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Special Section: THE CELL: Unraveling the Double Helix and the Secret of Life

17 minute read
TIME

Wildly excited, two men dashed out of a side door of Cambridge University’s Cavendish Laboratory, cut across Free School Lane and ducked into the Eagle, a pub where generations of Cambridge scientists have met to gossip about experiments and celebrate triumphs. Over drinks, James D. Watson, then 24, and Francis Crick, 36, talked excitedly, Crick’s booming voice damping out conversations among other Eagle patrons. When friends stopped to ask what the commotion was all about. Crick did not mince words. “We,” he announced exultantly, “have discovered the secret of life!”

Brave words—and in a sense, incredibly true. On that late winter day in 1953, the two unknown scientists had finally worked out the double-helical shape of deoxyribonucleic acid, or DNA. In DNA’s famed spiral-staircase structure are hidden the mysteries of heredity, of growth, of disease and aging—and in higher creatures like man, perhaps intelligence and memory. As the basic ingredient of the genes in the cells of all living organisms, DNA is truly the master molecule of life.

The unraveling of the DNA double helix was one of the great events in science, comparable to the splitting of the atom or the publication of Darwin’s Origin of Species. It also marked the maturation of a bold new science: molecular biology. Under this probing discipline, man could at last explore—and understand—living things at their most fundamental level: that of their atoms and molecules. Once molecular biology was sardonically defined as “the practice of biochemistry without a license.” Now it has become one of science’s most active, exciting and productive arenas, taking the limelight (and some of the best talent) from that longtime favorite, nuclear physics.

Using laboratory skills that were unheard of a generation ago, scientists have isolated, put together and manipulated genes, and have come close to creating life itself. In 1967 Stanford University’s Arthur Kornberg synthesized in a test tube a single strand of DNA that was actually able to make a duplicate of itself. Kornberg’s “creation” was only a copy of a virus, a coated bit of genetic material that occupies a twilight zone between the living and inanimate. But many scientists have become convinced that they may eventually be able to create functioning, living cells.

Molecular biology, in part, is rooted in the science of genetics. Ever since Cro-Magnon man, parents have probably wondered why their children resemble them. But not until an obscure Austrian monk named Gregor Mendel began planting peas in his monastery’s garden in the mid-19th century were the universal laws of heredity worked out. By tallying up the variations in the offspring peas, Mendel determined that traits are passed from generation to generation with mathematical precision in small, separate packets, which subsequently became known as genes (from the Greek word for race).

Mendel’s ideas were so unorthodox that they were ignored for 35 years. But by the time the Mendelian concept was rediscovered at the turn of the century, scientists were better prepared for it. They already suspected that genetic information was hidden inside pairs of tiny, threadlike strands in cell nuclei called chromosomes, or colored bodies (for their ability to pick up dyes). During cell division they always split lengthwise, thereby giving each daughter cell a full share of what was presumed to be hereditary material.

A few years later, the suspicions were dramatically confirmed by the pioneering geneticist Thomas Hunt Morgan in Columbia University’s famed “Fly Room.” Through ingenious crossbreeding experiments with the fruit fly Drosophila melanogaster, Morgan and his students were able to map the relative positions of the genes along the insect’s four pairs of chromosomes. Still, the gene’s physical nature remained as great a mystery as ever. DNA had been discovered in the nuclei of cells by the Swiss biochemist Friedrich Miescher a few years after Mendel did his work on peas. But since the chromosomes in which the DNA was found also contained proteins—the basic building blocks of life—few scientists had any inkling that DNA might be playing an even more central role to life.

By the 1940s, however, the molecular biologists had come on the scene, and they insisted that fundamental life processes could be fully understood only on the molecular level. In their investigations, some used the electron microscope, which revealed details of structure invisible to ordinary optical instruments. Others specialized in X-ray crystallography, a technique for deducing a crystallized molecule’s structure by taking X-ray photographs of it from different angles. Physicist Max Delbrück turned to nature for his investigative tools: bacteriophages (literally, “bacteria eaters”), tiny parasitic viruses that invade their host bacteria and rob them of their genetic heritage.

