Cloning (biology only) — AQA GCSE Biology
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Cloning (biology only) explained
Tissue culture is a cloning method that uses small groups of cells taken from part of a plant, such as a shoot tip or leaf, to grow identical new plants.
Read the full explanation
The cells are grown on a sterile nutrient medium containing hormones, and because they divide by mitosis the new plants are genetically identical clones of the parent. Sterile conditions prevent microorganisms contaminating the culture. Tissue culture is valuable for preserving rare plant species, because many identical plants can be produced from a small amount of material, and commercially in nurseries, where large numbers of uniform plants can be grown quickly. Students should describe the method and give both reasons for its importance.
Cuttings: an older, but simple, method used by gardeners to produce many identical new plants from a parent plant.
Cuttings are a traditional form of asexual reproduction used by gardeners to make many genetically identical copies of a chosen parent plant. A healthy shoot is cut from the parent, lower leaves are removed, and the cut end may be dipped in rooting powder before being planted in damp compost. Because no gametes fuse, the new plant is a clone and inherits the parent's desirable features, such as flower colour or fruit yield. The method is simple, cheap and needs no specialist laboratory, but it relies on the parent being healthy and can spread a viral disease to every cutting. Large numbers of uniform plants can be raised quickly from one parent, which is why growers use cuttings for many ornamental shrubs and houseplants.
Embryo transplants: splitting apart cells from a developing animal embryo before they become specialised, then transplanting the identical embryos into host mothers.
Embryo transplants are an artificial cloning technique used in animal breeding. A donor female is treated with hormones so that she releases several eggs, which are fertilised by a chosen male. The resulting embryo is allowed to divide a few times, then it is split into separate cells or small groups of cells while they are still unspecialised. Each part can develop into a complete embryo because the cells have not yet differentiated. The identical embryos are transplanted into host mothers, where they develop and are born. The offspring are clones of one another, though not of either parent, and a single valuable mating can produce many identical animals. The method is quicker than waiting for several pregnancies and allows desirable characteristics to be multiplied.
Adult cell cloning: • 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.
Adult cell cloning produces a genetically identical copy of the adult that donated the body cell. First an unfertilised egg cell is taken from a female of the same species. Its own nucleus is removed, leaving the cytoplasm and cell membrane; this is called enucleation. The nucleus is then taken from an adult body cell, such as a skin cell, of the animal to be cloned. That nucleus is inserted into the empty egg cell, so the egg now contains a full set of chromosomes from the adult donor. Because the egg's original genetic material has gone, all the nuclear DNA in the reconstructed cell comes from the adult body cell. The egg cell is not fertilised, so no sperm nucleus is involved. For example, removing the nucleus from a sheep egg cell and inserting a skin-cell nucleus from a second sheep creates a cell whose nuclear genes match that second sheep.
An electric shock stimulates the egg cell to divide to form an embryo. • These embryo cells contain the same genetic information as the adult skin cell.
After the adult nucleus has been inserted into the enucleated egg cell, the reconstructed cell is given a small electric shock. This stimulus makes the egg cell start to divide by mitosis, as though it had been fertilised. Repeated division produces a ball of cells called an embryo. Because the embryo's cells arise by mitosis from the reconstructed egg, every one of them contains the same set of chromosomes as the inserted adult nucleus. The nuclear DNA therefore matches the adult body cell, such as the skin cell, that donated the nucleus. The embryo is later placed into a surrogate mother, where it develops into a clone of the adult donor. For example, the first cloned mammal, Dolly the sheep, was produced by this method, and her nuclear DNA matched the sheep that donated the udder cell nucleus.
When the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue its development.
This statement describes a stage in adult cell cloning, a technique used to produce a genetically identical copy of an animal. After the nucleus is removed from an egg cell and replaced with the nucleus from a body cell of the animal being cloned, the egg is stimulated to divide by mitosis. Once the resulting embryo has grown into a ball of cells, it is transferred into the womb of a surrogate adult female. The womb provides the temperature, nutrients and oxygen the embryo needs, so it can implant and continue developing into a full organism. The surrogate is not the genetic parent; the clone's nuclear DNA comes from the donor body cell. This is how Dolly the sheep was produced.
