Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the
learning and teaching of Biology at a variety of levels.
A clone is simply a group of individuals containing exactly the same genetic material.
Originally the term clone was used to cover plant material simply derived from asexual reproduction or vegetative reproduction - tubers, plantlets, offsets etc. and cuttings, grafts etc.
However, the term may also apply to animals which naturally reproduce asexually,
e.g. Amoeba reproduces solely by asexual reproduction to produce genetically identical offspring, and some animals alternate between sexual and asexual stages which result in clones being formed.
Viewed in this way, identical (non-fraternal) twins are fairly commonplace examples of a natural cloning process.
Curiously, the 9-banded armadillo (pictured above) routinely produces 4 offspring which are a clone.
Most bacteria reproduce asexually and so produce offspring which are a clone.
More modern developments
The principles of cloning have been applied to some more fundamental experimentation in plants and animals.
It has been discovered that plant cells which have apparently finished differentiation can be encouraged to revert to an unspecialised state called a callus, which can be caused to grow on special liquid/gel media. This has been done with many types of plants - from normal horticultural species such as carrots and cauliflowers to more exotic species such as new varieties of flowers and palm oil plants. Since the growth involves nuclear division by mitosis and cell division, the resulting cells are identical.
The mass of undifferentiated tissue can be divided into individual cells without damage, and then left to grow into more masses of tissue, similar to an embryo inside a seed. On transfer to different media, these embryos can be persuaded to develop into miniature plants inside special illuminated incubators in the laboratory. This process is called micropropagation.
These plants may then be transferred and grown on using standard horticultural techniques - pots in glasshouses, etc., before release.
Possible medical application
Cloning simpler organisms
It is known that in simpler organisms the differentiation process is less inflexible, and that damage to the body can be overcome due to cells re-growing lost tissue. Much experimental work along these lines has been carried out in in amphibians.
Experiments on amphibian eggs (frog spawn) have proved that it is relatively easy to transfer a new nucleus (from a body cell) into an egg cell which will then usually develop normally according to the genetic information in the transplanted nucleus. This technique has the potential for producing large numbers of genetically genetically identical individual organisms.
It had been thought that in mammals (including Man), the situation was somewhat different and that it was very difficult to persuade nuclei from differentiated cells to divide again, when inserted into other cells.
However, with the advent of techniques including nutritional and temperature conditioning of cells taken from the body of higher animals, it has proved possible to clone mammals, e.g. Dolly the sheep.
This picture shows a mammalian egg cell, held by a blunt pipette on the left hand side, with genetic material being introduced via a narrow pipette on the right hand side.
Several practical problems remain to be solved; the reliability of the process could be increased, and it has transpired that cloned offspring effectively age prematurely - due to progressive deterioration of structures called telomeres at the edges of chromosomes. However some believe that there are a variety of advantages in being able to clone agricultural animals by splitting early embryos.
Although some of the practical difficulties of cloning whole mammals have been overcome, there is little likelihood of applying this cloning technique to humans.
Cloning and embryo cells
In embryos, cells derived from a fertilised egg divide repeatedly to produce tissues for the developing foetus. In an embryo, some dividing cells are becoming differentiated according to their function, but there are also unspecialised stem cells which may be persuaded to divide into different types of cells, depending on the body's requirements.
In the laboratory, cells have been taken from human embryos (normally obtained via an abortion) or from foetal blood cells in umbilical cord. A more hopeful source is the use of "spare" early embryos fertilised in a dish by IVF (in vitro fertilisation) techniques.
There has been much controversy over these procedures, partly based on the origin of the biological material in the first place and partly due to misgivings over the implications of continuing cell division.
However, restrictions have been applied as to the use of these techniques in studying the development of these cells. The human embryology authority had limited research to early stages after fertilisation only. Cell biologists hoped that if allowed greater freedom then research into many genetic diseases, e.g. cystic fibrosis, could benefit.
The latest development in this field is that British MPs have voted to allow research into stem cells.
It is especially likely to be of use to researchers studying degenerative diseases, e.g. Alzheimer's, but also in the study and treatment of a variety of cancers.
There is no possibility that these cells will develop into a new individual unless implanted into a uterus. Human cloning "for reproductive purposes" remains illegal.
This topic has connections with other BioTopics units on:-
Cloning of human body parts for transplant surgery
The two methods of reproduction
ASEXUAL REPRODUCTION AND ARTIFICIAL PROPAGATION