The mammal copiers – advances in cloning
Key text
The cloning of Dolly the sheep has stimulated discussion on the benefits and risks of the development of cloning techniques.
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You will get more from this topic if you have mastered the basics of DNA and genes – these links will take you to an annotated list of sites with helpful background information. |
In nature, cloning is common among plants and is used extensively in plant propagation. The offspring produced by cloning (and other methods of asexual reproduction) simply develop from cells produced by the parent. Because the offspring have genetic information identical to the parent, they develop very similar characteristics to their parent (and to one another) as they mature. This means that an agriculturalist who grows new plants from pieces of an older plant ensures that the new crop will be a fairly uniform one (Box 1: Propagating plants). In contrast, an individual formed by sexual reproduction develops from a cell produced by the union of two cells, usually from different parents. Offspring produced in this way are not genetically identical to each other or to their parents, unlike offspring formed by asexual methods of reproduction.
The only clones produced naturally in vertebrate animals are identical twins. These are formed when cells produced by the early divisions of the fertilised egg separate and independently develop into new individuals. They are therefore genetically identical to each other but not identical to their parents.
Cloning mammals
Since the 1970s scientists have been able to clone mammals such as sheep and monkeys, using cells taken from embryos. What made the cloning of Dolly the sheep so remarkable was that she was cloned from the cell of an adult sheep – something that scientists thought couldn't be done.
To understand the reason that scientists thought it couldn't be done, you need to know a bit about animal cell differentiation.
Animal cell differentiation
As an embryo grows, it produces new cells that all contain the same DNA. At a certain point the cells start to differentiate – or become specialised. Some cells, for example, become nerve cells while others become muscle cells.
Scientists thought that this differentiation was irreversible and that once a cell had differentiated to become, say, a skin cell, it could not change into anything else. It was thought that somehow the DNA inside any particular cell had been chemically 'programmed' to produce only the range of proteins required for it to perform its specific tasks, and that 'reprogramming' was impossible.
The scientists that produced Dolly laid this notion to rest. They found a way to 'reprogram' the DNA of an adult cell so that it could start again from scratch – producing embryonic cells that eventually grew into a living, breathing animal (Box 2: How Dolly the sheep was cloned).
Whole animal cloning
While the cloning of Dolly was greeted with excitement, whole animal cloning (reproductive cloning) is unlikely to be used routinely until the success rate increases substantially – Dolly was the only successful clone from 277 attempts. Recent genetic engineering successes in sheep and cattle suggest that efficient gene transfer systems may soon be available to livestock breeders, without the need to clone from adult tissue.
| February 2003 update: Dolly was given a lethal injection after she was found to have a progressive lung disease; this happened only weeks after Matilda, the first sheep cloned in Australia, died unexpectedly. The deaths of these two relatively young sheep have increased concerns about the safety of cloning. For more information see Obituary: Dolly the sheep (Nature Science Update, 18 February 2003). |
Although there has been a lot of media coverage given to the possibility of cloning humans, scientists around the world have come out strongly against it on safety and ethical grounds (Box 3: On human cloning: A position statement from the Australian Academy of Science).
Explains what stem cells are and how they are used for cloning.
(The Royal Society, UK)
Therapeutic cloning
Cloning techniques may one day enable the production of 'spare parts' such as tissues and organs. These spare parts could then be used for the treatment of damaged or diseased tissue and organs. This is known as therapeutic cloning and involves using stem cells to grow new cell types.
One human organ, skin, is already grown in the laboratory. Healthy skin cells taken from a patient who has suffered severe burns can be grown to provide a self-compatible skin graft – this means that the risk of rejection by the patient's immune system is reduced to virtually zero.
Scientists are able to grow skin more readily than other organs or tissues because mature, differentiated skin cells retain the ability to divide and produce more cells. Most other types of adult cells need to be 'reprogrammed' before they are able to divide. Researchers are now attempting to 'reprogram' and grow other self-compatible cells, such as nerve cells for patients with spinal injuries or muscle cells for heart attack victims.
Boxes
1. Propagating plants
2. How Dolly the sheep was cloned
3. On human cloning: A position statement from the Australian Academy of Science
Related Nova topic
Stem cells – gateway to 21st century medicine
Page updated November 2004.