DNA & RNA are Polynucleotides
المؤلف:
Peter J. Kennelly, Kathleen M. Botham, Owen P. McGuinness, Victor W. Rodwell, P. Anthony Weil
المصدر:
Harpers Illustrated Biochemistry
الجزء والصفحة:
32nd edition.p335
2025-08-30
490
The 5′-phosphoryl group of a mononucleotide can esterify a second hydroxyl group, forming a phosphodiester. Most commonly, this second hydroxyl group is the 3′-OH of the pentose of a second nucleotide. This forms a dinucleotide in which the pentose moieties are linked by a 3′,5′-phosphodiester bond to form the “backbone” of RNA and DNA. The formation of a dinucleotide may be represented as the elimination of water between two mononucleotides. Biologic formation of dinucleotides does not, however, occur in this way because the reverse reaction, hydrolysis of the phosphodiester bond, is strongly favored on thermodynamic grounds. However, despite an extremely favorable ΔG, in the absence of catalysis by phosphodiesterases hydrolysis of the phosphodiester bonds of DNA occurs only over long periods of time. DNA therefore persists for considerable periods, and has been detected even in fossils. RNAs are far less stable than DNA since their 2′-hydroxyl groups (absent from DNA) can function as nucleophiles for the chemical hydrolysis of 3′,5′-phosphodiester bonds.
Posttranslational modification of preformed polynucleotides can generate additional structures such as pseudouridine, a nucleoside in which d-ribose is linked to C-5 of uracil by a carbon-to-carbon bond rather than by the usual β-N glycosidic bond. The nucleotide pseudouridylic acid (ψ) arises by rearrangement of a UMP of a preformed tRNA. Similarly, methylation by S-adenosylmethionine of a UMP of preformed tRNA forms TMP (thymidine monophosphate), which contains ribose rather than deoxyribose.
Polynucleotides Are Directional Macromolecules
Directional 3′ → 5′ phosphodiester bonds link the monomers of polynucleotides. Since each end of a polynucleotide thus is distinct, we refer to the “5′-end” or the “3′-end” of a poly nucleotide. Since the phosphodiester bonds all are 3′ → 5′, the representation pGpGpApTpCpA indicates that the terminal 5′-hydroxyl is phosphorylated. More concisely, the representation GGATC, which shows only the base sequence, is by convention written with the 5′-base (G) at the left and the 3′-base (C) at the right.
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