BUT THE HONORS for making the breakthrough discovery went to a traditional bacteriologist. Taking purified DNA extracted from the chromosomes of dead pneumonia bacteria, Rockefeller Institute’s Oswald T. Avery and his associates showed that it could transform other, normally harmless bacteria into virulent ones. The experiment indicated that it was DNA, and not protein, that carried the genetic message. So unexpected was that finding that even Avery was at first unwilling to accept it. Eight years later, Alfred Hershey and his assistant Martha Chase demonstrated that a virus’ DNA could, by taking over a bacterium, also nullify the cell’s genetic instructions and replace them with its own. Only then was DNA finally accepted as the magic substance of the genes.

Inspired by these experiments, Watson, then a young Ph.D. in biology from Indiana University, decided to take a crack at the complex structure of DNA itself. The same thought struck Crick, a physicist turned biologist who was preparing for his doctorate at Cambridge. Neither man was particularly well equipped to undertake a task so formidable that it had stymied one of the world’s most celebrated chemists, Linus Pauling. Watson, for his part, was deficient in chemistry, crystallography and mathematics. Crick, on the other hand, was almost totally ignorant of genetics. But together, in less than two years of work at Cambridge, these two spirited young scientists showed how it is possible to win a Nobel Prize without really trying.

In 1968 Watson himself produced a highly irreverent, gossipy bestseller, The Double Helix, which revealed the human story behind the discovery of DNA’s structure: the bickering, the academic rivalries, even the deceits that were practiced to win the great prize. Out of Pauling’s earlier work, Watson and Crick got the idea that the extremely long and complicated DNA molecule might take the shape of a helix, or spiral. From the X-ray crystallography laboratory at King’s College in London, where Biochemist Maurice Wilkins was also investigating the molecule’s structure, they quietly obtained unpublished X-ray data on DNA. Relying as much on luck as logic, they constructed Tinkertoy-like molecular models out of wire and other metal parts. To everyone’s astonishment, they suddenly produced a DNA model that not only satisfied the crystallographic evidence but also conformed to the chemical rules for fitting its many atoms together.

OUT OF THE architecture of their precisely constructed double helix emerged the secret of DNA’s awesome powers. The banisters of the staircase were fashioned of long links of sugars and phosphates; the steps between them were made of pairs of chemicals called bases, weakly joined at the center by hydrogen atoms. Only four different bases were used—adenine (A), thymine (T), cytosine (C) and guanine (G). But their sequence could vary so widely along the length of the staircase that they made up an almost limitless information-storage system, like the memory bank of a computer. In addition, because the bases were chemically complementary—that is, A paired off only with T, and C only with G—one side of the staircase was in effect a genetic mirror image of the other. Watson and Crick quickly recognized from the structure of their model how DNA worked. But their 900-word announcement in Nature, the international weekly published in Britain, concluded with one of the more coy statements in scientific literature. “It has not escaped our notice,” they said, “that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”

In a second letter, they described that mechanism: how the DNA molecule unwinds and unzips itself right down the middle during cell division, its base pairs breaking apart at their hydrogen bonds. Then by drawing on the free-floating material surrounding them in the nucleus of the cell, the two separated strands link up with complementary base-and-strand units along their entire length, forming two exact copies of the original double helix. Thus DNA faithfully passes its genetic information on to new cells and to future generations.

Ingenious as the theory was, scientists still demanded proof that the molecule actually replicated itself. That proof was quick to come. By 1956, Arthur Kornberg, then at Washington University in St. Louis, discovered an enzyme, or natural chemical catalyst (which he named “DNA polymerase”) that was apparently critical to some of the activities of the double helix. Once he obtained enough of the enzyme, he placed it in a test-tube brew with a bit of natural DNA, one of whose strands was incomplete, the four bases (A, T, C, G) and a few other off-the-shelf chemicals. True to his expectations—and the Watson-Crick theory—the incomplete segment picked up its complementary nucleotides from the brew to form a complete double helix.

