- 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 propagationor throughtissue propagation. Propagation is the process by which
existing organisms produce more offspring.
Vegetative propagationis 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 acallus. Callus cells grow, divide and
form various specialised cells such as roots and stems, eventually producing a grown plant.
Figure 5.38: Growing newPlumeriaplants from cuttings.Tissue culture propagationis a more recent practice which involves taking pieces of spe-
cialised 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 astissue
culture.
Artificial cloning of organisms
The technique used to clone whole animals, such as sheep is referred to asreproductive
cloning. In reproductive cloning, scientists remove a maturesomatic cell. 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 in which the nucleus has been removed (a denucleated cell). The clone produced can
then be transferred into asurrogate mother’swomb. 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.
Chapter 5. Plant and animal tissues 147