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.

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 . 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.

Double Helix

The double helix refers to the "spiral staircase" shape or structure of the deoxyribonucleic acid (DNA) molecule. DNA is the genetic material of all living organisms. Also described as a twisting ladder, this double helix model enabled scientists to finally account for both the similarities as well as the immense variety of life.

The discovery of the double helix molecular structure of DNA in 1953 by American biochemist James Dewey Watson and English biochemist Francis Harry Compton Crick was one of the major scientific events of the twentieth century and some would say in the history of the life sciences. Prior to this discovery, it was not understood how such a relatively simple nucleic acid as DNA could contain such a vast complex of hereditary material, and few, if any, believed that it did.

As a nucleic acid, DNA was known to be composed of only four different submolecules called nucleotides—adenine (A), guanine (G), thymine (T), and cytosine (C)—and Watson and Crick believed that if they could determine the structure of DNA, they could explain how DNA actually works. The major way of learning about the structure of a chemical is to crystallize it and x-ray the crystals. When x rays pass through the crystals, they bend or diffract and create a pattern that can be studied. Watson and Crick worked with English physicist Maurice H. F. Wilkins and his associate Rosalind Franklin, whose excellent x rays in

1951 provided important evidence that DNA had a spiral shape. Franklin's x rays showed that the DNA molecule was a double strand of twisted material which came to be called a double helix. (Helix is taken from the Greek word for spiral.)

Knowing this did not allow Watson and Crick to correctly describe the actual structure of a DNA molecule, and for some time they tried to build a model of what it might look like. Although, until they discovered that the four nucleotides always formed themselves into a definite pattern of pairs (A always pairs with T, and G always pairs with C), they were unable to make further progress. Once they knew that these "base pairs" were complementary (in other words they always paired up the same way), Watson and Crick designed and built a model in which the correctly paired bases were the "rungs" of a ladder that connected the two sides or "rails" of the ladder. These sides were then twisted in the shape of a compact spiral or coil. Watson and Crick also explained that the rungs on the ladder (called bases) were the coded instructions, and that the order of these four nucleotides (A,T,G,C) spelled out the instructions for all of the different characteristics of an organism.

In March 1953, the two scientists announced their discovery of the double helix structure of the DNA molecule, offering to science what was basically the explanation of the chemical basis of life itself. Unlike many discoveries in the life sciences, knowledge of the structure or shape (the double helix) of the DNA molecule was essential to explaining how it could carry all the information needed to make a living creature, as well as how it could make exact duplicates of itself. In 1962 Wilkins shared the Nobel Prize in Physiology or Medicine with Watson and Crick for their discovery. Rosalind Franklin would surely have been included as well, but she had died in 1958 and the prize is only given to living scientists.

[See alsoDNA ]

Double Helix

Described in 1953 by James Watson and Francis Crick, the double helix of DNA (deoxyribonucleic acid) is the cellular storehouse of genetic information. This biopolymer consists of a pair of complementary chains approximately 2.4 nanometers (9.5×10⁻⁸ inches) in diameter and composed of

deoxyribose sugar molecules linked to each other by phosphoric acid, connecting the number three carbon of one sugar to the number five carbon of another. Attached to each sugar is a heterocyclic base: adenine , guanine , cytosine , or thymine . Each turn of the helix contains about 10.4 nucleotides.

These chains are said to be complementary: Guanine on one chain always pairs with cytosine on the opposite strand of DNA by forming three hydrogen bonds , and adenine on one chain pairs with thymine on the complementary chain, held in position by two hydrogen bonds. The interchain bonding forms an attraction between the two DNA chains and stabilizes the double helix against the strong repulsive force of the phosphoric acid residues. The chains are said to be antiparallel; that is, the two chains are held closely together but run in opposite directions, with the 3′ end of one chain matching the 5′ end of the other chain.

DNA performs two important functions. It contains the genetic code that provides directions for replication (synthesis of new DNA), and thus serves as a storehouse of genetic information, allowing the physical characteristics of parent organisms to be passed on to offspring. DNA also acts as a storehouse of synthetic information. The individual genes can be turned on or off, allowing the information of each codon within a gene to be converted into information contained in messenger RNA (a process known as transcription ). This information can then be transcribed into protein having catalytic or structural properties.

CHARGAFF'S RULES

Erwin Chargaff found that the ratios of adenine to thymine and of guanine to cytosine were always 1:1, suggesting that these bases form pairs. The fact that the ratios are 1:1 is referred to as Chargaff's rules.

see also Base Pairing; Codon; DNA Replication; Hydrogen; Nucleic Acids; Watson, James Dewey.

Dan M. Sullivan

Bibliography

Boyer, Rodney F. (2002). Concepts in Biochemistry. New York: Wiley.

Devlin, Thomas M., ed. (2002). Textbook of Biochemistry: With Clinical Correlations, 5th edition. New York: Wiley-Liss.

McKee, Trudy, and McKee, James R. (2003). Biochemistry: The Molecular Basis of Life, 3rd edition. Boston: McGraw-Hill.

dou·ble he·lix • n. a pair of parallel helices intertwined about a common axis, esp. that in the structure of the DNA molecule.