The ability to create a clone used to be science fiction. Dr. Ian Wilmut's group changed that in 1997 with the creation of Dolly the sheep. Since then the debate on applying the technique to clone human beings has been ongoing.
What are clones? Simply put, clones are organisms that have identical genetic material. In other words, the sequence of bases in their DNA is exactly the same.
Long before the birth of Dolly the sheep, clones had been observed in both nature and in the laboratory. When a couple has identical twins (or identical triplets, etc.), the children are clones of one another. A plant cutting can also be used to generate a clone.
Prior to 1996, it was thought that cloning an entire animal could only be done with embryonic cells – cells present in the early stages of an organism's development. In the 1950s, scientists generated entire frogs from embryonic frog cells.
After a small number of cell divisions, embryonic cells start to change into the different types of cells that an organism needs, including cells that form muscle, blood, liver, etc. This process is called differentiation. Although each of these cells has the same genetic material, each cell can only access the genes needed for its particular function.
Before the experiment at the Roslin Institute, it was thought that once cells differentiated, they could not be used to generate an entire organism. For instance, in a sheep, udder cells could generate other udder cells, but not an entire sheep. The scientists at the Roslin Institute solved this problem by growing sheep udder cells under starvation conditions. This put the cells in a state similar to embryonic cells. This is called the G0 state.
An egg cell was taken from another sheep. The nucleus (which contains the genetic material) was removed from the egg cell using a very fine needle. They then used electric shock to fuse one starved udder cell with one nucleus-free egg cell. They made 277 of these fused cells. Although the egg cell came from a black-faced sheep, notice that the nucleus with the genetic material came from the white-faced sheep. These fused egg cells were then inserted into several different sheep. These surrogate mothers were also black-faced.
Of the 277 fused cells, only one progressed to form a developed lamb. Dolly was born on July 5, 1996. Scientists found that Dolly had the same DNA as the udder cells she came from. She is a clone of these udder cells. Dolly has given birth to a lamb named Bonnie, produced the natural way.
Other lambs have been born at the Roslin Institute through their cloning process; some carry genes that will produce usable human drugs.
A laboratory in Hawaii run by Dr. Ryuzo Yanagimachi was the second group to successfully clone an animal from an adult cell. They cloned mice using cumulus cells, a cell type found in the ovaries.
The cloning method used by the lab in Hawaii was different in two ways from the method used to clone Dolly. First, the cells used to clone the mice were not grown in culture, but instead were used immediately. Second the nucleus was removed from the cumulus cell and then directly injected into the egg cell. This egg cell's nucleus had already been removed.
The Yanagimachi lab used coat color to track genetic heritage. The cumulus cell comes from an agouti (brown) mouse, and the egg cell comes from a black mouse. The egg cell now had the same genetic information as the nucleus donor mouse. The egg cell was then activated and implanted into a white host mother. On October 3, 1997, the host mouse gave birth to Cumulina, named after the cumulus cells she was cloned from. Cumulina is the same color as the mouse that donated the nucleus. DNA fingerprinting confirmed that Cumulina had the same DNA as the nucleus donor.
The scientists have taken cells from Cumulina to make more clones. They have successfully made several generations of clones and all mice seem normal.
Dolly the sheep died at the age of six. Since the world said hello to Dolly, several other animals have also been cloned. Both Dolly and Cumulina were cloned from cells in the female reproductive system; cows have also been cloned using ovary and cumulus cells with the same method that was used to clone Dolly. Pigs have been added to the cloned animal menagerie. Scientists hope to use cloned pigs to grow organs that can be transplanted into humans.
clone, cloning, Dolly, sheep, ethics, genetic modication
