By Mohamed Nasr
Have you ever wondered what could happen if there were several exact copies of yourself? What would life be like if every biological entity had several exact copies somewhere in the world? If these copies are genetically identical, they are called clones, and the process by which these identical copies are made is called cloning. Clones can occur either naturally or through artificial cloning. In nature, asexual reproduction results in offspring which have identical genetic makeup similar to that of their parents and affects organisms such as bacteria. However, in this article, we are going to focus on artificial cloning.
Most of the time, when someone is asked about artificial cloning, what first comes to mind is generating genetically-identical entire organisms. However, artificial cloning is not limited to producing genetically-identical copies of an entire organism. It also includes production of several copies of a specific gene, cell, or tissue. Artificial cloning is classified into three types: genetic cloning, therapeutic cloning, and reproductive cloning, based on what is being cloned.
Genetic cloning aims to produce multiple copies of a specific gene or DNA segment. Genetic cloning starts with isolating the gene of interest from your sample, amplifying it through polymerase chain reaction (PCR), inserting it into a plasmid which acts as a genetic vector, and finally inserting such vectors in bacteria to replicate and produce millions of copies of the insert. These gene clones can then be used for biomedical research investigations or even to produce proteins of therapeutic importance. For example, cloning the insulin gene into bacteria produces huge amounts of insulin, which is then purified and given to diabetic patients.
Therapeutic cloning aims to produce embryonic stem cells (ESCs) that have the same genetic makeup of a donor cell, a cell whose DNA or even the whole cell itself is being used to generate these ESCs. These embryonic stem cells are often used for regenerative medicine. They can be induced to adopt characteristics of a certain cell type (called “differentiating”). These cells can then be grown up to generate tissues that are implanted into a patient. Because these cells were initially harvested from the patient and then implanted again after genetic manipulation, the patient’s body will recognize and not destroy them. This type of cloning can also be used to study early developmental events. But is it possible to generate ESCs from donors who are already adults?! The answer is yes. This can be accomplished through creating induced pluripotent stem cells (iPSCs) or through Somatic Cell Nuclear Transfer (SCNT) technique. Generation of iPSCs is done by transferring specific proteins (Oct4, Sox2, c-MYC, and Klf4) to a donor somatic cell which results in the reprogramming of the somatic cell into a pluripotent stem cell, iPSC (figure1). In SCNT technique, a somatic cell nucleus is transferred from a donor cell to an oocyte whose nucleus has been removed (enucleated oocyte). This results in the reprogramming of the somatic cell nucleus in its recipient ovum, followed by oocyte division into an embryo from which ESCs can be isolated (figure 2).
Figure 1: Generation of iPSCs. Adapted from http://www.sajbl.org.za/index.php/sajbl/article/view/418/411
Figure 2: Demonstration of Somatic Cell Nuclear Transfer (SCNT). Adapted from http://www.sajbl.org.za/index.php/sajbl/article/view/418/411
Reproductive cloning also uses iPSCs and SCNT techniques. However, in reproductive cloning, instead of isolating ESCs for downstream application, the embryo produced by SCNT/ iPSCs is kept intact and is then implanted into a female uterus of the same species (figure 3). Reproductive cloning will result in a genetically identical organism to that of the nucleus donor. The most famous example of reproductive cloning is Dolly the sheep which was cloned at the Roslin Institute, 1997 (figure 4 )
Figure 3: Reproductive and therapeutic cloning. Adapted from http://www.sajbl.org.za/index.php/sajbl/article/view/418/411
Figure 4: Photo of Dolly the Sheep in a field at The Roslin Institute. (© Photo courtesy of The Roslin Institute, The University of Edinburgh)
Other than Dolly, many other species have been cloned, including mice, frogs, tadpoles, rats, horses, camels, cats, pigs, Macaque monkeys, Rhesus monkeys, and cattle. Reproductive cloning of all of these species raised the possibility of cloning extinct species if we have their DNA remnants as in the sci-fi movie Jurassic Park. Yet, scientists are facing difficulties due to technical problems in addition to the fact that reproductive cloning itself has been shown to have a low success rate. This low success rate is caused by growth defects or developmental abnormalities that may affect embryos, leading to their death. The question now is can we clone humans? Till now, no one cloned human embryos. Moreover, this possibility has raised a lot of ethical concerns where some countries rendered human cloning illegal.
So, to put all things together, cloning is not a sci-fi anymore. Rather, it has been involved in several biological aspects, ranging from molecular cloning to reproductive cloning, with huge impacts in the biomedical sciences fields. Cloning future aspects may therefore be focused on solving the technical problems, including the low success rates of reproductive cloning, in addition to addressing cloning ethical concerns.