Cloning is the process by which a genetically identical copy of an organism is produced. In nature, cloning occurs when organisms such as plants, insects or bacteria reproduce asexually. The copied material is referred to as a clone. There are three main types of cloning:
- Gene cloning: involves cloning of small sections or regions of DNA.
- Reproductive cloning: produces copies of whole animals or cells.
- Therapeutic cloning: produces stem cells for experiments to attempt to replace injured or diseased tissues.
Some plants have been producing identical clones of themselves through natural processes for millions of years. Through the production of a ‘runner’ (stolon) for instance, strawberry crops produce genetically identical offspring. The new plant is referred to as a clone. Similar cloning occurs in grasses, potato crops and onions. Artificial cloning occurs through either vegetative propagation or through tissue propagation. Propagation is the process by which existing organisms produce more offspring.
Vegetative propagation is an ancient form of cloning plants. It involves taking a leaf cutting from a plant and growing it into a new plant. Vegetative propagation occurs because of the presence of a mass of unspecialised cells known as a callus. Callus cells grow, divide and form various specialised cells such as roots and stems, eventually producing a grown plant.
Tissue culture propagation is a more recent practice which involves taking pieces of specialised roots, isolating the cells and growing them in a nutrient-rich culture. In the culture, the specialised cells become transformed into undifferentiated cells. These are similar to the calluses formed above. The calluses then get treated with chemicals that trigger the growth of new plants that are identical to the original plant from which the root pieces were taken as shown in the diagram below. This method of cultivating new plants is known as tissue culture.
Artificial Cloning of Organisms
The technique used to clone whole animals, such as sheep is referred to as reproductive cloning. In reproductive cloning, scientists remove a mature somatic cell from the organism that is to be cloned. A somatic cell is any cell in the body that does not serve a reproductive purpose. In these cells, both sets of chromosomes (from the mother and father) are present. An example of a somatic cell is a skin cell. The nucleus is removed from the `donor’ somatic cell and added to a `recipient’ cell.
The recipient cell is usually an egg cell, from which the nucleus has been removed, so that only the cytoplasm remains (a denucleated cell). The clone produced can then be transferred into a surrogate mother’s womb. A surrogate organism is one which acts as a substitute for another. In this case, the clone is transferred to a surrogate so the embryo can develop.
The cloning processes for reproductive and therapeutic cloning. Reproductive cloning is used for cloning of whole organisms e.g Dolly the Sheep.
Video: Cloning Dolly The Sheep
Learn how Dolly the Sheep was cloned in this video below.