wild

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

• This is asexual reproduction: no gametes fuse and the new cells are made by mitosis, so every cutting has the same alleles as the parent

• Plants grown from seed vary because they come from sexual reproduction (meiosis, and mixing of alleles from two parents through cross-pollination)

• 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 as follows:

  1. Egg cells from the desired cow are artificially fertilised using sperm cells taken from the desired bull

  2. The resulting zygote develops into an embryo

  3. The embryo is then split apart into serval individual cells

     • This is done before the cells of the embryo become specialised

  4. Each cell forms a separate embryos; all embryos developed from the divided cells are genetically identical

  5. The cloned embryos are transplanted into host mothers

  6. 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)