Your focus
- Describe how tissue culture produces identical new plants
- Explain why sterile conditions and hormones are needed
- Evaluate the importance of tissue culture for conservation and nurseries
Show all 18 objectives
- Describe how a cutting is taken from a parent plant and grown into a new plant.
- Explain why plants grown from cuttings are genetically identical clones.
- Evaluate the advantages and limitations of using cuttings to propagate plants.
- Describe the sequence of steps in embryo transplantation.
- Explain why splitting the embryo before specialisation allows identical embryos to form.
- Discuss why embryo transplants are used in animal breeding.
- Describe the steps of adult cell cloning in the correct sequence.
- Identify the source of the nucleus and the role of the unfertilised egg cell.
- Explain why the reconstructed cell contains only the adult body cell's nuclear genetic information.
- Describe how an electric shock stimulates the reconstructed egg cell to divide and form an embryo.
- Explain why the embryo cells are genetically identical to the adult body cell.
- Relate the process to the production of a clone and the role of a surrogate mother.
- Describe the transfer of a ball of cells into the womb during adult cell cloning.
- Explain why the embryo must divide by mitosis before it is inserted into the womb.
- State that the surrogate female is not the source of the clone's nuclear DNA.
Cloning (biology only) exam tips
Marking Points
- Small groups of cells are taken from part of a plant, such as a shoot tip
- Cells are grown on a sterile nutrient medium with hormones
- The cells divide by mitosis to form identical new plants
- Tissue culture preserves rare plant species because many clones can be grown from limited material
- Commercially, nurseries use tissue culture to produce large numbers of uniform plants quickly
- Cuttings are a form of asexual reproduction, so no gametes fuse and no fertilisation occurs.
- A shoot or piece of stem is cut from the parent plant and planted so that it grows into a new plant.
- The offspring are genetically identical clones of the parent because all cells arise by mitosis from the parent tissue.
- Gardeners use cuttings because the method is simple, inexpensive and produces many identical plants quickly.
- Removing lower leaves and keeping the cutting in warm, damp conditions encourages root growth.
- A limitation is that any disease present in the parent can be passed to all the cuttings.
- Embryo transplants begin with fertilisation of eggs from a donor female by a chosen male.
- The developing embryo is split into separate cells or groups of cells before the cells become specialised.
- Each separated cell or group can grow into a complete embryo because the cells are still unspecialised.
- The identical embryos are transplanted into host mothers, which carry them to birth.
- The resulting offspring are genetically identical to one another, so they are clones.
- The technique allows many identical offspring to be produced from one valuable mating.
- An unfertilised egg cell is obtained from a female of the same species as the animal to be cloned.
- The egg cell's own nucleus is removed, a step called enucleation, leaving the cytoplasm and membrane intact.
- A body cell, for example a skin cell, is taken from the adult animal that is to be cloned.
- The nucleus is extracted from that adult body cell and inserted into the enucleated egg cell.
- The resulting cell contains nuclear DNA from the adult body cell only, because the egg's original nucleus was removed.
- No fertilisation occurs, so no sperm cell or second parent contributes genetic material at this stage.
- An electric shock is applied to the reconstructed egg cell to stimulate it to divide.
- The egg cell divides by mitosis to form an embryo, a ball of genetically identical cells.
- The embryo cells contain the same genetic information as the adult body cell that donated the nucleus.
- The embryo is implanted into a surrogate mother, where it continues to develop.
- The resulting offspring is genetically identical to the adult body-cell donor, so it is a clone.
- The electric shock replaces the trigger normally provided by fertilisation, but no sperm DNA is involved.
- The embryo is produced by mitosis after the donor nucleus is placed into an egg cell whose own nucleus has been removed.
- The embryo must reach the ball-of-cells stage before transfer; it is not inserted as a single cell.
- The embryo is inserted into the womb (uterus) of an adult female, often called the surrogate.
- The surrogate's womb allows the embryo to implant and continue its development to birth.
