RNA is a polymer of purine and pyrimidine ribonucleotides linked together by 3′,5′-phosphodiester bonds analogous to those in DNA (Figure 1). Although sharing many features with DNA, RNA possesses several specific differences:

1. In RNA, the sugar moiety to which the phosphates and purine and pyrimidine bases are attached is ribose rather than the 2′-deoxyribose of DNA.

 2. The pyrimidine components of RNA can differ from those of DNA. Although RNA contains the ribonucleotides of adenine, guanine, and cytosine, it does not possess thymine except in the rare case mentioned in the following discussion. Instead of thymine, RNA contains the ribonucleotide of uracil.

3. RNA can exist as a single strand, whereas DNA exists as a double-stranded helical molecule. However, given the proper complementary base sequence with opposite polarity, the single strand of RNA—as demonstrated in Figures 2 and 3—is capable of folding back on itself like a hairpin and thus acquiring double-stranded characteristics: G pairing with C, and A pairing with U. G-C base pairs form three H-bonds and A-U base form only two.

4. Since the RNA molecule is a single strand complementary to only one of the two strands of a gene, its guanine content does not necessarily equal its cytosine content, nor does its adenine content necessarily equal its uracil content.

 5. RNA can be hydrolyzed by alkali to 2′, 3′ cyclic diesters of the mononucleotides, compounds that cannot be formed from alkali-treated DNA because of the absence of a 2′-hydroxyl group. The alkali lability of RNA is useful both diagnostically and analytically.

Fig1. A segment of a ribonucleic acid (RNA) molecule in which the purine and pyrimidine bases—guanine (G), cytosine (C), uracil (U), and adenine (A)—are held together by phosphodiester bonds between ribosyl moieties attached to the nucleobases by N-glycosidic bonds. Note that negative charge(s) on the phosphodiester backbone are not illustrated and that the polymer has a polarity as indicated by the labeled 3′- and 5′-attached phosphates.

Fig2. RNA Secondary Structure. (Left) Diagrammatic representation of the secondary structure of a hypothetical single stranded RNA molecule in which a stem loop, or “hairpin”, has been formed. Formation of this structure is dependent on the indicated intramolecular base pairing (colored horizontal lines between complementary bases). Note that in RNA G pairs with C as in DNA, but that A pairs with U. (Right) Schematic of A-U (top) compared to A-T base pairs (bottom).

Fig3. Structure of a mature, functional tRNA, yeast phenylalanyl-tRNA (tRNA^Phe). Shown are primary (1o), secondary (2o), and tertiary (3o) structures (top, lower left, and lower right, respectively) of tRNA^Phe. Numerals below the 76 nucleotide-long tRNA^Phe primary structure indicate nucleotide numbering from the 5′ (+1) to the 3′ end (+76) of the molecule. Note that the +1 nucleotide contains a 5′ phosphate moiety (P), while the 3′ nucleotide has a free 3′ hydroxyl group (OH). Bases underlined in bold type within the sequence of tRNA^Phe are heavily modified to the nucleotides shown in the 2o structural representation of tRNA^Phe. This structure is often referred to as a “cloverleaf.” Some of these nucleotides have noncanonical ribonucleotide names, as represented in the 2o structural model. Within tRNA^Phe nucleotides U16 and U17 are modified to D16 , D17 ; G37 to Y37 ; U39 and U55 to Ψ; and U54 to T54. Straight lines between bases within the tRNA secondary structure represent hydrogen bonds formed between bases (A–U; G–C). Note that these regions of secondary structure form with the same strand polarity (ie, 5′ to 3′ and 3′ to 5′) as base-paired regions of DNA. The three bases of the anticodon loop are shown in red. In the case of amino acid–charged tRNAs, an aminoacyl moiety is esterified to the 3′-CCAOH terminus (brown; in this case the amino acid would be phenylalanine; not shown). Blue type highlights nontraditional nucleotides introduced by posttranslational modification, abbreviated as follows: m2G = 2-methylguanosine; D = 5,6-dihydrouridine; m2 2 G = N2-dimethylguanosine; Cm = O2′-methylcytidine; Gm = O2′-methylguanosine; 357 T = 5-methyluridine; Y = wybutosine; Ψ = pseudouridine; m5C = 5-methylcytidine; m7G = 7-methylguanosine; m1A = 1-methyladenosine. Essentially all tRNAs fold into similar, characteristic, tertiary structures (3o) as shown, lower right. The distinct portions of the molecule in 2o (insert) and 3o configurations are color-coded in this image for clarity. tRNA^Phe was the first nucleic acid whose structure was determined by x-ray crystallography. Such distinct three-dimensional tRNA structures bind specifically to important functional sites on both aminoacyl tRNA synthetases and the ribosomes during protein synthesis. (Reproduced with permission from Transfer RNA/Wikipedia Commons https://en.wikipedia.org/wiki/Transfer_RNA.)

Information within the single strand of RNA is contained in its sequence (“primary structure”) of purine and pyrimidine nucleotides within the polymer. The sequence is complementary to the template strand of the gene from which it was transcribed. Because of this complementarity, an RNA molecule can bind specifically via the base-pairing rules to its template DNA strand (A–T, G–C, C–G, U–A; RNA base bolded); it will not bind (“hybridize”) with the other (coding) strand of its gene. The sequence of the RNA molecule (except for U replacing T) is the same as that of the coding strand of the gene (Figure 4).

Fig4. The relationship between the sequences of an RNA transcript and its gene, in which the coding and template strands are shown with their polarities. The RNA transcript with a 5′ to 3′ polarity is complementary to the template strand with its 3′ to 5′ polarity. Note that the sequence in the RNA transcript and its polarity is the same as that in the coding strand, except that the U of the transcript replaces the T of the gene; the initiating nucleotide of RNAs contain a terminal 5-triphosphate (ie, pppA-above).

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مواضيع ذات صلة

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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

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