X-ray diffraction patterns, of DNA

Concerning the helical stmcture of biological macromolecules in general, they relied on Linus Pauling s discovery of the alpha-helix stmcture of proteins and concerning the helical stmcture of DNA, they relied on Rosalind Franklin s X-ray diffraction pattern of DNA. 

RGURE 8-14 X-ray diffraction pattern of DNA. The spots forming a cross in the center denote a helical structure. The heavy bands at the left and right arise from the recurring bases. 

To account for the periodicities observed in the x-ray diffraction patterns of DNA fibers, Watson and Crick manipulated molecular models to arrive at a structure... 

Very shortly, the first question was answered in principle by Watson and Crick who proposed the three-dimensional structure of DNA in 1953. Their proposal that DNA is composed of two polynucleotide chains forming a double helix was based upon studies of x-ray diffraction patterns of DNA libers. 

Watson and Crick were the first to appreciate the significance of strong 3.4-A and 34-A spacings and the central crosslike pattern, which reflects a helix structure in the x-ray diffraction pattern of DNA. They interpreted this as arising from the hydrogen-bonded antiparallel double-helix structure. 

In 1953, James D. Watson and Francis C. Crick used X-ray diffraction patterns of DNA fibers to determine the molecular structure and conformation of DNA. They found that DNA contains two complementary polynucleotide chains held together by hydrogen... 

Watson and Crick based their model (known as the B-DNA helix) on x-ray diffraction patterns of DNA fibers, which provided information about properties of the double helix that are averaged over its constituent residues. The results of x-ray diffraction studies of dehydrated DNA fibers revealed a different form called A-D/VA, which appears when the relative humidity is reduced to less than about 75%. A-DNA, like B-DNA, is a right-handed double helix made up of antiparallel strands held together by Watson-Crick base-pairing. The A helix is wider and shorter than the B helix, and its base pairs are tilted rather than perpendicular to the helix axis (Figure 27.4). 

Most DNA in cells is a right-handed helix. The x-ray diffraction pattern of DNA indicates that the stacked bases are regularly spaced 0.36 nm apart along the helix axis. The... 

James Watson and Erancis Crick proposed the molecular structure of deoxyribonucleic acid (DNA) in 1952. Gathering a number of experimental findings on DNA, including x-ray diffraction patterns of DNA fibers, they proposed that DNA was a double-stranded helical molecule, with its hydrophobic bases occupying the interior of the molecule, and its hydrophilic... 

The X-ray diffraction pattern of DNA demonstrated the helical structure and the diameter. The combination of evidence from X-ray diffraction and chemical analysis led to the conclusion that the base pairing is complementary, meaning that adenine pairs with thymine and that guanine pairs with cytosine. Because complementary base pairing occurs along the entire double helix, the two chains are also referred to as complementary strands. By 1953, studies of the base composition of DNA from many species had already shown that, to within experimental... 

In 1953, James D. Watson and Francis C. Crick used X-ray diffraction patterns of DNA fibers to determine the molecular structure and conformation of DNA. They found that DNA contains two complementary polynucleotide chains held together by hydrogen bonds between the paired bases. Figure 23-26 shows a portion of the double strand of DNA, with each base paired with its complement. The two strands are antiparallel One strand is arranged 3 5 from left to right, while the other runs in the opposite direction, 5 3 from left to right... 

The x-ray diffraction patterns of DNA revealed two periodicities in the structure at 3.4 A and 34 A. Based on this observation. Crick and Watson ingeniously fit the information into a three dimensional struc-... 

A -DNA The Watson-Crick model of DNA is based on the x-ray diffraction patterns of B-DNA. Most DNA is B-DNA however, DNA may take on two other conformations, A-DNA and Z-DNA. These conformations are greatly favored by the base sequence or by bound proteins. When B-DNA is slightly dehydrated in the laboratory, it takes on the A conformation. A-DNA is very similar to B-DNA except that the base pairs are not stacked perpendicular to the helix axis rather, they are tilted because the deoxyribose moiety puckers differently. An A-DNA helix is wider and shorter than the B-DNA helix. 

