Scientists speak of the new gene engineering technology, without trying to exaggerate, as the single greatest tool of biological science to be created since the microscope.

It is a scientific advance of historic dimensions and has philosophical implications that go far beyond science. The revolution in biology prompted a bold reply from a Nobel Prize-winning French biologist who was questioned recently about the "secret of life".

"The secret of life? But in principle we already know the secret of life," he answered.

The secret is DNA, short for deoxyribonucleic acid. The series of discoveries that revealed what DNA is and how it may be put to use is a scientific advance that ranks with Galileo's confirmation of the movement of the planets, the quantum theory and Einstein's theory of relativity in its alteration of man's view of the universe and himself.

The planetary theories of Copernicus and Galileo challenged the concept that man was the center of the universe. Darwin's theory of evolution questioned man's superior place on earth, that he was uniquely created by God. The gene revolution takes the next step. Life, at least in its mechanics, is shown to be mere chemistry. The mechanical secrets of life, believed in the past to be secrets known only to God, are now open to man, at least in outline.

The formerly "sacred" mechanisms of life now are being manipulated by man. God's own tabernacle, some feel, has been rifled.

To scientists, it is no sacrilege, but the discovery of one of the most awesome of nature's intricacies.

During the first half of this century physics was the dominant science, as physicists broke through the surface of the material world to an entirely new level. They got at least a rough answer to a 2,500-year-old question--is the universe composed of simple basic units of some universal substance? What came of the revolution in physics was atomic theory and quantum mechanics--a body of work often called the single greatest achievement of the human mind.

Now scientists are establishing a stunning and equivalent body of work that has revolutionized the once quiet field of biology. The discovery of the structure and meaning of DNA in 1953 was followed rapidly by a series of discoveries that led finally to gene-splicing technology and exhilarated biologists with the excitement of the rush of new results.

But, unlike the 20th century advances in physics, the great discoveries of biology are not based on complicated mathematics. They are not obscure. They can be described and understood in plain English.

The advances give scientists the power to manipulate life at an entirely new level, the level of molecules, where the final, mechanical secrets of life reside.

The new technology gives rise to all sorts of possibilities because it makes possible the understanding of every body process run by genes. Thus, it may lead to a detailed understanding of how cancer or hundreds of genetic diseases work. It may lead to cures for them.

It also opens the possibility of adding new genes, which mean new traits, to any animal or plant. Among those currently being worked on are self-fertilizing plants, efficient oil-eating microbes, drought-resistant plants.

Adding new traits is easy with tiny organisms like bacteria. It is much more dfficult with higher animals, but adding new traits to humans is theoretically possible. Nor has human cloning, creating an identical copy of a human from the cell of another human, been proved impossible although scientists say it still is a distant and unlikely prospect.

Biologists can now read the code of life inscribed in DNA. They can artificially copy the code and make artificial genes. They can pluck specific genes from among millions in the human set. They can cut genes out of the DNA chain, and splice in new ones.

The resulting technology has transformed several fields, pushing ahead work by years in some areas, by decades in others, and made some once-impossible experiments now seem simple.Some evolutionary biologists now say we are the instruments of our genes, we are the tools that genes have shaped to ensure their survival over centuries. Our bodies are used and discarded each generation, but the genes that run them go on largely unchanged.

It has created a collective high throughout the field. It has stimulated sober businessmen to throw hundreds of millions of speculative dollars into the field, dreaming of spectacular commercial products, as well as spectacular profits.

Steven Clark is standing in a lab at the end of a hallway that is littered with furniture as movers begin to fill the row of empty labs. He is a senior scientist just hired at Genetics Institute in Boston, one of many new gene engineering firms hoping to take profitable advantage of the sudden wave of new knowledge in biology. They want to make interferon, the antiviral agent, and agricultural products. Other companies are making human growth hormone, insulin, foot-and-mouth vaccine, and are dreaming of a thousand other things.

Scratched on the blackboard in what will be Clark's office is a recipe for mixing the genes of different creatures. The recipe includes mixing chemicals in tubes, heating, spinning them in a centrifuge, and growing colonies of bacteria on a glass plate.

The lab recipes that constitute gene splicing technology depend on three elements.

First is the gene, the fundamental particle of life. Second is the peculiar helical shape of the DNA molecule. Third is the fact that nature itself cuts and splices, mixes and trades DNA on its own without help from man.

PARTICLES OF LIFE. There are, on average, a few thousand molecules lined up in a chain to make up one gene. Then the genes, linked together in a long, twisting chain, make up a strand of DNA. The great DNA strands are present in virtually every cell of the body.

Genes are the particles of life, the one indispensable element for all things alive on earth. They are passed through the sex cells to progeny, where they direct the construction of living bodies, determining which will become milkweed and which human.

Preserved in genes over the centuries are the instructions for making arms, legs, and wings. There are also bits of DNA whose mission is to make the Hapsburg lip and the Irish freckle.

