Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 59
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Theserelationships are now called Chargaff's rules:• The amount of adenine equals that of thymine: [A] = [T].• The amount of guanine equals that of cytosine:[G] = [C]• The amount of purine base equals that of pyrimidine bases:Although the chemical basis of these observations was not known at the time, one of the appealing features of theWatson-Crick structure of paired comple-Page 177Table 5.2 Base composition of DNA from different organismsBase (and percentage of total bases)Base composition(percent G + C)OrganismAdenineThymineGuanineCytosineBacteriophage T726.026.024.024.048.0Clostridium perfringens36.936.314.012.826.8Streptococcus pneumoniae30.229.521.618.740.3Escherichia coli24.723.626.025.751.7Sarcina lutea13.412.437.137.174.2Saccharomyces cerevisiae31.732.618.317.435.7Neurospora crassa23.022.327.127.654.7Wheat27.327.222.722.8*45.5Maize26.827.222.823.2*46.0Drosophila melanagaster30.829.419.620.239.8Pig29.429.620.520.541.0Salmon29.729.120.820.441.2Human being29.831.820.218.238.4BacteriaFungiHigher plantsAnimals*Includes one-fourth 5-methylcytosine, a modified form of cytosine found in most plants more complex thanalgae and in many animalsmentary strands was that it explained Chargaff's rules.
Because A is always paired with T in double-stranded DNA,it must follow that [A] = [T]. Similarly, because G is paired with C, [G] = [C]. The third rule follows by addition ofthe other two: [A] + [G] = [T] + [C]. In the next section, we examine the molecular basis of base pairing in moredetail.5.2—The Physical Structure of the Double HelixIn the three-dimensional structure of the DNA molecule proposed in 1953 by Watson and Crick, the moleculeconsists of two polynucleotide chains twisted around one another to form a double-stranded helix in which adenineand thymine, and guanine and cytosine, are paired in opposite strands (Figure 5.4). In the standard structure, whichis called the B form of DNA, each chain makes one complete turn every 34 Å. The helix is right-handed, whichmeans that as you look down the barrel, each chain follows a clockwise path as it progresses.
The bases are spacedat 3.4 Å, so there are ten bases per helical turn in each strand and ten base pairs per turn of the double helix. Eachbase is paired to a complementary base in the other strand by hydrogen bonds, which provide the main forceholding the strands together. (APage 178Figure 5.4Two representations of DNA illustrating the three-dimensional structure of the double helix. (A) In a "ribbon diagram" thesugar-phosphate backbones are depicted as bands, with horizontal lines used to represent the base pairs.(B) A computer model of the B form of a DNA molecule.
The stick figures are the sugar-phosphatechains winding around outside the stacked base pairs, forming a major groove and a minor groove.The color coding for the base pairs is A, red or pink; T, dark green or light green; G, dark brownor beige; C, dark blue or light blue. The bases depicted in dark colors are those attached to the bluesugar-phosphate backbone; bases depicted in light colors are attached to the beige backbone.[B, courtesy of Antony M.
Dean.]Page 179hydrogen bond is a weak bond in which two negatively charged atoms share a hydrogen atom.) The paired basesare planar, parallel to one another, and perpendicular to the long axis of the double helix. When discussing a DNAmolecule, molecular biologists frequently refer to the individual strands as single strands or as single-strandedDNA; they refer to the double helix as double-stranded DNA or as a duplex molecule. The two grooves spiralingalong outside of the double helix are not symmetrical; one groove, called the major groove, is larger than theother, which is called the minor groove. Proteins that interact with double-stranded DNA often have regions thatmake contact with the base pairs by fitting into the major groove, into the minor groove, or into both grooves.The central feature of DNA structure is the pairing of complementary bases, A with T and G with C. The hydrogenbonds that form in the adenine-thymine base pair and in the guanine-cytosine pair are illustrated in Figure 5.5.
Notethat an A–T pair (Figure 5.5A and B) has two hydrogen bonds and that a G-C pair (Figure 5.5C and D) has threehydrogen bonds. This means that the hydrogen bonding between G and C isFigure 5.5Normal base pairs in DNA. On the left, the hydrogen bonds (dotted lines) and the joined atoms areshown in red. (A, B) An A–T base pair. (C, D) A G-C base pair. In the space-filling models (B andD), the colors are C, gray; N, blue; O, red; and H (shown in the bases only), white. Each hydrogenbond is depicted as a white disk squeezed between the atoms sharing the hydrogen. The stickfigures on the outside represent the backbones winding around the stacked base pairs.[Space-filling models courtesy of Antony M.
