Last 10 Commits for turing.togger.com

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commit 48e965f494038b662fd777164cf9e86382302c7a
Author: ffff:173.198.69.239 <ffff:173.198.69.239@hub.scroll.pub> Date: Mon Nov 4 17:29:01 2024 +0000 Updated index.scroll diff --git a/index.scroll b/index.scroll index 63d680f..7adf314 100644 --- a/index.scroll +++ b/index.scroll @@ -14,6 +14,10 @@ printTitle printAuthors printDate +center +Source + https://www.cs.virginia.edu/~robins/Turing_Paper_1936.pdf + The "computable" numbers may be described briefly as the real numbers whose expressions as a decimal are calculable by finite means. Although the subject of this paper is ostensibly the computable numbers. ------------------------------------------------------------
commit 536fb2a316957429ae5119102a79f193df0e7a5d
Author: ffff:173.198.69.239 <ffff:173.198.69.239@hub.scroll.pub> Date: Mon Nov 4 17:28:18 2024 +0000 Updated index.scroll diff --git a/index.scroll b/index.scroll index 72be223..63d680f 100644 --- a/index.scroll +++ b/index.scroll @@ -9,7 +9,7 @@ metaTags theme gazette editButton /edit.html -container +container 500px printTitle printAuthors printDate ------------------------------------------------------------
commit 769a1aead2e8af9690b9b976956299fb53dae549
Author: ffff:173.198.69.239 <ffff:173.198.69.239@hub.scroll.pub> Date: Mon Nov 4 17:28:05 2024 +0000 Updated index.scroll diff --git a/index.scroll b/index.scroll index e762938..72be223 100644 --- a/index.scroll +++ b/index.scroll @@ -3,15 +3,14 @@ buildHtml date 5/28/1936 // [Received 28 May, 1936.—Read 12 November, 1936.] -container title ON COMPUTABLE NUMBERS, WITH AN APPLICATION TO THE ENTSCHEIDUNGSPROBLEM metaTags theme gazette editButton /edit.html +container printTitle - printAuthors printDate ------------------------------------------------------------
commit 2ffcc664b568b293a6ca910d0becffb49a39fc05
Author: ffff:173.198.69.239 <ffff:173.198.69.239@hub.scroll.pub> Date: Mon Nov 4 17:27:59 2024 +0000 Updated index.scroll diff --git a/index.scroll b/index.scroll index 55dd564..e762938 100644 --- a/index.scroll +++ b/index.scroll @@ -15,8 +15,6 @@ printTitle printAuthors printDate -container - The "computable" numbers may be described briefly as the real numbers whose expressions as a decimal are calculable by finite means. Although the subject of this paper is ostensibly the computable numbers. ------------------------------------------------------------
commit 010d4cbcc0401272012f5c37e9e11883a548b917
Author: ffff:173.198.69.239 <ffff:173.198.69.239@hub.scroll.pub> Date: Mon Nov 4 17:27:53 2024 +0000 Updated index.scroll diff --git a/index.scroll b/index.scroll index 2ba3d70..55dd564 100644 --- a/index.scroll +++ b/index.scroll @@ -2,6 +2,8 @@ authors A. M. TURING buildHtml date 5/28/1936 // [Received 28 May, 1936.—Read 12 November, 1936.] + +container title ON COMPUTABLE NUMBERS, WITH AN APPLICATION TO THE ENTSCHEIDUNGSPROBLEM metaTags ------------------------------------------------------------
commit de956a4a17f88c771fc90b958be7c14d1fbe6a7d
Author: ffff:173.198.69.239 <ffff:173.198.69.239@hub.scroll.pub> Date: Mon Nov 4 17:27:41 2024 +0000 Updated index.scroll diff --git a/index.scroll b/index.scroll index 975a13e..2ba3d70 100644 --- a/index.scroll +++ b/index.scroll @@ -6,7 +6,7 @@ title ON COMPUTABLE NUMBERS, WITH AN APPLICATION TO THE ENTSCHEIDUNGSPROBLEM metaTags theme gazette -viewSourceButton +editButton /edit.html printTitle ------------------------------------------------------------
commit fd6281aa91fcc9b58f86bf6e2a8e6c144daca4b5
Author: ffff:173.198.69.239 <ffff:173.198.69.239@hub.scroll.pub> Date: Mon Nov 4 17:27:33 2024 +0000 Updated index.scroll diff --git a/index.scroll b/index.scroll index 1d2da6b..975a13e 100644 --- a/index.scroll +++ b/index.scroll @@ -1,7 +1,8 @@ -authors J. D. Watson and F. H. C. Crick +authors A. M. TURING buildHtml -date 4/2/1953 -title MOLECULAR STRUCTURE OF NUCLEIC ACIDS +date 5/28/1936 +// [Received 28 May, 1936.—Read 12 November, 1936.] +title ON COMPUTABLE NUMBERS, WITH AN APPLICATION TO THE ENTSCHEIDUNGSPROBLEM metaTags theme gazette ------------------------------------------------------------
commit 4f7eda03a827d92734dfada72523b4f915004d79