But even more than carrying traits from generation to generation, genes run the machinery of living cells. There is no action or thought without a gene first having created a little chemical cascade in the cell. Genes direct the building of cell walls and then operate the metabolic "pumps" which draw through the membrane useful molecules (from food) into the cell. Genes direct the breakdown of these large molecules and the rearrangement of the bits to make the key substances for running the body.

For example, in bone marrow cells, a gene's code orders the production of hemoglobin, a chemical with the special ability to fold over and capture an oxygen molecule. In cells of the body's immune system, which defend against foreign substances invading the body, genes direct the manufacture of antibodies, those proteins which attach themselves to the foreign substances, thereby disabling them in the body.

In muscle cells, the key element that genes order up is myosin, a protein that has the ability to change its length and thus make muscles contract. In the brain, genes direct the making of the chemicals that transmit electrical impulses across the gap between nerve cells to create thought and action.

Altogether, the power of genes to run the daily machinery of life and get themselves passed from generation to generation over the ages is such a remarkable fact of life that is has led scientists to rethink the relation of genes to the creatures they define.

Some evolutionary biologists now say we are the instruments of our genes, we are the tools that genes have shaped to ensure their survival over centuries. Our bodies are used and discarded each generation, but the genes that run them go on largely unchanged over the centuries.

"We, and all the other animals are machines created by our genes," writes Oxford University zoologist Richard Dawkins, in his book, "The Selfish Gene." "We are survival machines . . . . An octopus is nothing like a mouse and both are quite different from an oak tree, yet in their chemistry they are rather uniform . . . in their genes are basically the same kind of molecule in all of us--from bacteria to elephants. We are all survival machines for the same kind of replicator or gene molecules called DNA . . . .

"But there are many different ways of making a living in the world, and the replicators have built a vast range of machines to exploit them. A monkey is a machine which preserves genes up trees; a fish is a machine which preserves genes in the water; there is even a small worm which preserves genes in German beer mats."

SHAPE AND ACTION OF DNA. Genes are nothing more than strands of the molecule DNA. DNA was discovered by a Swiss chemistry student, by accident, in 1839. He was assigned by his teacher to analyze the contents of some pus cells from war bandages, and in the process came on a substance found only in the cell's nucleus. He purified it and put the resulting white, gummy substance in a jar, there to remain with its true nature unknown until after 1953 when James Watson and Francis Crick began unraveling the nature of DNA.

When the chemical makeup of DNA became known before the work of Watson and Crick, it was first believed to be a boring, unimportant heap of molecules.

The magic is in the way they are put together. The double-stranded, twisting strand of DNA has extremely unusual dimensions. A lot of it must be stuffed into cells, but at the same time it has to remain a thin ribbon, as readable as a tape running past a recording head in a tape recorder.

The strand is 10 atoms wide and billions of atoms long. If the coiled and folded DNA from one single human cell were unknotted and stretched to its full length, it would not only be longer than the cell that contained it, but would reach completely out of the level of atom-sized things. It would be an invisible filament more than three feet long.

Each human cell has its bundle of DNA, about three million genes' worth, coiled and looped and folded so that all of it can fit into a pinpoint space.

The filament of DNA is very like a rope made up of two twisting strands. Embedded in the strands of the rope are the molecules that constitute the genetic code, the information that guides life.

It is the order of the molecules stuck between the strands of the DNA that create a code. There are only four such molecules--adenine, thymine, cytosine, and guanine--but together they make an alphabet.

Nature reads the long alphabetical sequences in three-letter words, such as TGT, TGA, CAC, and so on, to name the chemical bits it wants assembled in the cell to create important substances. There are 64 possible three-letter combinations and a few hundred of these three-letter words make a gene. Each gene is a sentence of command: Make insulin. Break down milk sugar. Make hemoglobin. Make adrenalin. The matter in outline is simple--and terrible. For example, hemoglobin is made at the direction of a sequence of 440 "letters" in the DNA chain. A change of a single letter turns normal red blood cells into mangled cells of the disease called sickle-cell anemia.

Biologists were both stunned and amused when they first learned that DNA actually operates by a code, one that is in principle exactly like the Morse code. Max Delbruck, one of the great biologists of recent decades, once said to a writer, "Absolutely nobody . . . had thought that the specificity of the gene might be carried in this exceedingly simple way, by a sequence, by a code. . . that the whole business was like a child's toy that you could buy at the dime store, all built in this wonderful way that you could even explain in Life magazine . . . that there was so simple a trick behind it. That was the greatest surprise for everyone."

Delbruck once sent a telegram to his friend George Beadle when Beadle won the Nobel prize for his work related to the DNA code. The text of the telegram consisted entirely of words in a DNA-like alphabet: ADB, ACB, BDB, ADA, CDC, BBA, BCB, CDA and so on.

Decoded, the telegram read: BREAK THIS CODE OR GIVE BACK NOBEL PRIZE.

