"EB - Edward L. Ferman - The Best From Fantasy & Science Fiction 23rd EditionUC - SS" - читать интересную книгу автора (Fantasy & Science Fiction Magazine)

In general, though, complex animals and, in particular, vertebrates do not clone but engage in sexual reproduction exclusively.
Why?ЧTwo reasons.
In the first place, as an organism becomes more complex and specialized, its organs, tissues, and cells become more complex and specialized as well. The cells are so well adapted to perform their highly specialized functions, that they can no longer divide and differentiate as the original egg cells did.}:
This seems a terrible disadvantage. Organisms that can clone, reproducing themselves asexually, would seem to be much better off than other organismsЧwho must go to the trouble of finding partners and who must engage in all the complex phenomena, both physical and chemical, involved in sexual reproduction. Think of ail the human beings who, for one slight flaw or another, can't have children Чa problem that would be unknown if we could just release a toe and have it grow into another individual while we grew another toe.
Here comes the second reason, then. There's an evolutionary advantage to sexual reproduction that more than makes up for all the inconveniences.* In cloning, the genetic contents of new organisms remain identical with those of the original organisms, except for occasional mutations. If the organism is very efficiently adapted to its surroundings, this is useful, but it is an extremely conservative mechanism that reduces the chance of change. Any alteration in the environment could quickly lead to the extinction of a species.
In the case of sexual reproduction, every new organism has a
t This is not mysterious. We see an analogy on the social plane. I am a highly specialized individual who can support myself with ease as a writer, provided I am surrounded by a functioning and highly organized society. Place me on a desert island and I shall quickly perish since I don't know the first thing about the simplest requirements for self-support
* Please don't write to tell me that the activities involved in sexual reproduction are not inconvenient at all, but are a lot of fun. I know that better than you do, whoever you are. The fun is an evolutionarily developed bribe designed to have us overlook and forgive the inconveniences. If you are a woman, you will see the point more quickly, perhaps, than a man wilt
Clone, Clone of My Own
105
brand-new mix of genes, half from one parent, half from another. Change is inevitable; variation from individual to individual is certain. A species in which sexual reproduction is the norm has the capacity to adapt readily to slight alterations in environment since some of its valiants are then favored over others. Indeed, a species can, through sexual reproduction, split with relative ease into two or more species that will take advantage of somewhat different niches in the environment
In short, a sexually reproducing species evolves much more quickly than a cloning species, and such difficult-to-evolve specializations as intelligence are not likely to arise in the entire lifetime of a habitable planet, without sexual reproduction.
Yet in one specialized way cloning can take place in even the most advanced animals-even in the human being.
Consider a human egg cell, fertilized by a human sperm cell. We now have a fertilized egg cell which contains a half set of genes from its mother and a half set from its father.
This fertilized egg cell cannot become an independently living organism for some nine months, for it must divide and redivide within its mother's womb and be nourished by way of its mother's bloodstream. It must develop, specialize, and grow larger until it has developed the necessary ability to live independently. Even after it emerges from its mother's womb, it requires constant and unremitting care for a period of time before it can be trusted to care for itself.
Nevertheless, the matter of necessary care is genetically irrelevant The fertilized egg is already a separate organism with its genetic characteristics fixed and unique.
The first step in the development of the fertilized egg is that it divides into two cells that cling together. Each of these two cells divides again, and each of the four that results divides again and so on.
If, after the first cell division, the two offspring cells, for any reason, should happen to fall apart, each offspring cell may then go on to develop into a complete organism of its own. The result is a pair of identical twins, each with the same genetic equipment and each of the same sex, of course. In a sense, each twin is a clone of the other.
There is no reason to suppose that this separation of offspring cells
106
Isaac Asimov
can't happen over and over, so that three or four or any number of organisms might not develop from the original fertilized egg. As a matter of practical fact, however, a mother's womb can only hold so much, and if there are multiple organisms developing, each is sure to be smaller than a single organism. The more organisms that develop, the smaller each one and, in the end, they will be too small to survive after delivery.
There are such things as identical triplets and quadruplets, but I doubt that any higher number of infants would survive long after birth without the advantages of modern medical technique. Even then it is hard enough.
Identical twins are very like each other and often display mirror-image characteristics. (I once had a chemistry professor with his nose canted to the left. His identical-twin brother had his nose canted to the right, I was told.)
It is also possible, however, though not usual, for a woman to bring two different egg cells to fruition at the same time. If both are fertilized, two children will be bom who are each possessed of genetic equipment different from the other. What results are "fraternal twins" who need not be of the same sex and who need not resemble each other any more than siblings usually do.