Author: ffff:173.198.69.239 <ffff:173.198.69.239@hub.scroll.pub> Date: Mon Nov 4 17:24:43 2024 +0000 Updated index.scroll diff --git a/index.scroll b/index.scroll index 50faea7..1d2da6b 100644 --- a/index.scroll +++ b/index.scroll @@ -9,61 +9,20 @@ viewSourceButton printTitle -# A Structure for Deoxyribose Nucleic Acid - printAuthors printDate -thinColumns 2 - -We wish to suggest a structure for the salt of deoxyribose nucleic acid (D.N.A.). This structure has novel features which are of considerable biological interest. - class dropcap - -A structure for nucleic acid has already been proposed by Pauling and Corey^pauling. They kindly made their manuscript available to us in advance of publication. Their model consists of three intertwined chains, with the phosphates near the fibre axis, and the bases on the outside. In our opinion, this structure is unsatisfactory for two reasons: (1) We believe that the material which gives the X-ray diagrams is the salt, not the free acid. Without the acidic hydrogen atoms it is not clear what forces would hold the structure together, especially as the negatively charged phosphates near the axis will repel each other. (2) Some of the van der Waals distances appear to be too small. - -Another three-chain structure has also been suggested by Fraser (in the press). In his model the phosphates are on the outside and the bases on the inside, linked together by hydrogen bonds. This structure as described is rather ill-defined, and for this reason we shall not comment on it. - -image https://breckyunits.com/dna.png - float left - width 150 - caption This figure is purely diagrammatic . The two ribbons symbolize the two phophate-sugar chains, and the horizonal rods the pairs of bases holding the chains together. The vertical line marks the fibre axis. - -We wish to put forward a radically different structure for the salt of deoxyribose nucleic acid. This structure has two helical chains each coiled round the same axis (see diagram). We have made the usual chemical assumptions, namely, that each chain consists of phosphate diester groups joining beta-D-deoxyribofuranose residues with 3',5' linkages. The two chains (but not their bases) are related by a dyad perpendicular to the fibre axis. Both chains follow right-handed helices, but owing to the dyad the sequences of the atoms in the two chains run in opposite directions. Each chain loosely resembles Furberg's ^furberg model No. 1 ; that is, the bases are on the inside of the helix and the phosphates on the outside. The configuration of the sugar and the atoms near it is close to Furberg's "standard configuration," the sugar being roughly perpendicular to the attached base. There is a residue on each every 3.4 A. in the z -direction. We have assumed an angle of 36° between adjacent residues in the same chain, so that the structure 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 the outside, cations have easy access to them. - -The structure is an open one, and its water content is rather high. At lower water contents we would expect the bases to tilt so that the structure could become more compact. - -The novel feature of the structure is the manner in which the two chains are held together by the purine and pyrimidine bases. The planes of the bases are perpendicular to the fibre axis. They are joined together in pairs, a single base from one chain being hydroden-bonded to a single base from the other chain, so that the two lie side by side with identical z -coordinates. One of the pair must be a purine and the other a pyrimidine for bonding to occur. The hydrogen bonds are made as follows: purine position 1 to pyrimidine position 1; purine position 6 to pyrimidine position 6. - -If it is assumed that the bases only occur in the structure in the most plausible tautomeric forms (that is, with the keto rather than the enol configurations) it is found that only specific pairs of bases can bond together. These pairs are: adenine (purine) with thymine (pyrimidine), and guanine (purine) with cytosine (pyrimidine). - -In other words, if an adenine forms one member of a pair, on either chain, then on these assumptions the other member must be thymine; similarly for guanine and cytosine. The sequence of bases on a single chain does not appear to be restricted in any way. However, if only specific pairs of bases can be formed, it follows that if the sequence of bases on one chain is given, then the sequence on the other chain is automatically determined. - -It has been found experimentally ^chargaff^wyatt that the ratio of the amounts of adenine to thymine, and the ratio of guanine to cytosine, are always very close to unity for deoxyribose nucleic acid. - -It is probably impossible to build this structure with a ribose sugar in place of the deoxyribose, as the extra oxygen atom would make too