In both cases, only a small part of the molecule is shown. Shortly after the Journal of the Chemical Society publications [ 3 , 20 , 26 ], Creeth's PhD thesis appeared [ 4 ] in which, on page 85, he presented a diagram showing his two-chain model for the structure, interpreted from the viscosity, birefringence and titration results.
This model is shown in Figure 3a , alongside the correct Watson—Crick two-chain double-helix model [ 1 ] for comparison Figure 3b. A putative ball-and-stick model for the Creeth structure constructed by the authors is shown in Figure 4.
The Creeth structure from page 85 of his thesis, and shown here as Figure 3a , has two chains and is a long linear molecule with the sugar-phosphate backbone clearly and correctly on the outside of the molecule. The constituent chains are united down their common length by hydrogen bonding between facing amino and hydroxyl groups of opposite chain bases correctly on the inside of the molecule.
All these features are consistent with the correct Watson—Crick structure produced 5 years later. Creeth's model, although close, falls short on two main grounds:. Creeth also estimated inaccurately the number of hydrogen bonds to be a maximum of two per four phosphorus atoms. Also neither he, nor Gulland or Jordan knew of the equivalence of A with T, and C with G, or of the correct tautomeric keto forms, although Gulland was aware of the work of A.
The breaks in the chain were considered consistent with the reduction in viscosity on scission of the hydrogen bonds [ 4 ]:. The action of acids and alkalis on this model is to sever the hydrogen bonds uniting the individual polynucleotide chains which are thus liberated.
Being relatively small and flexible they do not interact to form the network characteristic of the micellar state, and the solution is not very viscous. We now include a retrospective analysis by ourselves of the original Creeth data. Evidence for long linear molecules due to non-Newtonian or shear thinning behaviour of solutions resulting from molecular alignment under shear was clearly seen from both the streaming birefringence and from the effects of hydrostatic pressure on measured relative viscosities: this is evident from Figure 2 similar profiles are given in Figure 3 in ref.
The non-Newtonian effects made it very difficult at the time for Gulland, Creeth or Jordan to comment further on the structure, but with more recent hydrodynamic theory see, for example, ref. Triangles: original solution at 2. Circles: original solution at 0. Squares: solution at 2. As a check we can use a dataset obtained at a lower concentration 0.
Finally, also from Figure 4 of ref. The MHKS relation is. For a rod-shaped molecule, the expected reduction in intrinsic viscosity corresponding to a halving of the molar mass as the two chains come apart would from eqn 3 lead to a maximum reduction of 3. Bearing in mind that the single-chain molecules will be considerably more flexible than their double-chained counterparts — and hence their viscosities will be lower to a certain degree, this plus a halving in molecular mass through hydrogen bond disruption would not be an implausible explanation of the results.
We stress that this is our modern calculation, not a review of Creeth's from his thesis. So, the two-chain hydrogen-bonded model for DNA given in Creeth's PhD thesis could, on the basis of our calculation, have accounted for the viscosity data available at the time and without the need for alternating breaks in the chains.
The second scenario — that the action of titrating with acid or alkali could also lead to a fission of intra-chain hydrogen bonds between bases along the same chain — is also consistent with the drop in viscosity and loss of birefringence, and Creeth repeats this in his thesis. Although the molecular mass would then remain the same, the removal of the bonds could encourage the chain to take a more flexible, less-extended conformation resulting in a lowering of viscosity and loss of birefringence.
Besides the breaks, also missing from the Creeth two-chain model is, of course, the double helix and the correct pairing of the bases. Gosling see ref. Florence Bell and W. Astbury did not have access to this high-quality DNA when they published their diffraction images in [ 17 , 18 ].
It is interesting to speculate that if the Nottingham team had made available their high-purity DNA, could the Leeds team have produced images of the same quality as Rosalind Franklin's.
In , Gulland and Astbury both attended the same meeting — a meeting of the Society of Experimental Biology in Cambridge, and both contributed to the Proceedings published a year later [ 23 , 34 , 39 ] — this may have been an opportunity lost. Later, the Leeds team were able to produce a high enough quality image: in , E. Beighton, a researcher in Astbury's laboratory, produced a clear diffraction pattern with the characteristic helical cross see ref. But, by then, it was too late as the Nottingham team had all but gone: Creeth had finished his PhD and had become a research Fellow at the Courtauld Institute in London before moving to the Department of Physical Chemistry in Wisconsin [ 41 ].