Consider the fertilized egg again. Every time it divides and redi-vides, the new cells that form inherit the same genetic equipment possessed by the original fertilized egg.
Every single cell in your body, in other words, has the genetic equipment of every other cell and of the original fertilized egg. Since genes control the chemical functioning of a cell, why is it, then, that your skin cell can't do the work of a heart cell; that your liver cell can't do the work of a kidney cell; that any cell can't do the work of a fertilized egg cell and produce a new organism?
The answer is that though all the genes are there in every cell of your body, they aren't all working alike. The cell is an intricate assemblage of chemical reactions, chemical building blocks, chemical products, and physical structures, all of which influence one another. Some genes are inhibited and some are stimulated, in a variety of ways depending on subtle factors, with die result that different cells in your body have genetic equipment in which only characteristic parts are working at characteristic rates.
Clone, Clone of My Own
107
Such specialized development begins in the earliest embryo, as some cells come into being on the outside of the embryo, some on the inside; some with more of the original yolk, some with less; some with first chance at absorbing nutrients from the maternal bloodstream, some with only a later chance. The details are clearly of the greatest importance to human biology, and biologists just don't yet know them.
Naturally, the ordinary "somatic cells" of an adult human body, with their genetic equipment working only in highly specialized ways, cannot divide into a whole organism if left to themselves. Many body cells, such as those of the muscles or nerves, have become so specialized they can't divide at all. Only the sex cells, eggs and sperm, retain the lack of genetic specialization required to produce a new organism under the proper circumstances.
Is there any way of unspecializing the genetic structure of somatic cells so as to allow them to develop into a new organism?
Well, the genes are contained in the nucleus of the cell, which makes up a small portion of the total and is marked off by a membrane of its own. Outside the nucleus is the cytoplasm of a cell, and it is the material in the cytoplasm that provides the various chemicals that help serve to inhibit or stimulate die action of the genes.
Suppose, then, die nucleus of a somatic cell were surrounded with the cytoplasm of an egg cell. Would the genetic equipment in the nucleus unblock, and would the egg cell then proceed to divide and redivide? Would it go on to form an individual with the genetic equipment of the original somatic cell and, therefore, of the person from whom the somatic cell was taken? If so, the new organism would be a clone of the person who donated the somatic cell.
The technique has been tried on different animals. You begin with an unfertilized egg cell and treat it in such a way as to remove its nucleus, either by delicately cutting it out or by using some chemical process. In the place of the removed egg cell nucleus, you insert the nucleus of a somatic cell of the same (or, possibly, an allied) species, and then let nature take its course.
This has been successfully tried with animals as complex as a tadpole.
It stops being easy after the frog, though. Frog eggs are naked and can be manipulated easily. They develop in water and can just lie there after the micro-operation.
108
Isaac Asimov
The eggs of reptiles and birds, however, are enclosed in shells, which adds to the technical difficulty. The eggs of mammals are very small, very delicate, very easily damaged. Furthermore, even if a mammalian egg has had its nucleus replaced, it would then have to be implanted into the womb of a female and allowed to come to term there.
The practical problems of mammalian cloning are such that there is no chance of its happening for some time yet. Yet biologists are anxious to perform the feat and are trying hard. Eventually, they will no doubt succeed. What purpose will it serve?
If clones can be produced wholesale, a biologist can have a whole group of animals with identical genetic equipment; a set of ten thousand identical-twin mice, let us say. There are many animal experiments that can be conducted with the hope of more useful results if the question of genetic variation could be eliminated.
By the addition of other genetic-engineering techniques, it might be possible to produce a whole series of animals with identical genetic equipment, except that in each case, one gene is removed or alteredЧa different gene in each individual perhaps. The science of genetics would then advance in seven-league strides.
There would be practical uses, too. A prize bull or a champion egg-laying hen could be cloned, and the genetic characteristics that make the record-breaking aspects of the animal possible would be preserved without the chance of diminution by the interplay of genes obtained from a second parent.
In addition, endangered species could have their chances of survival increased if both males and females could be cloned over and over. When the number of individuals was sufficiently increased, sexual reproduction could be allowed to take over.
We might even dream of finding a frozen mammoth with some cell nuclei not entirely dead. We might then clone one by way of an elephant's womb. If we could find a male and a female mammothЧ
To be sure, if cloning is overdone, the evolutionary advantage of sexual reproduction is to some extent neutralized, and we might end up with a species in which genetic variability is too narrow for long-term survival.
It is important to remember that the most important genetic possession of any species is not this gene or that, but the whole mixed
Clone, Clone of My Own