close a van der Waals contact. - -The previously published X-ray data ^astbury^wilkins on deoxyribose nucleic 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 in the following communications. We were not aware of the details of the results presented there when we devised our structure, which rests mainly though not entirely on published experimental data and stereochemical arguments. - -It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material. - -Full details of the structure, including the conditions assumed in building it, together with a set of coordinates for the atoms, will be published elsewhere. - -We are much indebted to Dr. Jerry Donohue for constant advice and criticism, especially on interatomic distances. We have also been stimulated by a knowledge 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 King’s College, London. One of us (J. D. W.) has been aided by a fellowship from the National Foundation for Infantile Paralysis. - -**** - -- J.D. Watson -- F. H. C. Crick - -*** - -Medical Research Council Unit for the Study of the Molecular Structure of Biological Systems, Cavendish Laboratory, Cambridge. - -*** - -^pauling Pauling, L., and Corey, R. B., Nature, 171, 346 (1953); Proc. U.S. Nat. Acad. Sci., 39, 84 (1953). -^furberg Furberg, S., Acta Chem. Scand., 6, 634 (1952). -^chargaff Chargaff, E., for references see Zamenhof, S., Brawerman, G., and Chargaff, E., Biochim. et Biophys. Acta, 9, 402 (1952). -^wyatt Wyatt, G. R., J. Gen. Physiol. , 36, 201 (1952). -^astbury Astbury, W. T., Symp. Soc. Exp. Biol. 1, Nucleic Acid, 66 (Camb. Univ. Press, 1947). -^wilkins Wilkins, M. H. F., and Randall, J. T., Biochim. et Biophys. Acta, 10, 192 (1953). +container + +The "computable" numbers may be described briefly as the real +numbers whose expressions as a decimal are calculable by finite means. +Although the subject of this paper is ostensibly the computable numbers. +it is almost equally easy to define and investigate computable functions +of an integral variable or a real or computable variable, computable +predicates, and so forth. The fundamental problems involved are, +however, the same in each case, and I have chosen the computable numbers +for explicit treatment as involving the least cumbrous technique. I hope +shortly to give an account of the relations of the computable numbers, +functions, and so forth to one another. This will include a development +of the theory of functions of a real variable expressed in terms of computable numbers. According to my definition, a number is computable +if its decimal can be written down by a machine. \ No newline at end of file ------------------------------------------------------------
commit 82446dacc8eca809ce00248a5c280af68483e8f4
Author: ffff:64.180.84.41 <ffff:64.180.84.41@hub.scroll.pub> Date: Mon Oct 7 16:32:03 2024 +0000 Updated index.scroll diff --git a/index.scroll b/index.scroll index 09c7461..50faea7 100644 --- a/index.scroll +++ b/index.scroll @@ -1,69 +1,69 @@ -authors J. D. Watson and F. H. C. Crick -buildHtml -date 4/2/1953 -title MOLECULAR STRUCTURE OF NUCLEIC ACIDS - -metaTags -theme gazette -viewSourceButton - -printTitle - -# A Structure for Deoxyribose Nucleic Acid - -printAuthors -printDate - -thinColumns 2 - -We wish to suggest a structure for the salt of deoxyribose nucleic acid (D.N.A.). This structure has novel features which are of considerable biological interest. - class dropcap - -A structure for nucleic acid has already been proposed by Pauling and Corey^pauling. They kindly made their manuscript available to us in advance of publication. Their model consists of three intertwined chains, with the phosphates near the fibre axis, and the bases on the outside. In our opinion, this structure is unsatisfactory for two reasons: (1) We believe that the material which gives the X-ray diagrams is the salt, not the free acid. Without the acidic hydrogen atoms it is not clear what forces would hold the structure together, especially as the negatively charged phosphates near the axis will repel each other. (2) Some of the van der Waals distances appear to be too small. - -Another three-chain structure has also been suggested by Fraser (in the press). In his model the phosphates are on the outside and the bases on the inside, linked together by hydrogen bonds. This structure as described is rather ill-defined, and for this reason we shall not comment on it. - -image https://breckyunits.com/dna.png - float left - width 150 - caption This figure is purely diagrammatic . The two