Genes control the development of every living thing into a particular individual of a particular species, with a specific shape, size, color, set of instincts and natural abilities, and predisposition toward certain behaviors and health conditions.

Every cell in the body of an animal contains the genetic blueprint for the entire animal. However, as an embryo grows, certain genes are either activated or deactivated in certain cells, a process called differentiation, resulting in cells of certain types and functions, such as blood, brain, skin, etc.

For many years after differentiation was first described, the process in mammals was thought to be irreversible, meaning that a skin cell could never be de-differentiated, or reprogrammed, into a cell of another type or function. For this reason, cloning of mammals from adult (differentiated) donor cells was long thought to be impossible.

The birth of Dolly in 1996 changed all that. Dolly was created using a cloning technique called somatic cell nuclear transfer (SCNT), whereby an individual mammal is duplicated from a single differentiated cell. SCNT involves the following steps:

First, cells are first taken from the animal to be cloned in a process called gene banking. In the case of Dolly the sheep, cells were taken from her udder. These are normal body cells – called somatic cells - containing the full set of genes describing the animal to be cloned.

Next, an egg is obtained from a female of the same species as the animal to be cloned. This egg is then enucleated, meaning its nucleus is removed and discarded. The donor cell obtained in the gene banking process is then inserted into the enucleated egg in a process called recombination. The resulting couplet is shown in Figure 1.

Electricity is used to fuse the donor cell nucleus and the enucleated egg, and chemicals are used to activate the resulting embryo to begin to divide like a normal zygote, or single cell embryo. A single cell cloned embryo is showing in Figure 2. The dividing embryo is shown in Figure 3.

Once a cloned embryo has been successfully produced, it must be transferred into a fertile recipient – or surrogate – female of the same species, at just the right point in estrus (period of fertility). The embryo must also be at the right point in its growth when transferred. For some species, the ideal transfer point is the blastocyst stage; a blastocyst is a ball of about 100 cells, which requires about 5 days of growth. For dogs however, the ideal stage for embryo transfer is much earlier, just a few hours after production, when the embryo is only one or two cells in size.

After the embryo is successfully transferred into the oviduct of the surrogate dog, it develops just like a naturally conceived embryo, and eventually the surrogate gives birth to a cloned puppy just as it would its own offspring. The surrogate often looks different from the genetic donor, as you can see in the picture of the newborn Mira and her surrogate mom, shown in Figure 4. Although the surrogate generally exhibits strong maternal behavior toward her puppy, they are genetically unrelated.

Since Dolly was cloned in 1996, scientists have successfully cloned other mammals including cattle, goats, mice, pigs, rabbits, cats, mules and horses. For many years, however, dog cloning stumped numerous scientists. Check out the “Challenges Unique to Dog Cloning” section to learn why cloning dogs is so much more difficult than cloning other species.