This University of Florida colloquium presentation reviews the four seminal papers that established the double‑helix structure of DNA. It begins with Stokes and Wilson’s 1947 X‑ray diffraction work, which identified a helical repeat of 34 Å, a 3.4 Å internucleotide spacing, and suggested a universal helical form across species. The talk then details Rosalind Franklin and Raymond Gosling’s high‑resolution photographs of sodium DNA (structures A and B), highlighting the 34 Å pitch, 20 Å diameter, external phosphate backbone, and the 10‑base‑per‑turn geometry that directly informed Watson and Crick. The presentation proceeds to Watson and Crick’s 1953 model, describing antiparallel right‑handed helices, base‑pairing rules (A–T, G–C), hydrogen‑bond geometry, and the structural basis for complementarity. Finally, the speakers discuss the implications for DNA replication, proposing a mechanism of helix unwinding, hydrogen‑bond breakage, and template‑directed synthesis, and speculating on the role of a polypeptide (enzyme) in controlling coiling. Throughout, the narrative emphasizes how X‑ray diffraction patterns, Bessel‑function analysis, and measured layer‑line spacings converged to reveal the conserved double‑helix architecture of DNA.
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Thank you for watching Transformative Advances in Molecular Biology, a retrospective look at critical events in the history of the discipline. The presentations in the series were prepared by graduate students in a journal colloquium at the University of Florida supervised by Mark Settles and Kevin Folta.
For the second topic of this series, we will focus on the— and the 4 seminal papers that led to its elucidation, presented by Fodley and Brent. Molecular Structure of Deoxypentose Nucleic Acids by Stokes and Wilson, Provides experimental evidence for the polynucleotide chain being helical, which is the natural configuration. They also provide preliminary evidence that DNA structure is the same in all species, even though base ratios differ across species. Additionally, this research demonstrates that polynucleotide chains may assemble in crystalline, semi-crystalline, and paracrystalline forms. All forms with 2 distinct regions in X-ray diffraction photos. With one region being due to regular spacing of nucleotides on the chain, And the other due to longer spacings of chain configuration. If you look to the figure on the right of an X-ray diffraction photo of E. coli DNA, you'll notice an absence of reflection at the meridian, which suggests a helical structure. The strong 3.4 angstrom reflection is due to internucleotide repeat along the axis, and 34 angstrom layer lines are due to repeats of chain configuration. The illustration to the left demonstrates how X-ray diffraction photos of helical structures are interpreted. The intensity distribution in the diffraction pattern of a series of points equally spaced along a helix is given by the squares of Bessel functions. A uniform continuous helix gives a series of layer lines of spacing corresponding to the helix pitch. A straight line may be drawn through the innermost maxima of each Bessel function and the origin. The angle this line makes with the equator is roughly equal to the angle between an element of the helix and the helix axis. Layer line widths suggest intensities correspond more closely to a single helix, indicating that the dominant helix has a pitch of 34 angstroms. The angle of the helix indicates that its diameter is 20 angstroms. Considering these observations, 2 or 3 intertwined coaxial helices with 10 nucleotides per turn would be required to obtain a reasonable number of nucleotides per unit length of fiber. Additionally, the absence of reflection on or near the meridian is a direct consequence of helical structure, and the empty region near the equator is a result of radial distribution of nucleotide shape. Observations of structures in vivo indicate that the structure of DNA is likely conserved across species. These observations were that X-ray spectra from trout semen is determined by the helical form function. X-ray data from bacteriophage show the main features of paracrystalline sodium nucleate, and active deoxypentose nucleate has the same crystalline structure as sodium thymonucleate.
