Double Helix

Double Helix

The term double helix refers to the structure of deoxyribonucleic acid (DNA), which consists primarily of two linear strands of building blocks, termed nucleotides, which are linked to each other in a defined pattern. The result is visually similar to a ladder; the rungs of the ladder are the linkages between the nucleotides. As well, the nature of the nucleotide linkage imparts a right-handed twist to the ladderlike sturcture, so that the final structure looks something like a sprial staircase.

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 so-called 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 double-helix molecular structure of deoxyribonucleic acid (DNA) was published in 1953 by 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. Prior to Watson and Crick’s discovery, it was known that DNA contained four kinds of nucleotides. A nucleotide contains a five-carbon sugar called deoxyribose, a phosphate group, and one of four nitrogen-containing bases: adenine (A), guanine (G), thy-mine (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 (2× 10⁹ meters). 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. 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 poly-peptide 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.