Fig. 7 X-ray diffraction patterns of a the as-cast film, b the stretched film of the DNA-lipid complex in water moisture, and c the stretched DNA-lipid film in the dry state. Open, two-headed arrows show the stretching direction of the film. Closed arrows show the incident beam of the X-rays. X-ray photographs of a and c were enlarged to show diffraction more clearly...

Fig. 25 (a) DNA release from EDOPC-DNA lipoplexes after addition of negatively charged lipid dispersion, as monitored by FRET (CM, oleic acid DOPA, dioleoyl phosphatidic acid DOPG, dioleoyl phosphatidylglycerol CL, cardiolipin DOPS, dioleoyl phosphatidylserine PI, phospha-tidylinositol). (b) Fraction of released DNA from EDOPC lipoplexes 10 min after addition of the respective anionic liposomes (c) X-ray diffraction patterns of mixtures of EDOPC and anionic liposome dispersions the respective structures are shown schematically on the left side (reproduced with permission from [98] copyright (2004) Biophysical Society)... 

The Watson-Crick model was based on molecular modeling and two lines of experimental observations chemical analyses of DNA base compositions and mathematical analy ses of X-ray diffraction patterns of crystals of DNA. 

The first X-ray diffraction studies were done on purified DNA preparations, and one could argue that the molecular regularity observed under those conditions is an artifact, and that the DNA present in the intact cell may not exhibit such characteristics. This objection was overcome when it was observed that the X-ray diffraction patterns of sperm heads and viruses were compatible with the existence of doublehelical DNA in the native state. 

When the histone octamer is mixed with purified, double-stranded DNA, the same x-ray diffraction pattern is formed as that observed in freshly isolated chromatin. Electron microscopic studies confirm the existence of reconstituted nucleosomes. Furthermore, the reconsti-mtion of nucleosomes from DNA and histones H2A, H2B, H3, and H4 is independent of the organismal or cellular origin of the various components. The histone HI and the nonhistone proteins are not necessary for the reconstitution of the nucleosome core. 

The DNA-lipid cast film was stretched ca. three times in length (ca. 20 p.m thick) in the wet state, and X-ray diffraction patterns are shown in Fig. 7b. When the incident beam was irradiated parallel to the top edge of the stretched film, the circular reflection of 41 A was observed. When the beam was exposed parallel to the side edge and perpendicular to the film plane, the diffraction on the equator appeared as two spots of 41 A indicating a distance between the DNA-lipid strands and the diffraction on the meridian of 3.4 A. The distance between parallel stacked base-pairs was clearly observed [2-4]. These findings clearly show that DNA strands are ahgned... 

Host of our knowledge of the detailed three-dimensional conformation of nucleic acids came from x-ray diffraction studies. The earlier studies of natural and synthetic DNA polymers were carried out on fibers (1). However, fiber diffraction studies have intrinsic limitations as their x-ray diffraction patterns provided only a limited amount of experimental information. In general, it is impossible to solve the molecular structure of a... 

To shed more light on the structure of DNA, Rosalind Franklin and Maurice Wilkins used the powerful method of x-ray diffraction (see Box A-A) to analyze DNA fibers. They showed in the early 1950s that DNA produces a characteristic x-ray diffraction pattern (Fig. 8-14). From this pattern it was deduced that DNA molecules are helical with two periodicities along their long axis, a primary one of 3.4 A and a secondary one of 34 A The problem then was to formulate a three-dimensional model of the DNA molecule that could account not only for the x-ray diffraction data but also for the specific A = T and G = C base equivalences discovered by Chargaff and for the other chemical properties of DNA. 

While searching for the meaning of these equalities, James Watson noted that hydrogen-bonded base pairs with the same overall dimensions could be formed between A and T and between G and C (fig. 25.4). The A-T base-paired structure has two hydrogen bonds, whereas the G-C base pair has three. The hydrogen-bonded pairs are formed between bases of opposing strands and can only arise if the directional senses of the two interacting chains are opposite or antiparallel (fig. 25.5). With this notion in mind Francis Crick took a closer look at the x-ray diffraction pattern produced by DNA and was able to interpret the diffraction pattern in terms of a helix (see Methods of Biochemical... 

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