Cloning is a term used both in traditional biology and in biotechnology. In traditional biology, cloning happens naturally in nature among many asexual species of plants, bacteria, and insects, where identical genetic copies of an organism are created as a form of procreation. In asexual cloning, the only differentiation that occurs happens as a result of random mutation, as opposed to the mingling of different DNA. In biotechnology the term is used to refer to the intentional cloning of entire organisms, or part of the DNA or organism. It is this latter definition that most people mean when they talk about different types of cloning.
There are three main kinds of cloning within biotechnology: reproductive cloning, therapeutic cloning, and DNA cloning. Although it is reproductive cloning that has captured the imaginations of most people and media outlets, because of the size of the organisms involved, and the very vocal opposition to it, in fact the other types of cloning are much more common in the world today.
Reproductive cloning is cloning of an entire organism, or at least the nuclear DNA of that organism. This allows a plant, animal, or other organism to be essentially recreated from the same foundational material, although of course environmental factors can change the organism itself in the short or long term. Reproductive cloning is one of the most exciting types of cloning for most people, as it is the one that gave us animals such as the famous sheep Dolly, and is what most excites peoples dreams and fears of cloning technology in general.
Reproductive cloning works by taking a bit of genetic material from a source cell, and transferring it to an egg which has had its nucleus removed. This makes the egg a sort of blank slate, upon which the transferred genetic material can be imprinted. This process is called somatic cell nuclear transfer, and although it is still far from perfect, great strides have been made in recent years, allowing it to be undertaken with a relatively high rate of success when compared to even a few years ago. Reproductive cloning has met fierce opposition from many groups, some of whom oppose it for religious reasons, some of whom oppose it citing a lack of full understanding of the consequences, and some of whom oppose it because they believe it to be unnatural in a general sense.
Another of the kinds of cloning, therapeutic cloning, is the type designed to give us a ready source of stem cells, which can be used in a wide range of therapeutic situations. Therapeutic cloning involves cloning human embryos, which can then be harvested of their stem cells after about five days. Because the process of harvesting these cells destroys the embryo, this is one of two of the types of cloning that has been most contentious, as some people view it as the destruction of a potential life, and therefore immoral.
DNA cloning, or recombinant DNA cloning, is the least argued of the types of cloning, in which only a small fragment of DNA is cloned. DNA cloning takes place within something like a bacterial plasmid, which replicates on its own. DNA cloning is an important part of researching things like the human genome, and since it doesn’t touch on thorny issues like destroying an embryo or creating a form of higher life, it has remained relatively unproblematic in the larger world since its development in the 1970s.