Strictly speaking, cloning is the creation of a genetic copy of a sequence of DNA or of the entire genome of an organism.
The copied material is called a clone.
Geneticists have cloned cells, tissues, genes and entire animals.
In the latter sense, cloning occurs naturally in the birth of identical twins and other multiples.
But cloning can also be done artificially in the laboratory via embryo twinning or splitting: an early embryo is split in vitro so that both parts, when transferred to a uterus, can develop into individual organisms genetically identical to each other.
In the cloning debate, however, the term ‘cloning’ typically refers to a technique called somatic cell nuclear transfer (SCNT).
SCNT involves transferring the nucleus of a somatic cell into an oocyte from which the nucleus and thus most of the DNA has been removed. (The mitochondrial DNA in the cytoplasm is still present).
The manipulated oocyte is then treated with an electric current in order to stimulate cell division, resulting in the formation of an embryo.
The embryo is (virtually) genetically identical to, and thus a clone of the somatic cell donor.
Dolly was the first mammal to be brought into the world using SCNT.
Dolly, however, was not 100% genetically identical to the donor animal.
Genetic material comes from two sources: the nucleus and the mitochondria of a cell. Mitochondria are organelles that serve as power sources to the cell.
They contain short segments of DNA. In Dolly’s case, her nuclear DNA was the same as the donor animal; other of her genetic materials came from the mitochondria in the cytoplasm of the enucleated oocyte.
For the clone and the donor animal to be exact genetic copies, the oocyte too would have to come from the donor animal (or from the same maternal line – mitochondria are passed on by oocytes).
In biomedical research, cloning is broadly defined to mean the duplication of any kind of biological material for scientific studies, such as a piece of DNA or an individual cell.
With the advent of recombinant DNA technology in the 1970s, it became possible for scientists to create transgenic clones—clones with genomes containing pieces of DNA from other organisms.
Cloning happens all the time in nature—for example when a cell replicates itself asexually without any genetic alteration or recombination.
Prokaryotic organisms (organisms lacking a cell nucleus), such as bacteria and yeasts, create genetically identical duplicates of themselves using binary fission or budding.
In eukaryotic organisms (organisms possessing a cell nucleus) such as humans, all the cells that undergo mitoses, such as skin cells and cells lining the gastrointestinal tract, are clones; the only exceptions are gametes (eggs and sperm), which undergo meiosis and genetic recombination.
Mitosis is the division of a cell into two daughter cells that are genetically identical to the parent cell.
Meiosis is the division of a germ cell into four sex cells (e.g. egg or sperm), each with half the number of chromosomes of the parent cell.
Mitosis is a means of asexual reproduction, whereas meiosis is necessary for sexual reproduction.