Dean.]Page 180Connection The Double HelixJames D. Watson and Francis H. C. Crick 1953Cavendish LaboratoryCambridge, EnglandA Structure for Deoxyribose Nucleic AcidThis is one of the watershed papers of twentieth-century biology. After its publication, nothing in genetics was thesame. Everything that was known, and everything still to be discovered, would now need to be interpreted in termsof the structure and function of DNA. The importance of the paper was recognized immediately, in no small partbecause of its lucid and concise description of the structure. This excerpt is unusual in that it includes theacknowledgment.
Watson and Crick benefited tremendously in knowing that their structure was consistent with theunpublished structural studies of Maurice Wilkins and Rosalind Franklin. The same issue of Nature that includedthe Watson and Crick paper also included, back to back, a paper from the Wilkins group and one from the Franklingroup detailing their data and the consistency of their data with the proposed structure. It has been said thatFranklin was poised a mere two half-steps from making the discovery herself, alone. In any event, Watson andCrick and Wilkins were awarded the 1962 Nobel Prize for their discovery of DNA structure.
Rosalind Franklin,tragically, died of cancer in 1958 at the age of 38.We wish to suggest a structure for the salt of deoxyribose nucleic acid (DNA) . . .. The structure has two helicalchains each coiled round the same axis . . .. Both chains follow right-handed helices, but the two chains run inopposite directions . . ..
The bases are on the inside of the helix and the phosphates on the outside . . .. There is aresidue on each chain every 3.4 A and the structure repeats after 10 residues . . .. TheIf only specific pairs of bases can be formed, it follows that if the sequences of bases on one chainis given, then the sequence on the other chain is automatically determined.novel feature of the structure is the manner in which the two chains are held together by the purine and pyrimidinebases. The planes of the bases are perpendicular to the fiber axis. They are joined together in pairs, a single basefrom one chain being hydrogen-bonded to a single base from the other chain, so that the two lie side by side. Oneof the pair must be a purine and the other a pyrimidine for bonding to occur .
. .. Only specific pairs of bases canbond 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 assumptionsthe other member must be thymine; similarly for guanine and cytosine.
The sequence of bases on a single chaindoes not appear to be restricted in any way. However, if only specific pairs of bases can be formed, it follows that ithe sequence of bases on one chain is given, then the sequence on the other chain is automatically determined . . ..It has not escaped our notice that the specific pairing we have postulated immediately suggests a plausible copyingmechanism for the genetic material . .
.. We are much indebted to Dr. Jerry Donohue for constant advice andcriticism, especially on interatomic distances. We have also been stimulated by a knowledge of the general natureof the unpublished experimental results and ideas of Dr. Maurice H. F. Wilkins, Dr. Rosalind Franklin and their coworkers at King's College, London.Source: Nature 171: 737–738stronger in the sense that it requires more energy to break; for example, the amount of heat required to separate thepaired strands in a DNA duplex increases with the percent of G + C.
Because nothing restricts the sequence of basesin a single strand, any sequence could be present along one strand. This explains Chargaff's observation that DNAfrom different organisms may have different base compositions. However, because the strands in duplex DNA arecomplementary, Chargaff's rules of [A] = [T] and [G] = [C] are true whatever the base composition.Each backbone in a double helix consists of deoxyribose sugars alternating with phosphate groups that link the 3'carbon atom of one sugar to the 5' carbon of the next in line (Figure 5.3).
The two polynucleotide strands of thedouble helix are oriented in opposite directions in the sense that the bases that are paired are attached to >sugars lyingabove and below the plane of pairing, respectively. The sugars arePage 181offset because the phosphate linkages in the backbones run in opposite directions (Figure 5.6), and the strands aresaid to be antiparallel. This means that each terminus of the double helix possesses one 5'-P group (on one strand)and one 3'-OH group (on the other strand), as shown in Figure 5.6.The diagrams of the DNA duplexes in Figures 5.4 and 5.6 are static and so some-Figure 5.6A segment of a DNA molecule showing theantiparallel orientation of the complementarystrands. The overlying blue arrows indicate the5'-to-3' direction of each strand.
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