Clones are abundant in nature – when a zygote splits in two identical twins are formed, when bacteria divideasexually by binary fission the resulting bacteria are clones of the original bacterium, when plants reproduce asexually by producing runners the resulting plants are clones and when a singles cell divides during mitosis the two daughter cells are clones. These clones produced by asecual reproduction are advantageous as the process is quick, can be carried out when sexual reproduction cannot and all of the offspring have the genetic information to allow them to survive in their environment. However, asexual reprodcution generates no genetic variation, unless a mutation occurs, and so all of the offspring are equally suscebtible to envorinomental changes such as the introduction of a new disease-causing pathogen.
There are two main types of cloning – reproductive and non-reproductive. Reproductive cloning is the production of offspring which are genetically identical to either their mother, if created by nuclear transfer, or the other offspring, if created by splitting embryos. Non-reproductive cloning is the use of stem cells in order to generate replacement cells, tissues or organs which may be used to treat particular diseases or conditions in humans.
Splitting embryos is the process of seperating out cells from a developing embryo and so producing separate, genetically identical organisms:
Nuclear transfer is when the nucleus of a differentiated adult cell is placed in an enucleated egg cell:
Advantages of artficially cloning aniamls:
High value animals, such as cows with a high milk yield, can be cloned in large numbers.
- Rare animals can be cloned in order to preserve the species.
- Genetically modified animals, such as goats which produce spider silk in their milk, can be quickly produced.
Disadvantages of artifically cloning animals:
- As with asexual reproduction, the genetic unformity of cloning means that all of the offspring will be susceptible to environmental changes.
- Animal welfare is not always taken into account, for example chickens with a high meat yield, yet are unable to walk, have been produced.
- It is not yet known if animals cloned using nuclear material from adult cells will develop any long term health problems.
An example of natural vegetative propogation is the English Elm tree. After damage to the parent plant, such as disease or burning, root suckers (basal sprouts) begin to grow from the meristem tissue in the trunk close to the ground, as this is where the lest damage is likely to have occured. This response to stress or trunk death helps the elm to spread, for example, when an elm is felled during copicing, the root suckers grow into a circle of new elms, called a clonal patch, around the old trunk. These new elms produce their own root suckers, and so the clonal patch continues to expand where resources permit it. However, this adaptation can also be disadvantageous, in particular when it is in response to Dutch elm disease. The roots of an elm infected with dutch elm disease will produce many root suckers, but as these suckers are clones of the original plant they have no resistance to the fungal attack and so as they continue to grow they also begin to show symptoms of the disease. Many plants we take for granted also use vegetative propogation as a survival mechanism – potatoes form tubers which are underground stems swollen with nutrients from which new plants grow, onions and daffodils form bulbs which are condensed shoots containing nutrients and from which new bulbs can fom and strawberries have specialised stems, called runners (see below), which grow along the ground, forming new roots and shoots at the tips. These adaptations all mean the plant can reproduce even if it becomes isolated and there is no reliance on wind, insects or other pollinating agents, but the tubers and bulbs are also a disadvantage as they are an attractive foood source for certain animal including us humans.
Plants can also be propogated aritifically. Traditionally there were two methods of doing this; taking cuttings and grafting:
Taking cuttings is where a stem is cut between nodes and its lower leaves are removed. The cut end is then treated with plant hormones to encourage root growth before planting. The cuttingss are clones of the parent plant. Commercially this techniques is used to quickly produce large numbers of plants such as geraniums.
Grafting is where a shoot section of a woody plant, such as a rosebush or fruit tree, is joined to a rootstock (a root and stem already growing). The graft then grows and is a clone of the original plant, but the rootstock is genetically different.
Although these methods are useful they cannot easily produce high numbers of plants and some plants struggle to reproduce successfully in these ways. The more mordern method of artificial vegetative propogation is micropropogation by callus tissue culture. This method can quickly produce very large stocks of a plant from a small amount of plant tissue, and it has an added advantage that the stock is disease free. Many household plant, such as orchids, are produced using the following method:
- A small piece of tissue (an explant) is taken from the shoot tip of a plant.
- The explant is placed on a nutrient growth medium and cells in the tissue divide to form a callus (a mass of undifferentiated cells).
- Single callus cells are seperated from the mass and placed on a growing medium with plant hormones that encourage shoot growth.
- These growing shoots are then transferred to another medium with hormones encouraging root growth.
- Growing plants are then transferred to a greenhouse to acclimatise and grow further, before being planted outside.
Plant cloning in agriculture has both advantages and disadvantages. Lots of genetically identical plants can be produced from one plant – you know what the plants will be like and the process is faster than selective breeding. Also, costs are reduced as the crop is all ready at the same time and plants can be produced at any time of the year instead of having to wait until their natural growing season. However, the process is arguably more labour intensive as it’s harder to replant small plants than sow seed. Most importantly, environmental change such as the arrival of a new disease could damage the whole crop as their identical genetics means that they would all be equally susceptible.