Cloning animals
Complex biotechnological procedures have enabled scientists to successfully clone mice, sheep, cows and other mammals.
The technology is still at early stages and currently, one in three cloned animals is born abnormally large or with other developmental problems.
Scientists at the Monash Institute of Reproduction and Development in Melbourne believe these problems could be linked to a process called gene 'imprinting'. Embryos contain two copies of each gene – one from each parent. It is thought that about 60 genes are ‘imprinted' with instructions to switch one copy on or off to allow for normal growth and development.
If this doesn’t happen correctly and both copies are switched on, or both copies are switched off, it results in problems in growth and development, both prenatal and postnatal. We do not understand this imprinting process in cloned embryos.
The closest scientists have come to cloning a non-human primate occurred in October 2004. Biologists successfully transferred cloned monkey embryos into monkey mothers. None of the resulting pregnancies lasted more than a month.
Did you know flies are the latest animals to be cloned? Fruit flies have long been a ‘model’ to study reproductive biology and researchers think the insect may help science understand why cloning is often flawed.
Somatic cell nuclear transfer
Roslin Institute, Edinburgh
Dolly, the first animal to be cloned, was created using the technique of somatic cell nuclear transfer (SCNT).
To do this, cells are taken from the animal that is going to be cloned. In the case of Dolly the sheep, a cell was taken from her udder. These are normal body cells – somatic cells - and the nucleus is removed.
The nucleus contains all of the genetic material to make the animal and so, is termed the donor cell.
Egg cells are used for cloning because of their ability to grow rapidly. The egg cell’s nucleus is removed and the nucleus from the donor cell is inserted in its place.
The egg is then stimulated to grow by numerous stimulants which activates the reconstructed embryo to divide and grow. The division of the egg cell follows the same process that would occur if the egg was fertilised by sperm during natural reproduction.
The cell division continues for 5 days until a blastomere forms. A blastomere is a ball of nearly 100 cells all with the same genetic material as the donor.
Once a cloned embryo reaches the blastomere stage of development, it can follow two paths. Either it can be implanted into a uterus of a female to create a whole organism. This is called reproductive cloning. When Dolly was born, she was the only lamb born from 277 attempts. She was a clone of the sheep whose udder cell was used.
Blastomeres can also be used as a source of stem cells.
You can read more about cloning in the Human Uses chapter.
Try cloning your favourite dog - interactive
Did you know that dogs are particularly difficult to clone? The first cloned dog, an Afghan hound called Snuppy, was cloned by South Korean researchers and was shown to the world in August 2005.
Or, try bringing the Tasmanian tiger back by cloning - interactive
Embryo splitting
© University of Idaho, Phil Schofield 2003
Another cloning technique is embryo splitting. Using microsurgery (surgery conducted under a microscope), an embryo is split while it still consists of only a few cells. Genetically identical individuals develop from each portion in the same way as identical twins are formed in nature. This technique has been used to successfully create cloned embryos and cloned animals.
Chromatin transfer
There are a number of problems associated with nuclear transfer - the method used to clone Dolly and almost all cloned animals since then.
When the nucleus is transferred to a new egg cell, the egg reprograms the incoming nucleus to allow it to go back to its undifferentiated state. Because it has come from an adult cell, it no longer needs to produce the proteins, hormones and other molecules associated with it being an embryo and growing to produce all the different tissues in a whole body.
© Victor Fisher, Genetic Savings and Clone
Incomplete reprogramming of the donor cell is thought to be a leading factor in the low success rate of animal cloning.
Chromatin transfer is a new cloning technique aimed at reducing these problems. It involves treating the cell of the animal to be cloned to remove molecules associated with cell differentiation before the nucleus is removed.
This is a very new method created by Genetic Savings and Clone, a company in the USA that clones pets.
For a news release on this method, go to: www.savingsandclone.com/news/press_releases_09.html
More information on cloning is found in the Human Uses chapter.