II how scientists clone cells

Scientists initially made cloned cells in the laboratory by letting a single cell divide into a population of genetically identical cells. In this process scientists put the original cell in a laboratory dish containing culture medium (nutrients needed to keep a cell alive). The cell’s natural process of mitosis (cell division) then produces genetically identical offspring. This process mimics how cells multiply, for instance, in plants and in the human body.

Scientists later developed more complex cloning techniques using animal embryos. Every cell in an animal arises from a fertilized egg. The fertilized egg divides to form an embryo, and each cell in the embryo has the same genetic makeup. At some point in the embryo’s growth and development, cells differentiate and become specialized. For instance, a heart cell only functions in the heart and not the liver, even though the genes of a heart cell and liver cell are the same.

In the 1950s scientists began to experiment with embryo cells that were undifferentiated—that is, they had not yet specialized into a particular type of cell. Scientists found that such embryo cells are totipotent (able to give rise to all the different cell types in the body). Exploiting this characteristic, scientists developed three techniques to clone embryo cells: blastomere separation, blastocyst division, and somatic cell nuclear transfer.

Medical procedures using stem cells still remain experimental. In 2001 the first clinical trial that injected stem cells into the brains of patients suffering from Parkinson disease produced mixed results. Although the injected cells grew, the treatment produced no obvious benefits for patients aged 60 and older. Some of the patients under age 60 said they felt better after the treatment, but about 15 percent of these younger patients acquired irreversible side effects, including twitching and other uncontrollable movements.

Cloned stem cells could pose other risks. For example, the cloning process—producing large numbers of cells from one starting cell—could create genetic errors in the cells. If something went wrong in cell division during cloning, the error could be replicated in many other cells—even all of them if the error existed in the original cell. Nevertheless, in 2002 scientists at Rutgers University found few genetic mutations in embryonic stem cells cloned from mice. In fact, the study’s investigators found those stem cells were better able to resist mutation than some adult cells.

Some scientists worry that cloned stem cells could carry disease. For example, when cloning stem cells, scientists typically mix human stem cells with mouse cells in culture. The mouse cells produce an as yet unidentified nutrient or growth factor that helps keep the human stem cells alive. Scientists worry that infected mouse cells could just as easily transfer viruses to the human stem cells. They hope to develop new methods of cell culture that do not rely on such “feeder cells.”