ribbons symbolize the two phophate-sugar chains, and the horizonal rods the pairs of bases holding the chains together. The vertical line marks the fibre axis. - -We wish to put forward a radically different structure for the salt of deoxyribose nucleic acid. This structure has two helical chains each coiled round the same axis (see diagram). We have made the usual chemical assumptions, namely, that each chain consists of phosphate diester groups joining beta-D-deoxyribofuranose residues with 3',5' linkages. The two chains (but not their bases) are related by a dyad perpendicular to the fibre axis. Both chains follow right-handed helices, but owing to the dyad the sequences of the atoms in the two chains run in opposite directions. Each chain loosely resembles Furberg's ^furberg model No. 1 ; that is, the bases are on the inside of the helix and the phosphates on the outside. The configuration of the sugar and the atoms near it is close to Furberg's "standard configuration," the sugar being roughly perpendicular to the attached base. There is a residue on each every 3.4 A. in the z -direction. We have assumed an angle of 36° between adjacent residues in the same chain, so that the structure 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 the outside, cations have easy access to them. - -The structure is an open one, and its water content is rather high. At lower water contents we would expect the bases to tilt so that the structure could become more compact. - -The novel feature of the structure is the manner in which the two chains are held together by the purine and pyrimidine bases. The planes of the bases are perpendicular to the fibre axis. They are joined together in pairs, a single base from one chain being hydroden-bonded to a single base from the other chain, so that the two lie side by side with identical z -coordinates. One of the pair must be a purine and the other a pyrimidine for bonding to occur. The hydrogen bonds are made as follows: purine position 1 to pyrimidine position 1; purine position 6 to pyrimidine position 6. - -If it is assumed that the bases only occur in the structure in the most plausible tautomeric forms (that is, with the keto rather than the enol configurations) it is found that only specific pairs of bases can bond together. These pairs are: adenine (purine) with thymine (pyrimidine), and guanine (purine) with cytosine (pyrimidine). - -In other words, if an adenine forms one member of a pair, on either chain, then on these assumptions the other member must be thymine; similarly for guanine and cytosine. The sequence of bases on a single chain does not appear to be restricted in any way. However, if only specific pairs of bases can be formed, it follows that if the sequence of bases on one chain is given, then the sequence on the other chain is automatically determined. - -It has been found experimentally ^chargaff^wyatt that the ratio of the amounts of adenine to thymine, and the ratio of guanine to cytosine, are always very close to unity for deoxyribose nucleic acid. - -It is probably impossible to build this structure with a ribose sugar in place of the deoxyribose, as the extra oxygen atom would make too close a van der Waals contact. - -The previously published X-ray data ^astbury^wilkins on deoxyribose nucleic 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 in the following communications. We were not aware of the details of the results presented there when we devised our structure, which rests mainly though not entirely on published experimental data and stereochemical arguments. - -It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material. - -Full details of the structure, including the conditions assumed in building it, together with a set of coordinates for the atoms, will be published elsewhere. - -We are much indebted to Dr. Jerry Donohue for constant advice and criticism, especially on interatomic distances. We have also been stimulated by a knowledge 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 King’s College, London. One of us (J. D. W.) has been aided by a fellowship from the National Foundation for Infantile Paralysis. - -**** - -- J.D. Watson -- F. H. C. Crick - -*** - -Medical Research Council Unit for the Study of the Molecular Structure of Biological Systems, Cavendish Laboratory, Cambridge. - -*** - -^pauling Pauling, L., and Corey, R. B., Nature, 171, 346 (1953); Proc. U.S. Nat. Acad. Sci., 39, 84 (1953). -^furberg Furberg, S., Acta Chem. Scand., 6, 634 (1952). -^chargaff