The mapping of the human genome is almost complete. It is a great accomplishment, involving as it does the sequencing of three billion nucleotide base pairs and the mapping of about thirty thousand genes.

The relatively small size of that second number is a recent surprise, discovered within the past two years. It may seem at first sight something of a blow to the human ego. Ten years ago we expected that the gene count for humans would certainly exceed a hundred thousand. Thirty thousand is no more than those possessed by a mouse or an octopus. All our vaunted complexity, all the genetic intellectual potential that can give rise to a Shakespeare or a Beethoven or an Einstein is packed into genes that barely outnumber those possessed by a worm or a wasp. Apparently intelligence, which we value so much and which seems to separate us from the rest of creation, is from Nature's point of view no more than a superficial add-on to the basic structure of life.

Sometimes writers give the impression that with the mapping of the genome we will come to the end of the story; that once we know the full sequence of nucleotide bases that comprise an individual's DNA, and how the genes formed out of that DNA interact with each other, there will be little left to learn. We will hold the key to curing all genetic diseases. We will understand the source of our unique intelligence. We could well be on the way to the indefinite extension of life itself.

It's a rosy prospect, but unfortunately a misleading one. It is true that an individual, be it human, mouse, octopus, worm, or wasp, is defined by its DNA (or, for some viruses, its RNA). However, there is more to life and reproduction than DNA. We can show that this is so easily enough, by taking, for example, the nucleus of a wolf's cell, which contains the wolf-defining DNA, and injecting it into an ovum of a sheep from which the nucleus has been removed. The new cell develops into neither wolf nor sheep. It dies.

You began life not just as a DNA sequence, but as a DNA sequence embedded within the environment of a human ovum. An ovum is a special type of cell, containing within it complex mixtures of chemicals, three-dimensional structures of proteins and non- proteins, and mitochondria which possess their own DNA. Ova for every species are different.

Which, then, is more important in shaping the unique you: your human DNA, or the human ovum in which that DNA develops? That's a meaningless question. Both are essential. A human will not develop if you place the DNA of a mouse inside a human ovum, and a human will not develop if you place human DNA inside a mouse ovum. You may say, but what about a virus? A virus is just a package of DNA or RNA, without a cell of its own in which to grow.

Even a virus is picky. It requires not just any old cell, but the cells of certain specific species whose internal machinery it can take over and employ to its own reproductive ends. Lucky for us that this is so, since otherwise viruses would hop at will from any species to any other. We would be susceptible to a thousand or a million more virus-borne diseases. As it is, the acquired ability of a virus to move from one species to another is the suspect in some important diseases, particularly AIDS.

The removal of a cell nucleus of one species and its insertion into the denucleated cell of another is a tricky procedure, although today it is routine in many labs. However, we don't need modern technology to make the point that the ovum in which a nucleus develops may be as important as the nucleus itself. Long before the study of genetics, a male horse would sometimes accidentally mate with a female donkey (accidental from the owner's point of view; the horse undoubtedly knew exactly what it was doing). The result of such a union is an undesirable offspring known as a hinny. The hinny is not useful as a work animal, and it is weak and prone to disease.

By contrast, a mule is the offspring of a male donkey and a female horse. It is a product of deliberate breeding, because the mule is strong, hardy, and a good (if obstinate) work animal. Yet both the mule and the hinny contain the same amount of horse and donkey genetic material. Their differences are likely to arise because of the different ova in which they develop, as well as through the DNA. It is difficult to tell which by further crossbreeding, because both mule and hinny are generally sterile.

Another example is the mating of a lion and a tiger. This does not occur in nature, because the two animals live on different continents. A male tiger and a female lion produce a tigon. A male lion and a female tiger produce a liger (the naming here shows a certain lack of imagination). Again, the two types of offspring display substantial differences, but again the DNA for each derives in equal amounts from both species.

It is common today to measure the "distance" between two species by the differences between their DNA. However, for closely related species (horse and donkey, lion and tiger, and, dare I say it, human and chimpanzee) as good a measure may be the ease with which offspring can be produced. The ovum presumably plays a significant part in this.

No doubt about it, DNA is of absolutely central importance if we are to understand life and reproduction. We are certainly working the problem hard. Twenty years from now we may possess genome maps of hundreds of species, together with an understanding of how these lead to distinct genes, and how in turn those genes interact with each other. However, our knowledge will remain inadequate unless we also understand how the ovum, with its particular species-specific structure, serves as a home in which DNA can flourish and reproduce.

Note: I wanted a cute title for this column, but in the interests of full disclosure I have to admit that to my knowledge no one has ever shown by experiment that a nucleus taken from a wolf will not develop in a sheep's ovum. However, I am willing to take substantial bets.

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