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History of Cloning Animals

Instructor: Joanne Abramson

Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.

Animal cloning is one of the most fascinating topics in biology. This lesson covers the history of cloning, from the first animal cloned over 100 years ago up to the most recent developments in genetic engineering.

Dolly the Sheep

Dolly the Sheep, born in 1996, is by far the most famous cloned animal, but she was nowhere near the first. Scientists had been cloning animals for more than 100 years before Dolly was born. In fact, Dolly isn't the first cloned mammal, and she isn't even the first cloned sheep! So why is Dolly so important? Let's look at the history of cloning to see why Dolly's conception was so groundbreaking.

The remains of Dolly the Sheep (1996-2003) are on display at the National Museum of Scotland.
A photograph of Dolly the Sheep taxidermy remains.

Twinning

A clone is an exact genetic copy of another organism. Cloning is found throughout nature. Species of jellyfish, starfish, worms, insects, frogs and lizards create clones of themselves as a form of reproduction. Identical twins are also clones. Artificial cloning, which does not occur in nature, is the production of genetically identical animals through scientific intervention.

Twinning was first accomplished in 1885 with sea urchins. Twinning refers to splitting an embryo in the very early stages of development into two or more identical cells. The term 'twinning' is used since this is the process that occurs naturally to create identical twins.

Sea urchin embryos in various stages of development.
Microscope image of sea urchin embryos.

Twinning was originally demonstrated by splitting a two-cell sea urchin embryo. The cells developed into two, complete, genetically identical sea urchins. Seventeen years later, the same process was completed on a two-cell embryo of a more advanced organism, the salamander, and then again on a sixteen-cell salamander embryo. Twinning in more advanced embryos, however, did not successfully develop into adults.

Nuclear Transfer

Nuclear transfer involves injecting the nucleus of a cell to be cloned into an egg cell which has had its nucleus removed. In other words, the nucleus of one cell is transferred into another cell. The new egg cell is then allowed to develop normally, creating a clone of the original animal.

Nuclear transfer was used to create Dolly.
A diagram of nuclear transfer.

The first successful nuclear transfer occurred in 1952 when scientists removed a nucleus from an early tadpole embryo and inserted it into a denucleated frog egg. The new egg cell developed into a fully functioning tadpole. Again, similar attempts with more advanced embryos did not develop normally.

Until 1958, any attempts at cloning cells beyond those in the very early embryonic stages were not successful. Finally, a tadpole was cloned from another tadpole, not an embryo, via nuclear transfer of an intestinal cell. This was the first time a clone of a fully developed animal was created.

Cloning of Mammals

Scientists soon moved on to cloning mammals. The first successful nuclear transfer of an embryonic cell from a rabbit occurred in 1975. However, in order for the egg to complete its development it needed to be implanted into a rabbit's womb. Since this technology was still in its early stages at the time, this final step did not happen. By 1984, however, in vitro fertilization was well-developed, and the first lamb cloned from an embryo was born to a surrogate mother. In 1987 a cow was cloned using the same method.

Dolly had three mothers: one donated the egg cell, another donated the mammary cell, and a third was the surrogate.
A diagram illustrating how nuclear transfer was used to create Dolly the Sheep.

So now it is 1996, and Dolly is about to come on stage. Before Dolly, all successful clones of mammals, and most successful clones of any animal, could only be completed using the cells from early embryos. Dolly changed everything. The nucleus of an udder cell from an adult sheep was transferred to a denucleated egg cell. The new egg was given a short electric shock to stimulate cell division. The embryo was implanted in a surrogate sheep, and on July 5th, 1996, Dolly, the first clone of an adult animal, was born.

Many mammals have since been cloned using this technique, including cats, cattle, deer, dogs, horses, mules, oxen, rabbits and rats. In fact, a South Korean biotech firm now specializes in cloning beloved pets for around $100,000. Additionally, rhesus monkeys have been cloned from embryonic cells.

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