Human cloning is a form of cloning which is designed to result in a copy of a human being or a human body part. There are a number of ethical and religious issues surrounding cloning, and many nations have specifically banned it, discouraging research into procedures which are designed to result in cloned humans.
In therapeutic cloning, the goal is to clone a part of the human body for use in research. Therapeutic cloning could also be theoretically used to make clones of organs, skin, and tissue for the purpose of transplant. For example, someone who needs a new heart could have a cloned heart made, rather than waiting for a donor heart to become available, and the use of a cloned organ would eliminate the problem of rejection after transplant.
With reproductive human cloning, a copy of a human being would be produced. This practice has been deemed controversial in many parts of the world, as there is a great deal of ethical unease about creating cloned humans. Many major religions have spoken out firmly against human cloning, arguing that it interferes with the natural processes of nature, and could be construed as a form of playing God. In addition, interesting ethical questions have been raised about ensoulment of clones and other philosophical issues related to reproductive cloning.
Whether therapeutic or reproductive, human cloning would theoretically start with somatic cell nuclear transfer, in which the nucleus of a cell from the body of the human to be cloned would then be transferred into a human egg which had been relieved of its nucleus. The egg would be stimulated so that it began to divide and grow, producing stem cells which would eventually develop into another human being.
Recombinant DNA technology could also play a role in human cloning. Using this technology, a scientist could replace selected areas of the genome to create a specifically desired outcome. This is one of the reasons why human cloning has attracted so much controversy, due to concerns that genetic manipulation could be used to create “improved” clones of existing individuals, potentially reducing genetic diversity in the human race over the course of repeated manipulations for desired traits.
The ethical issues over human cloning have created some legal tangles. For example, scientists in some nations are not allowed to use cloning to produce a new stem cell line for research under laws which ban human cloning. For researchers who want to work with stem cells, this can be extremely frustrating.