The chemical nature of the monomeric deoxynucleotide units of DNA—deoxyadenylate, deoxyguanylate, deoxycytidylate, and thymidylate—is described in Chapter 32. These monomeric units of DNA are held in polymeric form by 3′,5′ phosphodiester bonds constituting a single strand, as depicted in Figure 1. The informational content of DNA (the genetic code) resides in the sequence in which these monomers— purine and pyrimidine deoxyribonucleotides—are ordered. The polymer as depicted possesses a polarity; one end has a 5′-hydroxyl or phosphate terminus while the other has a 3′-phosphate or hydroxyl terminus. The importance of this polarity will become evident. Since the genetic information resides in the order of the monomeric units within the polymers, there must exist a mechanism of reproducing or replicating this specific information with a high degree of fidelity. That requirement, together with x-ray diffraction data from the DNA molecule generated by Franklin, and the observation of Chargaff that in DNA molecules the concentration of deoxyadenosine (A) nucleotides equals that of thymidine (T) nucleotides (A = T), while the concentration of deoxyguanosine (G) nucleotides equals that of deoxycytidine (C) nucleotides (G = C), led Watson, Crick, and Wilkins to propose in the early 1950s a model of a double-stranded (ds) DNA molecule. The model they proposed is depicted in Figure 2. The two strands of this double-stranded helix are held in register by both hydrogen bonds (H-bonds) between the purine and pyrimidine bases of the respective linear molecules and by van der Waals and hydrophobic interactions between the stacked adjacent base pairs. The pairings between the purine and pyrimidine nucleotides on the opposite strands are very specific, and are dependent on hydrogen bonding of A with T and G with C (see Figure 2). A–T and G–C base pairs are often referred to as Watson-Crick base pairs.

Fig1. A segment of one strand of a DNA molecule in which the purine and pyrimidine bases guanine (G), cytosine (C), thymine (T), and adenine (A) are held together by a phosphodiester backbone between 2′-deoxyribosyl moieties attached to the nucleobases by an N-glycosidic bond. Note that the phosphodiester backbone is negatively charged and has a polarity (ie, a direction). Convention dictates that a single-stranded DNA sequence is written in the 5′ to 3′ direction (ie, pGpCpTpAp, where G, C, T, and A represent the four bases and P represents the interconnecting phosphates).

Fig2. A diagrammatic representation of the Watson and Crick model of the double-helical structure of the B form of DNA.The horizontal arrow indicates the width of the double helix (20 Å), and the vertical arrow indicates the distance spanned by one complete turn of the double helix (34 Å). One turn of B-DNA includes 10 base pairs (bp), so the rise is 3.4 Å per bp. The central axis of the double helix is indicated by the vertical rod. The short arrows designate the polarity of the antiparallel strands. The major and minor grooves are depicted. (A, adenine; C, cytosine; G, guanine; P, phosphate; S, sugar [deoxyribose]; T, thymine.) Hydrogen bonds between A/T and G/C bases indicated by short, red, horizontal lines.

This common form of DNA is said to be right handed because as one looks down the double helix, the base residues form a spiral in a clockwise direction. In the double-stranded molecule, restrictions imposed by the rotation about the phosphodiester bond, the favored anti-configuration of the glycosidic bond , and the predominant tautomers  of the four bases (A, G, T, and C) allow A to pair only with T, and G only with C, as depicted in Figure 3. These base-pairing restrictions explain the earlier observation that in a double-stranded DNA molecule the content of A equals that of T and the content of G equals that of C. The two strands of the double-helical molecule, each of which possesses a polarity, are antiparallel; that is, one strand runs in the 5′ to 3′ direction and the other in the 3′ to 5′ direction. Within a particular gene in the double-stranded DNA molecules, the genetic information resides in the sequence of nucleotides on one strand, the template strand. This is the strand of DNA that is copied, or transcribed, during ribonucleic acid (RNA)synthesis. It is sometimes referred to as the noncoding strand. The opposite strand is considered the coding strand because it matches the sequence of the RNA transcript (but containing uracil in place of thymine) that encodes the protein.

Fig3. Classic Watson-Crick DNA base pairing between complementary deoxynucleotides involves the formation of hydrogen bonds. Two such H-bonds form between adenine and thymine, and three H-bonds form between cytidine and guanine. The broken lines represent H-bonds.

The two strands, in which opposing bases are held together by interstrand hydrogen bonds, wind around a central axis in the form of a double helix. In the test tube, double-stranded DNA can exist in at least six forms (A-DNA, through E-DNA and Z-DNA). These different forms of DNA differ with regard to intra- and interstrand interactions and involve structural rearrangements within the monomeric units of DNA. The B form is usually found under physiologic conditions. A single turn of B form DNA about the long axis of the molecule contains 10 bp. The distance spanned by one turn of B-DNA is 3.4 nm (34 Å). The width (helical diameter) of the double helix in B-DNA is 2 nm (20 Å).

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مواضيع ذات صلة

Overproduction of Pyrimidine Catabolites

Disorders of Purine Metabolism

Biochemical Characteristics and Biological Function of Cardiac Natriuretic Peptides

The Deoxyribonucleosides of Uracil & Cytosine are Salvaged

Biosynthesis of Pyrimidine Nucleotides

Hepatic Purine Biosynthesis is Stringently Regulated

Inosine Monophosphate (IMP) is Synthesized From Amphibolic Intermediates

DNA & RNA are Polynucleotides

Synthetic Nucleotide Analogs are Used in Chemotherapy

Chemistry of Purines, Pyrimidines, Nucleosides, & Nucleotides

Disorders of Bilirubin Metabolism

Catabolism of HEME Produces Bilirubin