Cloning can also take place in plants. In the 1950’s F.C. Steward of Cornell University obtained mature carrot root cells and cultured them. When they were exposed to the right conditions they developed into new genetically identical carrots. This meant that it would be possible to obtain cells of a plant with desirable characteristics and develop many of them on a large scale.
The same concept is used in cattle breeding. Cattle among other animals produce small numbers of offspring. As a result this restricts the rate of selective breeding. To over come this they now clone the embryos and transfer them to surrogate mothers. So a cow that shows desirable characteristics (produces a lot of meat or milk) is selected and she is given hormones so that she over ovulates. Then a suitable male is selected and the female either mates with the male or is artificially inseminated with his semen. The embryos are then washed from the uterus and transferred to the uteruses of other cows. Two embryos are given to each surrogate mother; any left over are then frozen. By doing this the reproductive rate is increased, the chosen cow is never put at risk through pregnancy and the embryos can be sexed and checked for genetic diseases.
It has been found that an embryo in the early stages of development can be removed, split and then the separate parts are then placed in a surrogate uterus, where they will develop further into identical twins. A variety of mammals, such as mice and sheep, have been produced by this method, known as embryo splitting. However more recently it has been discovered that a whole can be taken from a cell and injected into a fertilised egg that has already had it’s nucleus removed. When the egg divides it also divides the nucleus, and so these nuclei can, in turn, be injected into eggs. After this has been repeated a few times, the nuclei may then be able to develop into organisms genetically identical to the organism that the original nucleus was taken.
A lot of research has been done in producing clones of animals and in1975 Gurgon managed to clone tadpoles from the gut cells of an adult frog. To do this he took out the nucleus of a frog oocyte (an unfertilised egg) creating an enucleated oocyte. He then transferred the nucleus from a frog’s gut cell in to this enucleated oocyte. This is known as nuclear transfer. Many of these cells acted like zygotes (recently fertilised eggs) and so the divided like a normal embryo and differentiated. These developed in to tadpoles that were all clones of each other. However he later tried to do the same with mammals but he didn’t succeed and none of them would differentiate properly.
Then in 1997 a group of scientists at the Rosiln Institute in Edinburgh realised that the reason for these mammal cells not developing may have something to do with the cell cycle. There is a point in the cell cycle called quiescence where the cell has left the cell cycle; the cell is a quiescent cell.
These scientists thought that these quiescent cells would be better donors, as they wouldn’t divide straight away.
They decided to try this out with sheep. They took cells from the mammary glands for the donor nucleus, they grew these cells in tissue culture where they ‘starved’ the cells of important nutrients which meant that the cell stopped growing and dividing. They became quiescent. They then removed the nucleus of from an oocyte (from a Scottish Blackface ewe, it has a black face) and injected the nucleus from a quiescent mammary cell (taken from a Finn Dorset, it is a white breed). By using small electric pulses they fused the nucleus with the cytoplasm, the pluses also made the cell begin to divide. This new cell was placed in a surrogate mother.
After repeating the process 276 times, Dolly was born. She is a healthy Finn Dorset, which shows that Dolly was a clone, as the Backface ewe wouldn’t have been able to produce a white sheep no matter who was the father. They also carried out DNA tests to be sure.
However Dolly doesn’t possess all the genetic characteristics of the Finn Dorset sheep from which they took the nucleus. This is because there are a few genes in the mitochondria in the cytoplasm, which were supplied by the donor egg from the original Scottish Blackface ewe.
This huge progress in cloning has led to the possibilities of genetically changing animals to product organs for transplant into humans.