His supervisor Doj Jordan moved to take up an appointment at the University of Adelaide, and tragically Masson Gulland was killed in a train crash in October [ 6 ], not long after the publication of the hydrogen bond finding. The Nottingham team also would not have had access to the findings of Chargaff et al. Davidson's classic text [ 42 ] used by Watson. As famously recalled by J. Watson in his book [ 8 ], he and Crick had been struggling to get the T—A and C—G base pairs to fit into the helical structure.
Donohue, a Guggenheim Fellow sharing an office with them at the Cavendish — and an expert on tautometric forms — was able to point out to Watson that Davidson was wrong, and the T and G bases were primarily in the keto form —CO—NH— at physiological pH's [ 8 ]. This enabled Watson and Crick to complete their model, with the bases hydrogen bonded in their correct tautomeric forms Figure 6 , allowing a regular stacking of bases within the antiparallel double helical frame of the sugar-phosphate backbone and at the correct spacing.
The precise hydrogen bond link between the bases made by Watson and Crick followed from the Chargraff base-pair rules and from J. O'Donohue's identification of the correct keto tautomeric form. Adapted with permission from Booth and Hey [ 6 ]. Copyright American Chemical Society. To the modern-day molecular biologist, it is not hard to recognise that the diagram drawn by Creeth [ 4 ] in his PhD thesis Figure 3a bears similarities to sketches that might be drawn today for the design of PCR polymerase chain reaction experiments using multiple annealing primers.
It also resembles current textbook diagrams depicting mechanisms by which some viruses integrate into host chromosomes during their replication cycles using staggered cuts in the duplex and the resulting production of short single-stranded segments e. Of course, in , the processes of DNA replication or indeed PCR amplification were not yet discovered; the correct semi-conservative replication model of Watson and Crick was not proposed until [ 45 ] and not confirmed until using analytical ultracentrifugation [ 46 ].
Nor was it yet known in what the mechanism of heredity was. So, although Creeth's model is a depiction of DNA structure alone, he could not have realised how it resembles what we now understand about steps in some replicative mechanisms for the molecule of life that he was working on. Top left: J. Gulland, from a photograph taken at the Symposia Nucleic Acids and Nucleoproteins.
Top right: D. Bottom: J. Creeth, photograph taken ca. We thank Cochranes Ltd. We thank Professor James D. These replication forks are the actual site of DNA copying.
Helix destabilizing proteins bind to the single-stranded regions so the two strands do not rejoin. Enzymes called topoisimerases produce breaks in the DNA and then rejoin them in order to relieve the stress in the helical molecule during replication.
As the strands continue to unwind and separate in both directions around the entire DNA molecule, the hydrogen bonding of free DNA nucleotides with those on each parent strand produce new complementary strands. Supporting Information. Cited By. This article is cited by 27 publications. Stanley T. Crooke, Punit P. Seth, Timothy A. Vickers, Xue-hai Liang. Journal of the American Chemical Society , 35 , Seth, Stanley T.
Journal of the American Chemical Society , 21 , Lacroix, Yong Liu, Steven H. Inorganic Chemistry , 58 21 , Muriph, Wei Zhang. The Journal of Organic Chemistry , 84 9 , ACS Catalysis , 7 11 , Accounts of Chemical Research , 50 9 , Brad Wan and Punit P. The Medicinal Chemistry of Therapeutic Oligonucleotides. Journal of Medicinal Chemistry , 59 21 , The Journal of Organic Chemistry , 81 8 , Chemical Reviews , 2 , Guanine has 1 H-bond acceptor and 2 H-bond donors. Can you find one H-bond donor and 2 H-bond acceptors in cytosine?
Examine the molecule yourself, then click the button below to see the relevant atoms blink green. Guanine and cytosine make up a nitrogenous base pair because their available hydrogen bond donors and hydrogen bond acceptors pair with each other in space.
Guanine and cytosine are said to be complementary to each other. This is shown in the image below, with hydrogen bonds illustrated by dotted lines. The button below the image highlights the hydrogen bonds between guanine and cytosine in a DNA double helix. Adenine and thymine similarly pair via hydrogen bond donors and acceptors; however an AT base pair has only two hydrogen bonds between the bases.
Examine the image and click the button below to explore hydrogen bonding in an AT base pair. Hydrogen bonds are weak, noncovalent interactions, but the large number of hydrogen bonds between complementary base pairs in a DNA double helix combine to provide great stability for the structure. The same complementary base pairing discussed here is important for RNA secondary structure, transcription, and translation.
In these applications, uracil takes the place of thymine, forming a complementary A-U base pair. Examine the structure of uracil in the image above, and compare it to thymine. How many hydrogen bonds will form in an A-U base pair? Nitrogenous Bases Chemical structures of the five nitrogenous bases are shown below.
Adenine Guanine Cytosine Thymine Uracil.
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