The first mammal to be cloned was Dolly, the sheep. This was a breakthrough achievement by Ian Wilmut and Keith Campbell, colleagues at the Roslin Institute, Scotland, in 1997. It carries genetic material solely from ewes; that is, Dolly does not have a father. The property of totipotency of cells was exploited for this procedure and nuclear transfer was performed to create this clone.
The ability of a cell to differentiate, divide and develop into all the cells and organs in an organism is called as totipotency. It is one of the specialized characters of embryonic cells. They can form all the organs in the organism and finally, the whole organism. Once differentiated into adult cells, the cells lose their totipotency. For this procedure, totipotency was induced in the cells.
For the process of cloning, a mammary gland (udder) cell was isolated from a donor Finn Dorset ewe. It was subjected to nutrient deprivation for five days to induce a state of dormancy. The growth cycle was discarded and the property of totipotency was reinstated. An ovum (egg) was taken from another ewe, a Scottish Blackface and the nucleus was removed to obtain an enucleated ovum. Then, the dormant mammary cell and the enucleated ovum were fused by electric pulse. This disrupted the outer cell membrane of the mammary gland cell thus exposing the nucleus. This allowed the enucleated ovum to envelop the dormant nucleus. This fused cell now had the power of totipotency. This cell could now multiply and form an embryo or a blastocyst.
This embryo was monitored for normal development which was confirmed for six days. It was then implanted in the womb of a surrogate mother. Wilmut and Campbell performed two hundred and seventy seven of these implantations of which they achieved thirteen successful pregnancies. Of these, only one resulted in live birth. Since she carried the nucleus from the Finn Dorset ewe, Dolly was a Finn Dorset too.
Dolly was born on 5th July 1996. Most genetically cloned animals are born with birth defects. However, no such problem was observed in this sheep. She was observed trying to get on her feet and suckle within minutes of being born. Her birth was kept quiet for some time to allow the research team to put together the results of the experiment to be published. When the news was announced, it took the world by storm.
Dolly lived her entire life at the Roslin Institute, Edinburg. Dolly was allowed to breed with a Welsh Mountain ram that resulted in the birth of six offspring. At age five, Dolly began to develop complications in her joints and developed arthritis. But, anti-inflammatory drugs took care of the problem. Dolly survived for six years before she was euthanized because she was suffering from arthritis and lung problems. Several cloned animals had died of sheep pulmonary adenomatosis (SPA) which was a little known disease of the lungs that caused the growth of pulmonary tumors. SPA did not have any known cure. In spite of regular monitoring and health checks, tumors were found to be growing in Dolly's lungs. These problems could have arisen due to the indoor housing of the sheep. After this confirmation, it was decided to euthanize Dolly. So, on 14th February 2003, the six-year old Dolly was finally put to sleep. Today, the stuffed remains of Dolly are displayed in the Museum of Scotland, Edinburg.
The life expectancy of a Finn Dorset was about twelve years. It has been speculated that this shortened lifespan is due to the usage of somatic cells taken from six-year old sheep. It is possible that the cloned embryo was genetically six years old when implanted itself. This idea is supported by the discovery of shortened telomeres, indicators of ageing, found on Dolly.
Cloning of animals may be important in the future for production of transgenic livestock, although, the process has to yield viable offspring. Early cloning experiments involved injecting the DNA of interest into a fertilized egg and then allowing the embryo to grow. However, the success rate was quite low. Since the genetic material for the clones are obtained from existing species, there is no fear of introducing unfamiliar genes into the environment or increasing the gene diversity, unknowingly.
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