Chargaff, E., for references see Zamenhof, S., Brawerman, G., and Chargaff, E., Biochim. et Biophys. Acta, 9, 402 (1952). -^wyatt Wyatt, G. R., J. Gen. Physiol. , 36, 201 (1952). -^astbury Astbury, W. T., Symp. Soc. Exp. Biol. 1, Nucleic Acid, 66 (Camb. Univ. Press, 1947). -^wilkins Wilkins, M. H. F., and Randall, J. T., Biochim. et Biophys. Acta, 10, 192 (1953). +authors J. D. Watson and F. H. C. Crick +buildHtml +date 4/2/1953 +title MOLECULAR STRUCTURE OF NUCLEIC ACIDS + +metaTags +theme gazette +viewSourceButton + +printTitle + +# A Structure for Deoxyribose Nucleic Acid + +printAuthors +printDate + +thinColumns 2 + +We wish to suggest a structure for the salt of deoxyribose nucleic acid (D.N.A.). This structure has novel features which are of considerable biological interest. + class dropcap + +A structure for nucleic acid has already been proposed by Pauling and Corey^pauling. They kindly made their manuscript available to us in advance of publication. Their model consists of three intertwined chains, with the phosphates near the fibre axis, and the bases on the outside. In our opinion, this structure is unsatisfactory for two reasons: (1) We believe that the material which gives the X-ray diagrams is the salt, not the free acid. Without the acidic hydrogen atoms it is not clear what forces would hold the structure together, especially as the negatively charged phosphates near the axis will repel each other. (2) Some of the van der Waals distances appear to be too small. + +Another three-chain structure has also been suggested by Fraser (in the press). In his model the phosphates are on the outside and the bases on the inside, linked together by hydrogen bonds. This structure as described is rather ill-defined, and for this reason we shall not comment on it. + +image https://breckyunits.com/dna.png + float left + width 150 + caption This figure is purely diagrammatic . The two ribbons symbolize the two phophate-sugar chains, and the horizonal rods the pairs of bases holding the chains together. The vertical line marks the fibre axis. + +We wish to put forward a radically different structure for the salt of deoxyribose nucleic acid. This structure has two helical chains each coiled round the same axis (see diagram). We have made the usual chemical assumptions, namely, that each chain consists of phosphate diester groups joining beta-D-deoxyribofuranose residues with 3',5' linkages. The two chains (but not their bases) are related by a dyad perpendicular to the fibre axis. Both chains follow right-handed helices, but owing to the dyad the sequences of the atoms in the two chains run in opposite directions. Each chain loosely resembles Furberg's ^furberg model No. 1 ; that is, the bases are on the inside of the helix and the phosphates on the outside. The configuration of the sugar and the atoms near it is close to Furberg's "standard configuration," the sugar being roughly perpendicular to the attached base. There is a residue on each every 3.4 A. in the z -direction. We have assumed an angle of 36° between adjacent residues in the same chain, so that the structure 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 the outside, cations have easy access to them. + +The structure is an open one, and its water content is rather high. At lower water contents we would expect the bases to tilt so that the structure could become more compact. + +The novel feature of the structure is the manner in which the two chains are held together by the purine and pyrimidine bases. The planes of the bases are perpendicular to the fibre axis. They are joined together in pairs, a single base from one chain being hydroden-bonded to a single base from the other chain, so that the two lie side by side with identical z -coordinates. One of the pair must be a purine and the other a pyrimidine for bonding to occur. The hydrogen bonds are made as follows: purine position 1 to pyrimidine position 1; purine position 6 to pyrimidine position 6. + +If it is assumed that the bases only occur in the structure in the most plausible tautomeric forms (that is, with the keto rather than the enol configurations) it is found that only specific pairs of bases can bond together. These pairs are: adenine (purine) with thymine (pyrimidine), and guanine (purine) with cytosine (pyrimidine). + +In other words, if an adenine forms one member of a pair, on either chain, then on these assumptions the other member must be thymine; similarly for guanine and cytosine. The sequence of bases on a single