![]() ![]() Remember that the distance between phosphates in nucleoside diphosphate depends largely on Puckering that we have the two forms of DNA helices considered here. It is due to this difference in deoxyribose The closely similar C3'-exo-) conformation. It has been found that the sugar in the Aįorm of DNA has a C3'-endo-conformation, whereas in the B form it has a C2'-endo- (or Mononucleotide residues forming these helices. Such a significant difference between the A and B forms of DNA helicesīecomes better understood after analysis of the conformation of deoxyribose in the The axis of the helix (the angle with the perpendicular is about 20 0), and theīases themselves are not coplanar within a pair. The A form has the configuration of a spiral stair. Seen on the cross-sectional views of the helix in Fig. Nm (5 Å) off the axis and lie along the periphery of the macromolecule (which is clearly Passes through the center of complementary pairs, base pairs in the A form are nearly 0.5 However, it differs markedly from the B form in many ways. The latter is also a right-handed double helix with antiparallel chains. At a relative humidity below 80 per cent or at a lowĬoncentration of counter-ions in the sample, DNA exists in a crystalline form also knownĪs the A form. DNA exists predominantly in this form in the cell.Ĩ.1.2 Polymorphism of the Double Helix of DNAĪlthough the structure of DNA helices is extremely rigid, it can beĬhanged under certain conditions. Helix of DNA, whose model was proposed by Watson and Crick started being referred to as The adjacent complementary pairs) is 36 0. Neighboring base pairs (the angle between the straight lines connecting the C1' atoms in The only other thing to be added in conclusion of this briefĭescription of the Watson and Crick model is the fact that the angle between the This principle is an important corollary of the Watson and Crick modelĪs it provides a lucid explanation in chemical terms of the fundamental biological role ofĭNA - storage and faultless transmission of genetic information. Thus, the nucleotide sequence in one chain of the DNA molecule Of the other respectively (and vice versa). ![]() Pairs - that is, adenines and guanines of one chain are paired with thymines and cytosines Location of helix axis (x) relative to a base-pair.Įmphasized, this requirement is met by formation of complementary base (A) Watson and Crick model of B form of DNA and (B) The symmetry of the helix implies that for each purine base in oneĬhain there is a matching pyrimidine base in the other chain. In other words, the linearĭNA molecule terminates, in each direction, in the 5' end of one chain and the 3' end of Its polydeoxyribonucleotideĬhains are antiparallel: this means that if we move along the axis of the helixįrom one end to the other, the phosphodiester bonds in one chain will be passed in the 3' ® 5' direction and in the other, in the 5' ® 3' direction. Molecule points toward the outside of the helix, and two grooves differing in size can beĭiscerned on its surface, the major groove being about 2.2 nm (22 A) wide and the minor The planes of the carbohydrate moieties are almostĪs can be seen from Figure 8-1, the sugar-phosphate backbone of the The base planes are thus spaced about 0.34 nm (3.4 Å) apart. The pitch per helix turn, whichĬorresponds to its identity period, is 3.37 nm (33.7 Å). Over the entire length and equals 1.8 nm (18 Å). The diameter of the helix is virtually constant Symmetrical helix formed by two polydeoxyribonucleotide chains twisted one with respect to According to their hypothesis, the DNA molecule is represented by a Humidity in excess of 80 per cent and a high concentration of counter-ions (Li +) Nucleotide composition (Chargaffs rules), Watson and Crick decoded the X-ray fiberĭiffraction patterns representing the paracrystalline form of DNA emerging at a relative Proceeding from the available data on the structure of heterocyclicīases, conformation of nucleosides, the internucleotide linkage in DNAs and their Important discoveries of our times had turned out to be seminal in many fields of natural With a description of a new model of DNA structure. Watson and E Crick published a short paper Principles of spatial organization of DNA and RNA macromolecules and those of theirįeatures that offer an insight into their chemical properties and interactions with Here, we shall give a brief outline of the current concepts regarding the Some of the books and reviews dealing with the subject are listed at the end of thisĬhapter. 8 Macromolecular Structure of DNA and RNAĪ large body of literature is concerned with the structure of DNA and ![]()
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