3. Cloning of Organisms

What is an Organismal Clone ? Two or more individuals with identical genetic makeup formed from one parent by asexual reproduction are or­ganismal clones.

Examples of Organismal Clones. Daughter plants produced by strawberry runners and whole plants produced by tissue culture are clones. Similarly, the individuals formed by budding on the same parent Hydra are also a clones.

Identical Twins are Clones. The identical twins (also called monozygotic, uniovular, or one-egg twins) develop from one egg by splitting of the early embryo. They contain identical genetic infor­mation and are of very similar appearance. Thus, they are an example of a clone, and are always of the same sex. Any physical or mental difference notable between identical twins arises from environmental influence before and after birth.

Methods of Organismal Cloning. Plant clon­ing is easy due to totipotency of nearly all of their cells. Animal cloning is difficult because animal cells lose their totipotency at gastrula stage. How­ever, the cells of early embryos and tumour cells have been cultured.

Clones of Xenopus through Nuclear Transplantation. Gurdon in 1962 placed a piece of the intestine of a tadepole in a dissociating medium which dissociated its epithelial cells. An intestinal cell was sucked into a transplant micropipette, in which it burst, releasing the nucleus. The liberated nucleus was then injected into the unfertilized egg of Xenopus. The nucleus of the egg had been destroyed by irradiating it with ultraviolet rays. The irradiation treatment activates the egg. The egg developed normally, passed through cleavage, gastrulation, and metamorphosis, finally forming a normal adult toad. The latter was a nuclear transplant toad, a clone of the toad whose tadpole provided the nucleus.

The donor nucleus was obtained from a mutant strain of Xenopus that has only one nucleolus per nucleus. The host egg was from a wild-type strain with 2 nucleoli per nucleus. The nuclei of the nuclear transplant toad had one nucleolus only. This showed that development was due to the donor nucleus.

World's First Mammalian Clone. Ian Wilmut and Coworkers produced in February, 1997 the world's first clone of an adult at Edinburgh's Roslin institute. The clone is a sheep developed from an enucleated egg into which the nucleus of a somatic (udder) cell was introduced. The egg turned into an embryo which was then transplanted in a surrogate mother, i.e., a substitute mother. The latter gave birth to a baby sheep called "Dolly". She has no father.

Dolly, aged six, was put to sleep on February 14, 2003 by verterinarians after they failed to cure her of severe lung infection. Dolly's death suggests that clones may age prematurely because their DNA source is older. This suggests futility of human cloning efforts. Human cloning is being opposed all over the world.

Cloning of Cattle. Japanese scientists have produced eight clonal calves from one zygote. A cow was mated with a bull. A fertilized egg developed in her womb, producing 8-celled embryo. The embryo was carefully removed from the womb and kept in a nutrient medium. The 8 blastomeres were separated with the help of an enzyme. Each blastomer was later skillfully im­planted into the womb of a separate surrogate cow. Each blastomere, being still totipotent, developed into a normal calf. The surrogate cows must be hormonically fit to accept the cell and make it develop further.

Clone of Asian Gaur. Massachusetts (USA) scientists have recently cloned an endangered species, the Asian gaur (Bos gaums), a humped, horned mammal native to India and Myanmar. It was cloned from a single skin cell taken from a dead gaur. The skin cell was fused with a cow's egg whose genes had been removed. The fused cell was transferred to the womb of another cow. The gaur calf (named Noah) was due to be born in November, 2000.

Asian gaur is the first endangered species to be cloned, and also the first cloned animal to gestate in the womb of another species.

Massachusetts scientists have plans to clone some other endangered animals also.

Technique Suggested for Human Cloning.

Artificial fertilization should be achieved in a glass dish, the zygotes formed should be enucleated, and then the nuclei of bone marrow cells of a desired individual should be introduced into the eggs. If the renucleated eggs develop into individuals, the latter will be genetically identical. This method, if success­ful, will produce a human clone.

Purpose of Cloning. The cell or organism cloning is aimed at preserving the genotype of a cell or organism. In a way, it counteracts death, which results in the loss of the cell's or organism's specific genotype.