Reproductive and non-reproductive cloning
Reproductive cloning is the production of offspring which are genetically identical to either the mother (nuclear transfer), or the other offspring (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 of humans.
Natural clones in plants, vegetative propagation
Molecular evidence has shown that all specimens of the English Elm tree, Ulmus procera, form a genetically isolated clone. English Elms developed from a variety of elm brought to Britain from Rome in the first centure A.D. Although English Elm trees make pollen, they rarely produce seeds. Instead they spead by developing structures known as suckers from their roots. Each sucker can grow into a new tree. This tendancy of elms to create suckers has been exploited by humans, who have separated the sucks, with roots attached, and used them to plant hedges and establish new woodlands. The English Elm clone is genetically isolated from other varieties of elm because they do not produe seens and they only reproduce asexually. The production of suckers is an example of vegetative propagation. When the tree is damaged, the suckers can grow around the old trunk, forming a clonal patch, and grow into new trees.
In 1967, a new, virulent strain of an elm disease fungus arrived in Great Britain on imported timber. Beetles that lived under the bark of elm trees spead the fungus. The saws used to cut down dead branchs were not sterilised after use. When the saws were used to prune healthy trees, these trees became infected. Approximately 25 million trees, most of the English Elm population, died within a few years of the arrival of this fungus. This happened because they were all susceptible to this disease as, being clones, they are genetically identical. The beetles fly from tree to tree actog as a vector and as the trees grow close together and the disease also sperads through the suckers it spread rapidly.
Artificial clones of plants from tissue culture.
Use a leaf, stem, root, bud (i.e. meristem or the dividing region at tip of plant) using aseptic technique. Cut the plant material into small pieces called explants and sterilisd them using bleach, sodium hypochlorite or alcohol.
Place the explants on agar which contain glucose, amino acids, nitrates and phosphates where they form a callus (mass of undifferentiated cells) by treating the explant with high auxin and cytokinin concentrations.
Subdivide the callus and treat to induce roots and shoots by changing the plant hormone ratio. Transfer to a greenhouse or soil in a non-sterile environment to be acclimatised and grown further before they are planted outside.
Advantages and disadvantages of cloning
- Disease-free stock is created
- Uniform plants are created
- It is possible to reproduce infertile plants
- It is possible to reproduce plants that are hard to grow from seed
- It is possible to create whole plants from genetically modified cells
- Production is not determined by seasons so it can take place anytime, anywhere in the world
- Plantlets are small and so can be transported easily and grown in small space
- Can save rare species from extinction
- It is an expensive and labour intensive process
- The process can fail due to microbial contamination
- All offspring are susceptible to the same pest, disease or environmental factor (e.g. drought)
- There is no genetic variation
Artificial clones of animas
Nuclear transfer: A nucleus from an adult differentiated cell is placed in an enucleated egg cell. The egg then goes through the stages of development using the genetic information from the inserted nucleus.
Splitting embryos: Cells from a developing embryo are separated out, with each other going on to produce a separate, genetically identical organism.
Advantages and disadvantages of cloning animals
- High value animals, e.g. cows giving a high milk yield, can be cloned in high numbers
- Rare animals can be cloned to preserve the species
- Genetically modified animals, e.g. sheep that produce pharmaceutical chemicals in their milk - can be quickly reproduced
- High value animals are not necessarily produced with animal welfare in mind. Some strains of meat producing chickens have been developed that are unable to walk
- As with plants, excessive genetic uniformity in a species makes it unlikely to be able to cope with, or adapt to, changes in the environment
- It is still unclear whether animals cloned using the nuclear material of adult cells will remain healthy in the long term