The First Clone
Scientists have finally cloned a human embryo. The breakthrough promises cures for terrible diseases. Here's the inside story:
It's still too early to say whether the United States will accept or reject therapeutic cloning. Cibelli and colleagues still have mountains of work ahead of them. It takes not just an embryo but the nurturing of stem cells and the ability to transform those stem cells into specialized types before any clinical applications can be used in humans. "I'm overjoyed, but I'm not getting drunk yet," says Cibelli. "When I have neurons for Judson [Somerville] and islet cells for Pablo [Naumann], that's when I'll celebrate." If history is any guide, that is also when the public attitudes will warm toward this new and intimidating medical technology.
The art of cloning
The idea behind cloning are simple: Remove DNA from an egg and replace it with the DNA from a body cell. But actually getting that process to work is still as much a guessing game as an exact science, dependent on timing as well as technique. Here are some of the main challenges.
THE STEPS; THE CHALLENGES
(Step 1) Remove DNA from a human egg
The Challenges
(A) A human egg is very fragile. It can take hundreds of tries to extract the 2 meters of chromosomes with a microscopic needle without destroying the egg.
(B) There is a tiny window of time--just a few hours--during which an egg can be prepared for cloning. Then the egg loses its ability to repair and regenerate DNA.
(C) If one too many drops of cellular material are accidentally removed along with the DNA, the egg can be rendered useless.
(Step 2) Deliver new genes into the egg
The Challenges
(A) No one knows which body cells are the most amenable to cloning. The easiest cells to get from donors--skin cells--are not ideal because they are very large and hard to work with.
(B) Some scientists use an electric current to fuse an entire body cell into the egg cell, but the line between too little and too much current is perilously thin.
(C) Other scientists take the genes out of the body cell and inject them into the egg, but the chromosomes are often damaged by the process.
(Step 3) Trick the egg into action
The Challenges
(A) No sperm is involved in cloning. So scientists must find another way to make the egg think it has been fertilized.
(B) Some scientists use an electric current to make the egg grow. Others use a mixture of chemicals. But it is all too easy to kill the egg with either method.
(C) There appears to be only about a four-hour period during which an egg will respond to activation signals. Prodding the egg before or after that doesn't work.
(4) Collect stem cells
The Challenges
(A) Dozens of nutrient mixtures have been developed to help embryonic cells proliferate, but no one yet knows which is best.
(B) Biologists disagree about how much oxygen the growing cells should be exposed to in order to develop the healthiest stem cells.
(C) Stem cells must be separated from the other cells without damage and kept alive until they become a self-sustaining colony.
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