Cloning
The phenomenon of identical twins has always attracted attention. After an egg is fertilized, it begins to divide repeatedly. If the egg completely separates during the two-cell stage, identical twins will result. Both individuals will have exactly the same combination of genes (genotype) and each will have the same physical characteristics (phenotype). This is an example of how exact duplicates can naturally occur through sexual reproduction.
A clone is a group of genetically identical cells descended from a single common ancestor. Science has capitalized on the mechanisms of cellular reproduction to produce clones. Advances in biotechnology since the 1970s have enabled livestock breeders to clone virtually unlimited numbers of identical animals from a single embryo, allowing the precise duplication of an animal with desired characteristics.
In 1979, veterinarian Steen Willadsen developed a way to divide sheep embryos in half at the two-cell stage, making clones possible. In the next few years, several scientists, including Willadsen, J. P. Ozil, C. Polge, Stephen Voelkel, and R. A. Godke, made further strides in this area with both sheep and cattle embryos. Willadsen and Godke, working together, developed a simplified method of dividing and cloning sheep embryos in 1984.
In one cloning technique, dairy farmers trying to clone a cow with high milk-producing qualities begin by artificially inseminating this desired cow with the sperm from a prize bull. The resulting embryo, which contains the entire genetic instructions needed to form a complete calf, develops in the desired cow. After some time, the embryo divides into a mass of 32 identical cells. It is then carefully removed from the desired cow and meticulously separated into 32 separate cells. Next, the nucleus from each embryonic cell is removed microsurgically. The genetic material (genes) from each of the embryonic cells of the desired cow is then inserted into the space once occupied by the genetic material in unfertilized eggs of 32 carrier cows. Finally, each new embryo is transplanted into the 32 carrier cows, where it develops fully. After a normal pregnancy, each carrier cow gives birth to a calf that is genetically identical to the 31 other calves derived from the original 32 cell embryo. Each calf is a clone. The trait for increased milk production has been cloned so that the farmer now has 32 high milk-producing cows instead of just one.
Cloning technology has enabled breeders to develop lines of cattle, sheep, and cotton plants that respectively produce more milk, wool, and cotton. These techniques have allowed us to increase productivity and quality control simultaneously. This work is also important to reproductive physiologists interested in how embryos develop and to researchers who require identical organisms for comparison during experimentation.
In another cloning technique, the animal is cloned from a cell taken from an adult rather than an embryo. Dolly, a sheep that was the first mammal produced this way, was born on July 5, 1996. This dramatic advance, achieved by Ian Wilmut and his colleagues at the Roslin Institute in Edinburgh, immediately caused a sensation within both the scientific community and the general public. In 1997, Roslin scientists announced two new additions to their high-tech flock: a pair of cloned lambs, named Molly and Polly, that carry a human gene in their cells. The goal was to create sheep whose blood would contain a substance that could be used to treat human patients with hemophilia B, a blood-clotting disorder.
Then in 1998, biologists James Robl and Steven Stice announced the creation of two other transgenic, or genetically altered, cloned animals: a pair of calves known as George and Charlie. These scientists claim that their cloning method has a failure rate much lower than the one used with Dolly, whose birth occurred only after 270 failures, many of which ended in miscarriage or prenatal deformity.
Cloning is one area of genetics that is advancing very rapidly, and it is therefore not without controversy. For example, sometimes the deoxyribonucleic acid (DNA) in the mitochondria of the carrier animal egg interferes with the transplanted genetic material from the desired animal. This may result in life-threatening complications in the carrier animal and/or its offspring. And if this technology is ever applied to humans, who will decide which genes are "desired" and should be cloned? This is only one of many important questions that have arisen as a result of the amazing advances in genetic cloning.
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