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Double Helix

The double helix refers to DNA's "spiral staircase" structure, consisting of two right-handed helical polynucleotide chains coiled around a central axis. Genes, which are specific regions of DNA, contain the instructions for synthesizing every protein. Because life cannot exist without proteins, the discovery of DNA's structure unveiled the secret of life: protein synthesis. In fact, the "central dogma" of molecular biology is that DNA is used to build ribonucleic acid (RNA), which is used to build proteins, which in turn play a role in building DNA and RNA.

The discovery of the double-helix molecular structure of deoxyribonucleic acid (DNA) in 1953, one of the major scientific events of the twentieth century, and some would say in the history of biology, marked the culmination of an intense search involving many scientists. But ultimately, credit for the discovery and the 1962 Nobel Prize in Physiology or Medicine went to James Dewey Watson (who was an American postdoctoral student from Indiana University at the time) and Francis Harry Compton Crick, a researcher at the Cavendish Laboratory in Cambridge University, England. Their work, conducted at Cavendish Laboratory, significantly impacted the emerging field of molecular biology.

Prior to Watson and Crick's discovery, it had long been known that DNA contained four kinds of nucleotides, which are the building blocks of nucleic acids, such as DNA and RNA. A nucleotide contains a five-carbon sugar called deoxyribose, a phosphate group, and one of four nitrogen-containing bases: adenine (A), guanine (G), thymine (T), and cytosine (C). Thymine and cytosine are smaller, single-ringed structures called pyrimidines; adenine and guanine are larger, double-ringed structures called purines. Watson and Crick drew upon this and other scientific knowledge in concluding that DNA's structure possessed two nucleotide strands twisted into a double helix, with bases arranged in pairs such as A T, T A, G C, C G. Along the entire length of DNA, the double-ringed adenine and guanine nucleotide bases were probably paired with the single-ringed thymine and cytosine bases. Using paper cutouts of the nucleotides, Watson and Crick shuffled and reshuffled combinations. Later, they used wires and metal to create their model of the twisting nucleotide strands that form the double-helix structure. According to Watson and Crick's model, the diameter of the double helix measures 2.0 nanometers (nm). Each turn of the helix is 3.4 nm long, with 10 bases in each chain making up a turn.

Before Watson and Crick's discovery, no one knew how hereditary material was duplicated prior to cell division. A model of the DNA double helix structure discovered by James Watson and Francis Crick. Paul Seheult; Eye Ubiquitious/Corbis. Reproduced by permission. Using their model, it is now understood that enzymes can cause a region of a DNA molecule to "unwind" one nucleotide strand from the other, exposing bases that are then available to become paired up with free nucleotides stockpiled in cells. A half-old, half-new DNA strand is created in a process that is called "semiconservative replication." When free nucleotides pair up with exposed bases, they follow a base-pairing rule which requires that A always pairs with T, and G always with C. This rule is constant in DNA for all living things, but the order in which one base follows another in a nucleotide strand differs from species to species. Thus, Watson and Crick's double-helix model accounts for both the sameness and the immense variety of life.

It is fair to say that Watson and Crick's discovery of the double helix would not have been possible without significant prior discoveries. In his 1968 book, The Double Helix, A Personal Account of the Discovery of the Structure of DNA, Watson wrote that the "race" to unveil the mystery of DNA was chiefly "a matter of five people:" Maurice Wilkins, Rosalind Franklin, Linus Pauling, Crick, and Watson. Wilkins, an Irish biophysicist who shared the 1962 Nobel Prize in Physiology or Medicine with Crick and Watson, extracted DNA gel fibers and analyzed them using x ray diffraction. The diffraction showed a helical molecular structure, and Crick and Watson used that information in constructing their double-helix model. Franklin, working in Wilkins' laboratory, between 1950 and 1953, produced improved x ray data using purified DNA samples, and through her work confirmed that each helix turn is 3.4 nm. Although her work suggested DNA might have a helix structure, she did not postulate a definite model. Pauling, an American chemist and twice Nobel laureate, in 1951 discovered the three-dimensional shape of the protein collagen. Pauling discovered that each collagen polypeptide or amino acid chain twists helically, and that the helical shape is held by hydrogen bonds. With Pauling's discovery, scientists worldwide began "racing" to discover the structure of other biological molecules, including the DNA molecule.

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