Nucleic Acid Structure:- DNA Is a Double Helix

To shed more light on the structure of DNA, Rosalind Franklin and Maurice Wilkins used the powerful method of x-ray diffraction (see Box 4–4) 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 Å and a secondary one of 34 Å. 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.

In 1953 Watson and Crick postulated a three dimensional model of DNA structure that accounted for all the available data. It consists of two helical DNA chains wound around the same axis to form a right handed double helix (see Box 4–1 for an explanation of the right- or left-handed sense of a helical structure). The hydrophilic backbones of alternating deoxyribose and phosphate groups are on the outside of the double helix, facing the surrounding water. The furanose ring of each deoxyribose is in the C-2 endo conformation. The purine and pyrimidine bases of both strands are stacked inside the double helix, with their hydrophobic and nearly planar ring structures very close together and perpendicular to the long axis. The offset pairing of the two strands creates a major groove and minor groove on the surface of the duplex (Fig. 8–15). Each nucleotide base of one strand is paired in the same plane with a base of the other strand. Watson and Crick found that the hydrogen-bonded base pairs illustrated in Fig ure 8–11, G with C and A with T, are those that fit best within the structure, providing a rationale for Chargaff’s rule that in any DNA, G=C and A=T. It is important to note that three hydrogen bonds can form between G and C, symbolized G≡C, but only two can form between A and T, symbolized A≡T. This is one reason for the finding that separation of paired DNA strands is more difficult the higher the ratio of G≡C to A=T base pairs. Other pairings of bases tend (to varying degrees) to destabilize the double-helical structure.

FIGURE 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.

FIGURE 8–15 Watson-Crick model for the structure of DNA. The original model proposed by Watson and Crick had 10 base pairs, or 34 Å (3.4 nm), per turn of the helix; subsequent measurements revealed 10.5 base pairs, or 36 Å (3.6 nm), per turn. (a) Schematic representation, showing dimensions of the helix. (b) Stick representation showing the backbone and stacking of the bases. (c) Space-filling model.

When Watson and Crick constructed their model, they had to decide at the outset whether the strands of DNA should be parallel or antiparallel—whether their 5,3-phosphodiester bonds should run in the same or opposite directions. An antiparallel orientation produced the most convincing model, and later work with DNA polymerases provided experimental evidence that the strands are indeed antiparallel, a finding ultimately confirmed by x-ray analysis. 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 in which the vertically stacked bases inside the double helix would be 3.4 Å apart; the secondary repeat distance of about 34 Å was accounted for by the presence of 10 base pairs in each complete turn of the double helix. In aqueous solution the structure differs slightly from that in fibers, having 10.5 base pairs per helical turn (Fig. 8–15).

As Figure 8–16 shows, the two antiparallel polynucleotide chains of double-helical DNA are not identical in either base sequence or composition. Instead they are complementary to each other. Wherever adenine occurs in one chain, thymine is found in the other; similarly, wherever guanine occurs in one chain, cytosine is found in the other.

The DNA double helix, or duplex, is held together by two forces, as described earlier: hydrogen bonding between complementary base pairs (Fig. 8–11) and base-stacking interactions. The complementarity between the DNA strands is attributable to the hydrogen bonding between base pairs. The base-stacking interactions, which are largely nonspecific with respect to the identity of the stacked bases, make the major contribution to the stability of the double helix. The important features of the double-helical model of DNA structure are supported by much chemical and biological evidence. Moreover, the model immediately suggested a mechanism for the transmission of genetic information. The essential feature of the model is the complementarity of the two DNA strands. As Watson and Crick were able to see, well before confirmatory data became available, this structure could logically be replicated by (1) separating the two strands and (2) synthesizing a complementary strand for each. Because nucleotides in each new strand are joined in a sequence specified by the base-pairing rules stated above, each preexisting strand functions as a template to guide the synthesis of one complementary strand (Fig. 8–17). These expectations were experimentally confirmed, inaugurating a revolution in our understanding of biological inheritance.

FIGURE 8–16 Complementarity of strands in the DNA double helix. The complementary antiparallel strands of DNA follow the pairing rules proposed by Watson and Crick. The base-paired antiparallel strands differ in base composition: the left strand has the composition A₃T₂ G₁ C₃; the right, A₂ T₃ G₃ C₁. They also differ in sequence when each chain is read in the 5→ 3 direction. Note the base equivalences: A=T and G=C in the duplex.

FIGURE 8–17 Replication of DNA as suggested by Watson and Crick. The preexisting or “parent” strands become separated, and each is the template for biosynthesis of a complementary “daughter” strand (in red).

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مواضيع ذات صلة

Nucleic Acid Structure:- Many RNAs Have More Complex Three-Dimensional Structures

Nucleic Acid Structure:- Messenger RNAs Code for Polypeptide Chains

Nucleic Acid Structure:- Certain DNA Sequences Adopt Unusual Structures

Nucleic Acid Structure:- DNA Can Occur in Different Three-Dimensional Forms

Nucleic Acid Structure:- DNA Molecules Have Distinctive Base Compositions

Nucleic Acid Structure:- DNA Stores Genetic Information

Carbohydrates as Informational Molecules: The Sugar Code:- Lectin-Carbohydrate Interactions Are Very Strong and Highly Specific

Carbohydrates as Informational Molecules: The Sugar Code:- Lectins Are Proteins That Read the Sugar Code and Mediate Many Biological Processes

Polysaccharides:- Glycolipids and Lipopolysaccharides Are Membrane Components

Polysaccharides: - Glycoproteins Have Covalently Attached Oligosaccharides

Polysaccharides:- Proteoglycans Are Glycosaminoglycan-Containing Macromolecules of the Cell Surface and Extracellular Matrix

Polysaccharides:- Glycosaminoglycans Are Heteropolysaccharides of the Extracellular Matrix