1- What are the two most significant differences between the structure for DNA a
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Question
1- What are the two most significant differences between the structure for DNA as proposed by Watson and Crick and that proposed by Pauling and Corey?
2. What important structural function do the purine and pyrimidine bases play? Besides the constraints on hydrogen bonding, what other reason must a purine pair with a pyrimidine in the structure proposed by Watson and Crick? (Hint: Look at the crude diagram of DNA presented in the paper.)
equipment, and to Dr. G·E. R. Deacon and the is a residue on each chain every 3.4 A, in the ainee. captain and officers of R.R.S. Discovery II for their tion. We have assumed an angle of 36 between adjacent residues in the same chain, so that the Young F. B, Gerrand, ., and Jevous, W., Phi. Mag, 0,ueture repeats after 10 residues on each chain, that is, after 34 A. The distance of a phosphorus atom from the fibre axis is 10 A. As the phosphates are on Higgins, M. 8., Afon. Nat. Rey. 4atroSec., Geplge, apr. on Ars, w. S., Woods Hole Papers in Phys, Oceanog. Meteor 11 the outside, cations have easy access to them. Ekman, Y.w. Arkie. Mat Astron. PviE. (Shock m)2c1) (005).is rather The structure is an open one, and its water eontent to tilt so that the structure could MOLECULAR STRUCTURE OF The novel feature of the structure is the manner in which the two chains are held together by the to the fibre axis. They are joined together in pairs, a single base from one chain being hydrogen-bonded to a single base from the other chain, so that the two lie side by side with identical One of the pair must be a purine and the other a pyrimidine for bonding to ooour. The hydrogen bonds are made as follows: purine position A Structure for Deoxyribose Nucleic Acid wish to suggest a structure for the salt of deoxyribose nucleic acidD.N.A This structure has novel features which are of considerable proposed by Pauling and Coney. They kindly made their manuseript available tou in advance of If it is assumed that the bases only ocour in the model consists of three inter structure in the most plausible tautomerie forms twined chains, with the phosphates near the fibre (that is, with the keto rather than the enol on axis, and the bases on the outside. In our opinion, figurations) it is found that only specifie pairs of 1) We bolieve that the material which gives the (purine) with thymine ( X-ray diagrams is the salt, not the free acid. Without (purine) with cytosine (pyrimidine). the acidie hydrogen atoms it is not elear what fors Inother words, if an adenine forms one momber of would hold the structure together, especially as the a pair, on either chain, then on these negatively charged phosphates near the axis will tho other member must bo thymine; repel each other. (2) Some of the van der Waals guanine and eytosine. The sequence of bases on a single chain doos not appear to be restrioted in any structure has also been sug way. However, if only specifie pairs of bases can be gosted by Fraser (in the press). In his model the formed, it follows that if the sequence of bases on are on the outside and the bases on the one chain is given, then the sequence on the other linked together by hydrogen bonds. This chain is automatically determined. structure as described is rather ill-defined, and for t has been found experimentally that the ratio this reason we shall not cormment of the amounts of adenine to thymine, and the ratio of guanine to eytosine, are always very close to unity We wish to put forward a for deoxyribose nucleie acid. radically different structure for It is probably impossible to build this strueture the salt of deoxyribose nucleie with a ribose sugar in place of tho deoxyribose, as acid. This structure has two the extra oxygen atom would make too close a van helical chains each coiled round der Waals contact. the same axis (see diagram). WeThe previously have made the usual chemical assumptions, namely, that each chain consists of phosphate di- ester groups joining -D-deoxy- ribofuranose residues with 3,5' linkages. The two chains (but in the following not their bases) are related by a ribose nueleio acid are insufficient for a rigorous test of our structure. So far as we can tell, it is roughly compatible with the experimental data, but it must be regarded as unproved until it has been checked against more exact results. Some of these are given We of the details of the results presented there when we devised our structure, which rests mainly though not entirely on published experimental data and stereo- axis. Both chains follow right- the dyad the sequenoos of the I has not esçaped our notice that the specific atoms in the two chains run pairing wo have postulated mmediately suggests a in opposite direcons. Each possible copying mechanism for the No. 1 that is, ditions assumed in building it, together with a set the bases are on the inside of of oo-ordinates for the atoms, be published the helix and the phosphates on elsewhere. ale guespurely the outside. The configurationWe are much indebted to Dr. Jerry Donohue for ibboss syabolae the of the sugar and the atoms constant advice and criticism, especially on inter- s atomio distances. We have also been stimulated by a Inowlodge of the general nature of the unpublished experimental results and ideas of Dr. M. H. F Wilkins, Dr. R. E. Franklin and their co-workers at outal rods the pairs of "standard configuration.Explanation / Answer
According to Pauling and Core
1. the DNA is a triple helix
2. They said that the phosphate group in the DNA molecule is neutrally charged.
While as watson and Crick said that
1.DNA is a double helix and the base pairs are stacked in the middle like rungs on a ladder.
2. They also realised that the phosphate group is a negatively charged.
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