The matter in outline is simple--and terrible. For example, the life-giving substance hemoglobin is made at the direction of a sequence of 440 "letters" in the DNA chain. A change of a single letter, from a T to a G in the 17th position, turns normal red blood cells into mangled, sickle-shaped cells of the disease called sickle-cell anemia. A few other letters changed and the thalassemias result--horrible diseases in which the body makes few or no blood cells. The victims usually die painfully as young children.

THE GENE-SPLICING TOOLS. The discovery of the great new tools of biology, the techniques of mixing DNA, began with the realization that DNA within cells is not perfectly set and stable. It can be broken by such environmental agents as radiation and cancer-causing chemicals. So, the cell has a series of molecules it uses to splice back together pieces of DNA.

Cells also have more than a hundred varieties of molecules designed to cut up DNA. This is because the body needs to defend itself against invading viruses and bacteria--they might inject their DNA into the human cells and foul the genetic machinery. The cutting molecules can identify foreign DNA and destroy it by latching onto it and breaking it into pieces.

Since the strands of DNA cannot be seen or manipulated directly, it is only by using these natural scissors-and-paste molecules that biologists can cut one gene from the millions in the human and animal chromosomes, study it, and then splice it back into another creature.

The ability to identify and pick out single genes from the millions in the human gene set is a power unlike any other in biology. It is as if astronomers could send probes to examine in minute detail every star, black hole, planet and interstellar cloud. Suddenly very old questions have answers, and whole new corridors of questions not imagined before are opened up.

The technique for pulling a single gene out of millions has, in rough outline, three steps:

First some cells are taken, perhaps from a drop of blood or bit of skin. Chemicals are applied to the cells to burst the cell walls and free the DNA. It is then separated from thousands of other substances in the cell by spinning it all in a centrifuge and pulling out the DNA, which ends up in the bottom of a tube.

The separated DNA is put into a test tube along with the substances that cut DNA up into fragments. The result is a test tube full of fragments of DNA, with each fragment about 20,000 "letters" long. Since a gene can be a few thousand letters long, each fragment is enough to have one or more genes in it. One of these genes, among the tens of thousands of fragments, is the one sought.

Second, the DNA fragments are inserted inside bacteria. They can be mixed with the bacteria's DNA easily. Or, they can be spliced into the DNA of a virus, which then in turn inserts itself into the bacteria. Either way, the foreign DNA is inserted into the bacteria's own DNA.

Thus, the bacteria will follow the instructions of the foreign DNA just as if it were the bacteria's own.

The bacteria continue to multiply normally, reproducing among their own genes the new, foreign one. After a short time this results in a whole colony of bacteria for each of the DNA fragments the researcher started with. Each colony is grown separately, multiplying its own DNA fragment, so in the end each colony is a collection of bacteria representing one DNA fragment in many copies. This is called "cloning" a piece of DNA or a gene, because, from one bacterium and one fragment of DNA, many identical copies are made by the multiplying bacteria.

Finally, since each colony represents many copies of one DNA fragment, each colony of bacteria can be tested to see if it is the one that has the desired gene fragment. For example, if a researcher wants to find the gene that makes interferon, each colony would be tested for its ability to stop viral infections. Only the bacterial colony with working interferon genes will pass the test.

So the gene has been isolated. Some such test can be devised for almost all genes, because every gene makes some substance--interferon, insulin, hemoglobin, myosin, and so on. Each of these substances is chemically quite different and there are methods to identify them.

When genes are plucked out by this method, said Harvard's Thomas Maniatis, who helped develop the techniques, the action never has to be repeated. Once a gene is pulled out and cloned or multiplied in bacteria, it can be grown up endlessly, in any amounts needed.

So gradually it will be possible to isolate and work with every human or animal gene of any interest. More than a hundred have already been pulled out and studied to the point that their exact "letter" sequence is known.

With genes teased out by this technology, Maniatis said, "You have a permanent, amplified version of something that is very, very rare. This is what constitutes the major breakthrough in molecular biology."

"Seven years ago, before any of this technology was developed, the prospect of isolating a single DNA fragment or gene out of three million was zero," Maniatis said. "There was just no way you could do it physically." The techniques were difficult and incomplete until about 1978, and the latest methods of identifying genes once they have been cloned in bacteria have just been developed in the past two years.

"But now all this technology is routine," Maniatis said. "For example, you really can take DNA from anyone with a genetic disease, very quickly isolate the gene and find the genetic abnormality. . . . This really is in the same class as the invention of the microscope, it has just opened up a whole area that had previously been impossible to approach."

To be able to discover that a disease results from the accidental movement of a single molecule, a single "letter" in the DNA chain, and how that altered letter causes the body to make a useless substance instead of, for example, blood cells, is an ability undreamed of only a few years ago.

For the first time, disease and other functions of life have been laid open at the level of pure molecular machinery.