Cloning broadly refers to creating genetically identical copies of a biological entity, from DNA fragments to entire organisms. Reproductive cloning specifically focuses on generating a new individual organism that is a precise genetic replica of an existing or previously existing organism. This process aims to produce a complete, living being with the same nuclear DNA as the donor.
How Reproductive Cloning Works
The primary scientific technique used for reproductive cloning is Somatic Cell Nuclear Transfer (SCNT). This method involves taking an unfertilized egg cell and removing its nucleus, which contains the egg’s genetic material. The next step involves acquiring a somatic cell, which is any cell from the body other than a sperm or egg cell, from the organism intended to be cloned.
The nucleus from this donor somatic cell is then transferred into the enucleated egg cell. This reconstructed egg cell is stimulated, often with an electrical pulse, to begin dividing as if it had been fertilized naturally. As the cell divides, it forms an early-stage embryo, called a blastocyst. This embryo is then implanted into the uterus of a surrogate mother, where it can develop to term, resulting in an offspring genetically identical to the somatic cell donor.
Distinguishing Reproductive and Therapeutic Cloning
While both reproductive and therapeutic cloning utilize Somatic Cell Nuclear Transfer (SCNT), their ultimate goals and outcomes differ significantly. Reproductive cloning aims to create a complete, living organism that is a genetic duplicate of the donor. The embryo created through SCNT is implanted into a surrogate to develop into a full individual.
In contrast, therapeutic cloning, also referred to as “embryo cloning,” focuses on generating embryonic stem cells for research and potential medical treatments. The SCNT-created embryo is not implanted into a surrogate; instead, it is allowed to develop only to the blastocyst stage, typically for about five days, at which point embryonic stem cells are harvested. These stem cells are genetically matched to the donor and possess the ability to differentiate into various cell types, offering potential for studying diseases or repairing damaged tissues, without the intention of producing a complete organism.
The Dawn of Reproductive Cloning
The concept of reproductive cloning gained significant public attention with the birth of Dolly the sheep in 1996. Dolly was the first mammal successfully cloned from an adult somatic cell, specifically a mammary gland cell. Her creation was a landmark achievement because it demonstrated that a specialized adult cell, previously thought to have a fixed developmental path, could be reprogrammed to generate an entire new organism.
Before Dolly, scientists believed that only embryonic cells held the full developmental potential required for cloning. Dolly’s existence challenged this understanding and showed that the genetic material from a differentiated adult cell retained all the necessary information to create a complete individual. Since Dolly, scientists have successfully cloned various other mammals, including mice, goats, cattle, pigs, and monkeys, illustrating the progression of this technology.
Where Reproductive Cloning Stands Today
Reproductive cloning remains a complex and technically demanding process. Despite advancements since Dolly the sheep, the efficiency of reproductive cloning is still remarkably low. For instance, the success rate for generating a living animal from a nuclear transfer typically ranges from 1 to 2 percent in mice and 1 to 5 percent in some other species.
This low success rate often stems from challenges in reprogramming the donor cell’s nucleus and issues with embryo development. Cloned animals can also experience health problems and have shorter lifespans compared to naturally bred animals. Consequently, reproductive cloning is largely confined to research settings and specific applications, such as replicating genetically valuable livestock or aiding in the preservation of endangered species, rather than widespread commercial use.