Much clearer evidence has been demonstrated by Rosalind Franklin and Gosling. The pictures show us the structure B of sodium DNA from calf thymus. There are 2 types of X-ray diagrams can be obtained from 2 different structures of sodium First, Structure A, which is crystalline and forms approximately at 75% humidity. Second, Structure B, which is less ordered and can be reasonably derived from Structure A when fibers take up more than 40% of their dry weight in water. Conversion of Structure A to Structure B is accompanied by a 30% increase in fiber length. 2 lines are drawn to show the double helix form on structure B. It is assumed that the structural units of structure B are relatively free from the influence of neighboring molecules due to protection by a sheath of water. If each unit in a long chain takes up its least energy configurations, then a helical structure is likely. Moreover, Franklin and Gosling mentioned several important cues in their X-ray diagram that stimulates the finding of Watson and Crick DNA helical model. First, the fiber axis period labeled with green line is 34 angstroms, and there is a strong reflection at 10th layer line about 3.4 angstroms. This suggests that there are 10 residues per turn of the helix. Second, radius measurement of several layer lines have a value of 10 angstroms. Which gives 20 angstrom diameter. Third, phosphorus atoms must lie on the outside of the helix, as the linear array of maxima are one of the diagram's strongest features. If there are 10 phosphorus atoms at 10 angstrom, the distance between each phosphorus atom is 7.1 angstrom, which corresponds to the expected distance In a fully extended molecule, sugar and base groups must face in the helical axis. Finally, a cylinder of radius 10 angstrom and height 34 angstrom would contain 32 nucleotides, and one repeating unit will contain 10 nucleotides on each of 2 or on each of 3 Coaxial molecule.
An accurate structural model for DNA was proposed by Watson and Crick in 1953. There had, however, been previous models. Pauling and Corey proposed a triple helix with phosphates near the fiber axis and bases on the outside of the structure. Fraser also proposed the triple helix. However, he accurately predicted that phosphates were on the outside with bases linked by hydrogen bonds near the axis. The structural model presented by Watson and Crick can be visualized in the illustration to the right. The leftmost figure depicts a basic chemical diagram of a single DNA chain. Nucleotides are linked 3' to 5' by sugar-phosphate bonds. The figure to the right shows 2 right-handed helical chains coiled around a common axis. The helix chains run antiparallel and are related by dyads perpendicular to the helix axis. The model also provides that Sugars are perpendicular to bases. Nucleotides are separated by 3.4 angstroms in the z direction. There is a 36-degree angle between adjacent bases. The structure repeats every 34 angstroms, or 10 residues. The helix has a diameter of 20 angstroms, and it is held together by hydrogen bonding of purine and pyrimidine bases. In Watson and Crick's base pairing model, a single base from one chain hydrogen bonds to a single base from the other chain. Due to conformational restrictions, one base must be a purine and the other must be a pyrimidine. Hydrogen bonds are made at purine position 1 to pyrimidine position 1, and purine position 6 to pyrimidine position 6. If bases exist in their most plausible tautomeric forms, then the only 2 pairs that can occur are adenine-thymine and guanine-cytosine. This is supported by the previous observation that the ratios of A to T and G to C are close to unity. According to this model, the sequence of one chain is complementary to the sequence of the other. However, the sequence of bases on a single chain is not restricted.
Considering a DNA model that is complementary, antiparallel, and not restricted in sequence of bases, Watson and Crick hypothesized that a long DNA molecule could have many permutations, and the sequence of bases code DNA is a pair of templates, one complementary to the other. Specific events must occur during replication. Based on their model, Watson and Crick identified the requirements of DNA self-replication in general. First, uncoiling of the helical chain structures, followed by breakage of hydrogen bonds. Then the chains are unwound and separate. In aqueous environment, free nucleotide hydrogen bonds to the template strand. It is important to note that each chain serves as a template to generate 2 pairs of identical chains. However, steric properties of the bases dictate that this method of replication is only possible if the resulting chain takes on the proper structure. They're also curious whether an enzyme is required for this process. The authors speculate that there are— there is room for a polypeptide chain to occupy the space between the pair of polynucleotide chains based on 2 observations. First, the 7.1 angstrom between phosphorus atoms is close to the repeat of a fully extended polypeptide. Second, the weakness of the second layer line of X-ray diagram is crudely compatible with this idea. The proposed functions of the polypeptide chains are actually to define the control of coiling and uncoiling of helical structures And how a single DNA strand can hold in helical conformation.
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