Implicit in the Watson-Crick model were the workings of DNA’s other essential function: how it orders the production of proteins. These are also long and twisted helical molecules, but they are the actual building blocks rather than the genetic blueprints for living things. As such, proteins are immensely varied; there are many thousands of different kinds in the human body alone. The distinctive proteins that make up the cells of the eye, for example, differ from those of the kidneys or muscles. Despite their variety, however, all proteins are built from some of only 20 smaller and simpler molecules, called amino acids. How then, scientists asked themselves, did the isolated double helix, locked in the nucleus of the cell, direct the assembly of amino acids into protein in other parts of the cell?

Scientists suspected that DNA had a helper, a single-stranded chemical first cousin called ribonucleic acid (RNA). Most of the cell’s RNA is found in ribosomes. These are globular bodies in the material outside the cell’s nucleus that seem to be highly active centers of protein synthesis. But if this ribosomal RNA played a role in protein making, how did it obtain and execute the instructions from the master molecule DNA inside the nucleus?

In 1955, after wrestling with the question, Francis Crick postulated (and Harvard Biochemists Paul Zamecnik and Mahlon Hoagland confirmed) a second form of RNA, which was later found to carry specific amino acids floating in the cytoplasm to the ribosomes; this substance became known as transfer RNA. Then in the early 1960s, biologists discovered a third kind of RNA—shortly after its existence had been theorized by Jacques Monod and François Jacob of France’s Pasteur Institute. Called messenger RNA, it provided the missing piece in the molecular puzzle. It was formed on an uncoiled strip of DNA in the nucleus, imprinted with the particular “message” encoded in that portion—or gene—of the staircase, and then sent off with these instructions to the protein-making ribosomes.

Neat as it was, this scheme still left unanswered one more question: How could DNA or RNA choose from among 20 amino acids to produce complex proteins by using an informational system that had only four code letters—the four bases—at its disposal? An answer to this intriguing problem was suggested by Physicist George Gamow, who likened the four bases to the different suits in a deck of playing cards.

If the cards are dealt one at a time, disregarding the order of the cards within the suits, the player encounters only one of four possibilities on each draw (a heart, diamond, spade or club); clearly, if DNA’s code worked this way, there would not be enough choices to encode 20 amino acids. If the cards are dealt in pairs, the number of combinations increases to 16 (since each card may combine with its own kind or one of three other suits). But such a two-unit system also would be inadequate. So Gamow reasoned that DNA’s four bases had to be taken at least three at a time: this would yield 64 possible combinations (4 X 4 X 4), more than enough to code for the existing amino acids.

IN 1961, CRICK’S team at Cambridge proved Gamow’s ingenious “triplet” theory. They demonstrated that RNA formed from only one or two base units could not effect the manufacture of proteins. But when they added a third base unit, protein formation began immediately. It remained, however, for an unknown young biochemist named Marshall Nirenberg, at the National Institutes of Health, to crack the code itself. That same year Nirenberg had succeeded in building up short, synthetic strands of RNA out of only one type of base. Invariably, this artificial RNA induced the manufacture of chains of proteins consisting of only one type of amino acid, phenylalanine. The conclusion was inescapable: in the genetic code, Nirenberg’s triplet had to signify phenylalanine.

Using this clue as their Rosetta stone, Nirenberg and other researchers eventually found one or more three-letter code words, or codons, that could call up every single amino acid—plus other words that acted as punctuation, marking the start or completion of a message ordering the production of a protein. Even more remarkable, they learned that the code was universal: the same four letters, taken three at a time to form a single genetic word, code the same amino acids in all living things. Thus by the mid-1960s, scientists finally understood how DNA passes on genetic information with exquisite precision, and the way it orders up the fabrication of new cellular protein.

That process, shown in the accompanying color chart, was summarized by Crick in a series of rules that became known as the Central Dogma. Most scientists interpreted the key rule of that dogma to be that genetic information flowed in one direction: from DNA to RNA to protein. To the surprise of many molecular biologists, however, it has recently been shown that part of the process can sometimes be reversed. This finding, in the opinion of molecular biologists like Columbia’s Sol Spiegelman, may offer an important clue to the workings of cancer cells (see box, page 44).

