15.11: Reproductive Cloning

Reproductive cloning is the process of producing a genetically identical copy—a clone—of an entire organism. While clones can be produced by splitting an early embryo—similar to what happens naturally with identical twins—cloning of adult animals is usually done by a process called somatic cell nuclear transfer (SCNT).

Somatic Cell Nuclear Transfer

In SCNT, an egg cell is taken from an animal and its nucleus is removed, creating an enucleated egg. Then a somatic cell—any cell that is not a sex cell—is taken from the animal to be cloned. The nucleus of the somatic cell is then transferred into the enucleated egg—either by direct injection or by fusion of the somatic cell to the egg using an electrical current.

The egg now contains the nucleus, with the chromosomal DNA, of the animal to be cloned. It is stimulated to divide, forming an embryo, which is then implanted into the uterus of a surrogate mother. If all goes well, it develops normally and the clone is born.

Although this process has been used to successfully clone many different types of animals—including sheep, cows, mules, rabbits, and dogs—its success rate is low, with only a small percentage of embryos surviving to birth. Cloned animals that survive to birth also appear to age and die prematurely. This is because their DNA comes from adult cells that have undergone telomere shortening—loss of a small portion of the protective ends of chromosomes with each cell division—as part of the normal aging process.

While the chromosomal DNA of the clone is the same as that of the nucleus donor, it may have different mitochondrial DNA, since the mitochondria come from the cytoplasm of the egg cell, which is usually from a different animal. Also, phenotypic differences can occur between the clone and the original animal, due to environmental and epigenetic factors. For example, the first cloned cat, Cc, looked very different from the original cat, because the coat pattern is due to random X-chromosome inactivation in different cells.

Despite the technical challenges, reproductive cloning has many potential uses including the production of genetically identical research animals, livestock with desired traits, and offspring of endangered species. It even has potential applications in human infertility and disease, although cloning of humans has not yet been done, and would raise ethical concerns.

Reproductive cloning is the production of a genetically identical copy, a clone of an organism. Dolly- the sheep, produced in 1996 was the first clone of an adult animal, but several other species have been cloned since, including dogs and cats.

The most common method of cloning adult animals is somatic cell nuclear transfer. First the nucleus is removed from an egg's cell, then a somatic cell such as a skin cell is taken from the animal to be cloned. The nucleus containing chromosomal DNA is removed and injected into the egg. The egg is stimulated to divide by chemical or electrical treatment, and forms an embryo, which is implanted into the uterus of an adult female, where it continues to develop until birth.

The resulting clone has identical chromosomal DNA to the original animal, but the mitochondrial DNA is often different since the mitochondria come from the cytoplasm of the egg, usually from a different individual.

Also phenotypic differences between the clone and the original can occur due to environmental and epigenetic factors during development. Just as identical twins are slightly different from each other, despite having the same DNA.