- The surrogate does not contribute nuclear genetic material, so the offspring is genetically identical to the donor of the body cell.
- This process is a form of cloning and is studied as adult cell cloning in animals.
Examiner Tips
- 💡Name the source of cells and the growth medium in your answer
- 💡State that mitosis makes the new plants identical clones
- 💡Give one conservation reason and one commercial reason for using tissue culture
- 💡State clearly that the offspring are genetically identical to the parent, using the term clone.
- 💡Link the method to a practical step, such as cutting a shoot and planting it in damp compost.
- 💡If asked to compare with sexual reproduction, mention the absence of gametes and fertilisation.
- 💡Use the phrase before they become specialised when describing when the embryo is split.
- 💡Make clear that the host mother does not pass on her genes to the offspring.
- 💡Link the technique to its purpose, such as producing many identical animals from one desirable pair.
- 💡Use the precise terms unfertilised egg cell, enucleation and adult body cell rather than vague words like egg and cell.
- 💡Describe the sequence in order: remove egg nucleus, take adult body-cell nucleus, insert it into the empty egg cell.
- 💡State clearly that the clone's nuclear DNA comes from the adult body-cell donor, which explains why the offspring is genetically identical to that donor.
- 💡Link the electric shock directly to the start of cell division, and name mitosis as the type of division.
- 💡Explain the genetic identity of the embryo by referring back to the removal of the egg nucleus and insertion of the adult nucleus.
- 💡Use the term surrogate mother when describing where the embryo develops, and state that the offspring is a clone of the adult donor.
- 💡Use the correct sequence words: nucleus removed, donor nucleus inserted, mitosis, ball of cells, inserted into womb, development continues.
- 💡Link the womb to its function: it supports implantation and supplies the embryo with what it needs to develop.
- 💡If asked to compare with sexual reproduction, state that no fertilisation occurs and the offspring is genetically identical to the donor.
Common Mistakes
- Confusing tissue culture with taking cuttings; correction: tissue culture uses small groups of cells grown on a medium, not detached stem pieces rooted in soil
- Saying the new plants are genetically different; correction: mitosis produces genetically identical clones
- Forgetting sterile conditions; correction: contamination by microorganisms would destroy the culture
- Thinking cuttings involve seeds or fertilisation; correct this by stressing that cuttings are asexual and produce clones.
- Believing the new plant is only partly like the parent; correct this by explaining that all its cells come from the parent by mitosis, so it is genetically identical.
- Assuming cuttings always succeed; correct this by noting that they need suitable warmth, moisture and rooting conditions.
- Saying the embryos are split after the cells become specialised; correct this by stating that splitting must happen before specialisation.
- Confusing embryo transplants with adult cell cloning; correct this by noting that embryo transplants start from a fertilised embryo, not from an adult body cell.
- Thinking the host mother contributes genes; correct this by explaining that she only provides the environment for development.
- Thinking the egg cell keeps its own nucleus and simply receives an extra one; the correction is that the egg nucleus must be removed first so only the adult nucleus remains.
- Believing the egg cell is fertilised by a sperm; the correction is that the egg is unfertilised and the adult nucleus replaces the egg nucleus.
- Saying the whole adult body cell is placed inside the egg; the correction is that the nucleus is removed from the body cell and inserted, not the entire cell.
- Saying the electric shock fertilises the egg; the correction is that it stimulates division, and no sperm is involved.
- Claiming the embryo cells contain a mixture of DNA from the egg donor and the body-cell donor; the correction is that the egg nucleus was removed, so the DNA matches the adult body cell.
- Confusing mitosis with meiosis in this context; the correction is that the embryo forms by mitosis, producing genetically identical cells.
- Thinking the surrogate mother is the genetic parent; correct this by stating that the nucleus comes from the donor body cell, so the clone's nuclear DNA matches the donor.
- Believing the embryo is inserted immediately after the nucleus is transferred; correct this by noting that it must first divide by mitosis to form a ball of cells.
- Confusing this with embryo cloning or with plant cloning; correct this by specifying that adult cell cloning uses a body cell nucleus and a surrogate womb.