chain does not appear to be restricted in any way. However, if only specific pairs of bases can be formed, it follows that if the sequence of bases on one chain is given, then the sequence on the other chain is automatically determined. + +It has been found experimentally ^chargaff^wyatt that the ratio of the amounts of adenine to thymine, and the ratio of guanine to cytosine, are always very close to unity for deoxyribose nucleic acid. + +It is probably impossible to build this structure with a ribose sugar in place of the deoxyribose, as the extra oxygen atom would make too close a van der Waals contact. + +The previously published X-ray data ^astbury^wilkins on deoxyribose nucleic 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 in the following communications. We were not aware of the details of the results presented there when we devised our structure, which rests mainly though not entirely on published experimental data and stereochemical arguments. + +It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material. + +Full details of the structure, including the conditions assumed in building it, together with a set of coordinates for the atoms, will be published elsewhere. + +We are much indebted to Dr. Jerry Donohue for constant advice and criticism, especially on interatomic distances. We have also been stimulated by a knowledge 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 King’s College, London. One of us (J. D. W.) has been aided by a fellowship from the National Foundation for Infantile Paralysis. + +**** + +- J.D. Watson +- F. H. C. Crick + +*** + +Medical Research Council Unit for the Study of the Molecular Structure of Biological Systems, Cavendish Laboratory, Cambridge. + +*** + +^pauling Pauling, L., and Corey, R. B., Nature, 171, 346 (1953); Proc. U.S. Nat. Acad. Sci., 39, 84 (1953). +^furberg Furberg, S., Acta Chem. Scand., 6, 634 (1952). +^chargaff Chargaff, E., for references see Zamenhof, S., Brawerman, G., and Chargaff, E., Biochim. et Biophys. Acta, 9, 402 (1952). +^wyatt Wyatt, G. R., J. Gen. Physiol. , 36, 201 (1952). +^astbury Astbury, W. T., Symp. Soc. Exp. Biol. 1, Nucleic Acid, 66 (Camb. Univ. Press, 1947). +^wilkins Wilkins, M. H. F., and Randall, J. T., Biochim. et Biophys. Acta, 10, 192 (1953). ------------------------------------------------------------
commit 233d4262c956432fff70fdc642b62862b44e1af6
Author: root <root@hub.scroll.pub> Date: Mon Oct 7 14:32:40 2024 +0000 Initial commit from paper template diff --git a/index.scroll b/index.scroll new file mode 100644 index 0000000..09c7461 --- /dev/null +++ b/index.scroll @@ -0,0 +1,69 @@ +authors J. D. Watson and F. H. C. Crick +buildHtml +date 4/2/1953 +title MOLECULAR STRUCTURE OF NUCLEIC ACIDS + +metaTags +theme gazette +viewSourceButton + +printTitle + +# A Structure for Deoxyribose Nucleic Acid + +printAuthors +printDate + +thinColumns 2 + +We wish to suggest a structure for the salt of deoxyribose nucleic acid (D.N.A.). This structure has novel features which are of considerable biological interest. + class dropcap + +A structure for nucleic acid has already been proposed by Pauling and Corey^pauling. They kindly made their manuscript available to us in advance of publication. Their model consists of three intertwined chains, with the phosphates near the fibre axis, and the bases on the outside. In our opinion, this structure is unsatisfactory for two reasons: (1) We believe that the material which gives the X-ray diagrams is the salt, not the free acid. Without the acidic hydrogen atoms it is not clear what forces would hold the structure together, especially as the negatively charged phosphates near the axis will repel each other. (2) Some of the van der Waals distances appear to be too small. + +Another three-chain structure has also been suggested by Fraser (in the press). In his model the phosphates are on the outside and the bases on the inside, linked together by hydrogen bonds. This structure as described is rather ill-defined, and for this reason we shall not comment on it. + +image https://breckyunits.com/dna.png + float left + width 150 + caption This figure is purely diagrammatic . The two ribbons symbolize the two phophate-sugar chains, and the horizonal rods the pairs of bases holding the chains together. The vertical line marks the fibre axis. + +We wish to put forward a radically different structure for the salt of deoxyribose nucleic acid. This structure has two helical chains each coiled round the same axis (see diagram). We have made the