DNA is as complex as the system it directs. Even after two decades of intensive study only about one-third of the genes have been mapped along the length of DNA in the chromosome of so elementary a creature as the digestive-tract bacterium Escherichia coli. The reason: just a teaspoon of E. coli DNA has information capacity approximately equal to that of a computer with a storage capacity of about 100 cu. mi.

MAN, FOR HIS PART, is even more generously endowed—with 1,000 times as much DNA as one E. coli in each of his reproductive cells. Even so, the cells of such relatively primitive animals as salamanders, lungfish and even certain one-celled algae contain far more DNA than man’s. Does this mean that such lowly beasts have a richer genetic capacity than man? The Carnegie Institution’s Roy Britten and David Kohne, after much painstaking investigation, may have found the answer to that embarrassing question. A few years ago they discovered that in the DNA of higher organisms many genes seem to be repeated. In calf cells, they calculated, up to 40% of the DNA consists of segments that are repeated as many as 100,000 times apiece. As a result of this work, some scientists are now convinced that in this seeming redundancy of genes, rather than in the total number, lies the secret of the genetic sophistication of higher organisms.

How would such genetic repetition help man? Some theorists suspect that the “spare” DNA plays a regulatory role, perhaps switching other genes on and off at just the right moment during the involved process of protein manufacturing. Harvard Biochemist Charles Thomas, however, supports a more radical idea. He thinks that the repeated segments are actually “slaves” of a “master” gene from which they have been copied. Working in tandem, explains Thomas, such “slaves” could produce proteins more quickly and efficiently—though, he admits, not necessarily in greater diversity.

Molecular biologists are also probing ever more deeply into the process of cell differentiation. It has long been known that the DNA in every body cell of an individual organism is identical; this DNA contains all the information necessary to construct the whole organism. Why then, in a human being, for example, is a liver cell so different from a hair cell, a heart cell so different from a skin cell? The answer, Jacob and Monod theorized in 1961, is that only a small percentage of the genes in any cell are giving instructions for the operation of that particular cell. The rest are “turned off” by protein repressers, which wrap themselves around long stretches of DNA and prevent them from transferring their coded information to messenger RNA.

A number of such repressers have since been found in bacteria. Scientists have also isolated enzymes that turn the genes back on. These inducers, as they are called, work by unlocking the repressers on the segment of DNA. But even in E. coli, such switching can become bafflingly difficult: the repressers and inducers, for example, require controlling enzymes of their own. These enzymes, in turn, apparently need the help of still other molecules, such as the recently discovered sigma, rho and psi factors, in recognizing the appropriate genes. In fact, it is because of the very complexity of these processes that leading molecular biologists like Crick find the questions arising from cell differentiation so fascinating. How in the human embryo, for instance, are certain genes switched on so that by the end of the first week after conception identical cells have begun to grow into cells with differing characteristics?

SO FAR THESE fundamental questions are largely unanswerable, although some clues have been uncovered. For one thing, it is thought that in higher, multicellular forms of life, repressers may be a special class of proteins called histones; these are not found in bacteria. When histones are removed, Rockefeller University’s Vincent Allfrey has found, RNA production soars by 400%, evidence that formerly repressed segments of DNA have become active. In addition, it has been learned that the cell membrane itself appears to play a crucial part in switching genes on and off. When a membrane is merely brushed by certain hormones—a large class of molecules that serve as intercellular messengers—the membrane will respond as though jolted by an electric probe. It will instantly send off a signal to the nucleus, triggering RNA production by the genes. That finding could eventually have medical application for diseases—like diabetes—resulting from vital genes that are inexplicably turned off.

Many more puzzles remain unsolved. Why are there small bits of DNA located outside the nucleus in energy-producing cell centers called mitochondria? Does this mean that there are other, unknown repositories of hereditary information? In spite of such questions and complications, the basic structure of DNA postulated by Crick and Watson 18 years ago has withstood the test of time remarkably well. More important, it has given man a profound new understanding of basic life processes—and the means to control and alter them.

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