WHEN the news broke a few weeks ago about Dolly, the cloned sheep, I thought of the day last summer when I called to see my mother. I found her cloning plants in the back garden. Had I been wearing my Superman spectacles, that allow me to see incredibly tiny things, I would have seen that the whole garden was a hive of cloning activity.
Bacteria were cloning themselves all over the place. I would have seen aphids busily cloning themselves by the hedge, and, down in the far corner, the bees and the wasps were also at it. Cloning is "old hat" to Mother Nature.
Biological organisms reproduce either by sexual or asexual methods, and some can employ both. In asexual reproduction, the new [individual (offspring) derives genetic material from a single parent only. In sexual reproduction, the new individual usually derives half its genetic complement from one parent and half from the other parent. The recent cloning of Dolly the sheep, in Scotland, is a special example of artificially contrived reproduction, where all the genetic content of the offspring comes from the mother.
The genetic content of an organism is contained in its chromosomes. A biological clone is defined as an individual that is genetically identical to its single parent.
For example, when simple single celled organisms wish to reproduce, they do so by dividing in two. The chromosomes of the "parent" duplicate prior to cell division, and each daughter cell receives an identical copy. Both daughters are genetically identical to the parent and are therefore clones of the original.
The asexual production of clones is also common in horticulture. The type of cloning my mother was doing involves sticking twigs ("cuttings"), from plants she landed while out for a stroll, into the ground. The cutting takes root and grows into a new plant identical to the one from which it was cut.
Asexual reproduction can also take place in multicellular animals. The more primitive the animal, the more likely it is that asexual reproduction can take place.
For example, you could cut a sponge or a flatworm into small parts, and each part, if kept in its usual environment, would grow into a complete organism. These new organisms could be considered clones of the original.
Many insects, including bees, wasps and aphids, also commonly produce clones of themselves.
Technically this is classified as a form of sexual reproduction, because a sex cell, the egg, is involved. However, the process differs radically from the normal form of sexual reproduction seen in the higher animals.
This special process in insects is called parthenogenesis. In aphids, an unfertilised egg cell containing, only a half set of chromosomes develops into an embryo, and subsequently into an adult insect, without the assistance of any sperm cell. The egg's half set of chromosomes duplicates itself to produce a full sea the egg divides and continues to divide to become a fully developed organism. All of the chromosomes come from the female parent. So, again we are dealing with a kind of clone.
I once heard a well known Irish feminist declare at a public meeting: "We (women) don't need men, we can have children by parthenogenesis." She may have been joking, but she sounded serious. If she was serious, she could scarcely have made a nastier statement.
Could a woman conceive by parthenogenesis? I don't know the answer to this question. In most forms of life an egg requires activation by a sperm in order to start dividing and to develop into an embryo. However, in several cases it has been shown that it is possible to spark this activation experimentally and to obtain parthenogenetic reproduction.
For example, sea urchin eggs can be stimulated to divide by placing them in strong salt water. Frog eggs have been stimulated similarly by pricking them with "needles. There is a report that "rabbit eggs were also successfully stimulated in this manner. A breed of white turkeys produce eggs (unfertilised) that sometimes develop into embryos when incubated. Some of these embryos eventually hatch and go on to produce adults - turkeys without fathers.
Conventional sexual reproduction is the norm for human beings and for organisms generally that are at our level of complexity. Sexual reproduction is effected by the union of a sperm cell from the father with an egg cell from the mother. The new individual receives an equal amount of genetic material from each.
The genetic material - genes - reside in the chromosomes, located in the nucleus of the cell. Our bodies are composed of cells of two types - somatic cells and germ cells. The somatic cells make up our tissues - including kidney, liver, etc. The germ cells are involved in procreation - sperm cells in the male, and egg cells in the female. Somatic cells contain 23 pairs of chromosomes, one set coming originally from the father and the other set from the mother.
In order for tissues to grow and maintain themselves, somatic cells increase by each dividing into two. Prior to cell division (mitosis), the, chromosomes duplicate themselves, and each daughter cell receives the full complement of 23 pairs of chromosomes.
However, when germ cells are formed, each cell receives, not 23 pairs, but 23 individual chromosomes. Union of the sperm with the egg restores the somatic situation of 23 pairs of chromosomes in the fertilised egg.
The subsequent development into an adult consisting of billions of cells is effected by countless cell divisions beginning with division of the fertilised egg cell. Each somatic cell in the body contains the same 23 pairs of chromosomes which constitute the entire genetic blueprint of the organism.
Cells of different tissues in the body look different from each other and perform different tasks, even though every cell contains identical genetic information. The explanation is that differentiated cells use only part of the entire genetic blue print information. Parts not in use in a particular tissue are switched off.
The question has long intrigued biologists - since an ordinary somatic cell has identical genetic information to that in the original fertilised egg, would it be possible to trigger a somatic cell to develop into a new organism, into a clone of the parent cell? Many attempts have been made over the years to achieve such cloning, and with, some success.
The phenomenon of parthenogenesis shows that the environment within the egg can trigger a programme of development in the chromosomes that results in the eventual formation of an adult organism. In attempting to clone an organism from a differentiated somatic cell, the approach has been to extract the nucleus from such a cell, then remove the nucleus from an egg cell and replace it with the nucleus from the differentiated somatic cell, and hope that the egg cell environment would trigger the developmental programme.
Success has been achieved in this manner in cloning tadpoles. However, not until Dolly had there ever been a success in triggering the chromosomes from a somatic cell from an adult mammal. Dolly was cloned from an udder cell taken from an adult sheep.
In an odd combination of the trite with the profound, it was decided to name the clone, Dolly, after the singer, Dolly Parton, who also displays impressive developments of mammary cells.
I will return next week to the subject of cloning to discuss ethical issues raised by this recent development.