usual chemical assumptions, namely, that each chain consists of phosphate diester groups joining beta-D-deoxyribofuranose residues with 3',5' linkages. The two chains (but not their bases) are related by a dyad perpendicular to the fibre axis. Both chains follow right-handed helices, but owing to the dyad the sequences of the atoms in the two chains run in opposite directions. Each chain loosely resembles Furberg's ^furberg model No. 1 ; that is, the bases are on the inside of the helix and the phosphates on the outside. The configuration of the sugar and the atoms near it is close to Furberg's "standard configuration," the sugar being roughly perpendicular to the attached base. There is a residue on each every 3.4 A. in the z -direction. We have assumed an angle of 36° between adjacent residues in the same chain, so that the structure 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 the outside, cations have easy access to them. + +The structure is an open one, and its water content is rather high. At lower water contents we would expect the bases to tilt so that the structure could become more compact. + +The novel feature of the structure is the manner in which the two chains are held together by the purine and pyrimidine bases. The planes of the bases are perpendicular to the fibre axis. They are joined together in pairs, a single base from one chain being hydroden-bonded to a single base from the other chain, so that the two lie side by side with identical z -coordinates. One of the pair must be a purine and the other a pyrimidine for bonding to occur. The hydrogen bonds are made as follows: purine position 1 to pyrimidine position 1; purine position 6 to pyrimidine position 6. + +If it is assumed that the bases only occur in the structure in the most plausible tautomeric forms (that is, with the keto rather than the enol configurations) it is found that only specific pairs of bases can bond together. These pairs are: adenine (purine) with thymine (pyrimidine), and guanine (purine) with cytosine (pyrimidine). + +In other words, if an adenine forms one member of a pair, on either chain, then on these assumptions the other member must be thymine; similarly for guanine and cytosine. The sequence of bases on a single chain does not appear to be restricted in any way. However, if only specific pairs of bases can be formed, it follows that if the sequence of bases on one chain is given, then the sequence on the other chain is automatically determined. + +It has been found experimentally ^chargaff^wyatt that the ratio of the amounts of adenine to thymine, and the ratio of guanine to cytosine, are always very close to unity for deoxyribose nucleic acid. + +It is probably impossible to build this structure with a ribose sugar in place of the deoxyribose, as the extra oxygen atom would make too close a van der Waals contact. + +The previously published X-ray data ^astbury^wilkins on deoxyribose nucleic 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 in the following communications. We were not aware of the details of the results presented there when we devised our structure, which rests mainly though not entirely on published experimental data and stereochemical arguments. + +It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material. + +Full details of the structure, including the conditions assumed in building it, together with a set of coordinates for the atoms, will be published elsewhere. + +We are much indebted to Dr. Jerry Donohue for constant advice and criticism, especially on interatomic distances. We have also been stimulated by a knowledge 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 King’s College, London. One of us (J. D. W.) has been aided by a fellowship from the National Foundation for Infantile Paralysis. + +**** + +- J.D. Watson +- F. H. C. Crick + +*** + +Medical Research Council Unit for the Study of the Molecular Structure of Biological Systems, Cavendish Laboratory, Cambridge. + +*** + +^pauling Pauling, L., and Corey, R. B., Nature, 171, 346 (1953); Proc. U.S. Nat. Acad. Sci., 39, 84 (1953). +^furberg Furberg, S., Acta Chem. Scand., 6, 634 (1952). +^chargaff Chargaff, E., for references see Zamenhof, S., Brawerman, G., and Chargaff, E., Biochim. et Biophys. Acta, 9, 402 (1952). +^wyatt Wyatt, G. R., J. Gen. Physiol. , 36, 201 (1952). +^astbury Astbury, W. T., Symp. Soc. Exp. Biol. 1, Nucleic Acid, 66 (Camb. Univ. Press, 1947). +^wilkins Wilkins, M. H. F., and Randall, J. T., Biochim. et Biophys. Acta, 10, 192 (1953).