Egg cells were extracted, and the nucleus from each was removed and discarded.
A mammary cell was placed next to the "empty" egg cell.
An electric shock opened up the cell membranes so that the cells fused. Cell division was triggered.
Biologists cloned a sheep by transferring a somatic cell nucleus from one animal into the egg cell of another. They implanted the resulting embryo into a surrogate mother, and the offspring that developed was a clone of the original nucleus donor.
In the 1990s, biologists began cloning whole organisms, such as sheep and mice. The name for this procedure is cloning by nuclear transfer, the introduction of a nucleus from a body cell into an egg cell to generate an organism identical to the nucleus donor. The first animal successfully cloned from an adult tissue was a sheep named Dolly in 1996.
As shown in Figure 13-12, scientists in Scotland isolated a mature, functioning mammary cell nucleus from an adult sheep. They also isolated an egg cell from a second sheep and removed the nucleus. They then fused the mammary cell with the "empty" egg cell. The egg was stimulated to divide and grew into an embryo. The researchers implanted this embryo into the uterus of a surrogate mother who gave birth to a lamb, which they called Dolly. Dolly's nuclear DNA was identical to the original donor of the mammary gland cell.
Despite the successful cloning, Dolly suffered premature aging and disease and died at age 6, only half of a normal sheep's lifespan. Researchers found that Dolly had short telomeres, or repeated DNA sequences at the ends of chromosomes that shorten with each round of cell division. Short telomeres may be associated with premature aging. Other cloned species, however, have not experienced similar telomere shortening.
The goal of most animal cloning is to alter the genome in some useful way. For example, researchers have altered and cloned goats so that they secrete human blood clotting factors into their milk. Cloned pigs have been altered in the hope that pig livers, hearts, and other organs might not trigger organ rejection if transplanted into human recipients. Some researchers are cloning animals as models for the study of human disease, such as cystic fibrosis.
The embryo developed in vitro and was later implanted into a surrogate mother.
A vaccine is a substance containing all or part of a harmless version of a pathogen that physicians introduce into the body to produce immunity to disease. The immune system recognizes the pathogen's surface proteins and responds by making defensive proteins called antibodies. A DNA vaccine is a vaccine made from the DNA of a pathogen but does not have disease-causing capability. The DNA vaccine is injected into a patient where it directs the synthesis of a protein. The immune system mounts a defense against the protein. If the vaccinated person contacts the disease agent in the future, his or her new immunity should provide protection. Researchers are currently working on developing DNA vaccines to prevent AIDS, malaria, and certain cancers.
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