Embryos are made of stem cells that divide to give rise to different types of cells, everything from skin to brain cells.
Scientists once thought that reproductive cloning – creating a genetically identical copy of an individual organism – would be impossible without using stem cells and that the path leading to mature ‘differentiated’ cells was irreversible. But clawed frogs proved them wrong...
What makes cloning possible?
In 1958, biologist John Gurdon inserted the nucleus from the mature gut cell of an adult frog into an egg whose nucleus had been removed. This produced tadpoles that were clones of the frog that donated the gut cell.
Gurdon’s experiment showed that mature cells can be reprogrammed to become stem cells. We now know that proteins within an egg cell are what control the genetic instructions in DNA that direct a cell to become a specific type. Reversing that programming let researchers sidestep some ethical issues over using embryonic stem cells, while also paving the way for artificial cloning of complex creatures.
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Why was Dolly the sheep so special?
Dolly was the first mammal to be produced by transferring the nucleus from a mature cell into an empty egg. Because the nucleus came from a body or ‘somatic’ cell from the udder of an adult sheep, the clone was named after buxom country singer Dolly Parton. The lamb, carried to term by a surrogate ewe, was born in 1996.
How is cloning carried out?
Somatic cell nuclear transfer (SCNT) is now the standard technique for cloning. A nucleus is often transplanted into an egg by injection, but Dolly’s creators, Ian Wilmut (pictured) and Keith Campbell, prompted the two cell parts to fuse using an electric pulse. The older way to clone animals is embryo twinning – splitting an embryo in early development, which is similar to the natural process that forms identical twins.
Are clones exact copies?
Yes... ish. The sequence of DNA letters from an original and replica should be the same. But like twins, those individuals don’t always look identical because the environment can influence which genes are switched on or off, which in turn impacts physical features.
The first cloned pet, a female calico cat nicknamed CC (for Carbon Copy or Copy Cat), had a coat pattern that was distinct from her ‘parent’ as genes for coat colour are inactivated at random during development.
Which mammals have been cloned?
Only about 25 species have been cloned by SCNT, from rats and rabbits to camels and coyotes. Individuals have survived to adulthood in 20 or so species. The reason that so few species have been cloned so far is partly because cloning isn’t efficient.
Horse embryos grown by in vitro fertilisation are three times more likely to develop into foals than those cloned in the lab. Based on figures from more than 1,500 domestic dogs that have been created since the first canine was cloned in 2005, cloning efficiency (puppies born after SCNT) is only 2 per cent.
Are clones healthy?
Dolly died aged six, half the normal life expectancy for a domestic sheep. Some people speculated that her death was related to cloning because chromosomes have caps called telomeres that protect genes from being lost (the very ends of DNA aren’t replicated when cells divide) and Dolly was cloned from a mature cell with shorter telomeres. But it turns out she was just unlucky and caught a lung disease, and she was euthanised to prevent further suffering. Clones are probably no more likely to die of genetic diseases and their offspring seem healthy, too.
Is cloning actually useful?
Many applications aim to benefit humans, such as using cloned pigs as models to study disease, and Dolly’s creators suggested that genetically-engineered sheep could add medicines to milk. Cloned livestock hasn’t really taken off, however: of 7 million dairy cattle in the US Department of Agriculture’s database, only 530 cows are clones.
Could cloning help conservation?
But cloning can benefit wildlife by boosting a population’s numbers with new individuals cloned from its remaining members. Such ‘conservation cloning’ has been used to aid endangered species such as North America’s black-footed ferret. Reintroducing extinct species could potentially help restore habitats, too, but of course this involves overcoming challenges beyond cloning.
