ON DNA AND CLONES

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ON DNA AND CLONES
ON DNA AND CLONES

A clone is an exact copy of a plant or animal that is produced from one of its cells by scientific methods.

Despite the horror stories and jokes about powerful people having themselves cloned, this technique has been limited to the study of the cell, the duplication of DNA, and the asexual reproduction of many kinds of plants, as well as the copying of some frogs, fish, and mammals.

            Cloning occurs frequently in the plant world, and for thousands of years humans have known about and used the plant’s ability to reproduce both sexually and asexually. Instead of waiting for the time-consuming process of pollination, growth, and seed production, a grower can take a cutting from a mature plant; its cells will soon differentiate into all the parts that will allow it to live on its own.

            For example, every time you give a cutting of your favourite houseplant to a friend, you begin a process that results in a clone, or identical genetic copy of the original plant. When gardeners plant chunks of potato, called seed potatoes, every spring, they are growing clones of some other potato plant that had desirable qualities. Roses, apples, and many other plants are routinely cloned by inserting a twig or bud from a closely related plant into a slit on a host plant in order to achieve the best fruit or flower possible. In fact, the term clone comes from a Greek word that means twig or branch.

            Today, scientists are able to clone complete plants by using single cells. Many plant cells that are not very specialised are totipotent (a totipotent cell has the capacity to form an entire organism). Human development begins when a sperm fertilises an egg and creates a single totipotent cell. In the first hours after fertilisation, this cell divides into identical totipotent cells. Approximately four days after fertilisation and after several cycles of cell division, these “totipotent” cells begin to specialize. Totipotent cells have total potential. They specialise into pluripotent cells that can give rise to most, but not all, of the tissues necessary for foetal development. Pluripotent cells undergo further specialisation into multipotent cells that are committed to give rise to cells that have a particular function. For example, multipotent blood stem cells give rise to the red cells white cells and platelets in the blood meaning they are undifferentiated. By contrast, a differentiated cell is specialised to perform only certain functions and cannot give rise to a complete organism. In other words, you can clone a mature carrot using one of its cells, but you can’t clone a grown woman using just any of her cells.

            It is possible, however, to clone some of the genetic instructions in the human cell. By splicing human DNA into that of bacteria or virus, researchers have developed ways to clone the genetic instructions for important human proteins like insulin, interferon, and human growth factor. When the microbes replicate, or clone their own genetic instructions, they also reproduce their human stowaway genes. Huge vats of human protein are then separated from their microbial products and made available to people who need them.

            The ability to clone larger animals has allowed livestock breeders to preserve and amplify the championship lines of prize bulls and cows, producing farm animals with the cookie-cutter sameness and quality of modern factory products. This process is achieved by artificially inseminating a cow with semen from a champion bull and then removing the embryos, each of which has already divided into a number of cells. Each cell nucleus contains the genetic plans for an entire beast and so, when nuclei of all the embryonic cells are transplanted into eggs from ordinary cows an implanted in their wombs, the surrogate mothers give birth to identical champion calves. Zoos have successfully used similar cloning technology to produce new members of endangered species, which are born to surrogate ordinary mothers.

            It is the scientists’ ability to clone mammals that makes some people nervous. If they can clone mice, sheep, and cattle, how difficult would it be to clone human beings? In fact, not difficult at all. The technology exists.

            Medicine’s ability to manipulate the human reproductive process through artificial insemination and in vitro (which means “in a glass dish”) fertilization anticipates, in some minds, the cloning of human beings.

            In theory, it would be possible to produce a human clone by removing the nucleus of a certain kind of reproductive cell that has a full complement of genetic material and exchanging it for the nucleus of an egg cell. In 1979, London B. Shettles reported in the American Journal of Obstetrics and Gynaecology that he had done just that. Three such eggs started to divide, but they were not implanted into a human uterus and allowed to develop.

            However, even though it may be possible to transfer human embryonic cell nuclei into other human eggs, it is not likely to happen for ethical and economic reasons. No responsible scientist would take on such an experiment, nor would any funding agency, such as the federal government, support a researcher intent upon manipulating human beings in that way. Such work would require the support of a sophisticated laboratory, plenty of money, and a distinct lack of scruples.

            At this writing, it is not possible to make clones from the DNA of mature humans. Despite several highly publicised claims, human cloning still appears to be fiction. There currently is no solid scientific evidence that anyone has cloned human embryos.

So we are not likely to see little copies of Albert Einstein or Abraham Lincoln any day soon. (Samples do exist of Einstein’s brain tissue, as does blood from the shirt sleeves of the surgeon who treated the assassinated president.)

            It’s not possible in this case, because there isn’t a complete copy of either man’s DNA available. Even if there were, scientists don’t understand enough about the human genome to make a whole human from DNA. Furthermore, human cloning won’t work because proteins in the nature cell restrict the developmental potential of DNA, making it an unsuitable recipe for a whole new human being. Scientists may someday figure out how to reverse these restrictions, but to what end? The nature adult is more than the sum of his or her parts. Experience, knowledge, and memory cannot be cloned.

            Nevertheless, ethicists tell us that the pace at which genetic research and engineering are advancing demands that we carefully discuss such issues and decide how we will use this knowledge when it becomes available.

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