BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 9. NUCLEIC ACIDS

9.5. Structure and Functions of Ribonucleic Acids

9.5.1. Chemical Nature of Ribonucleic Acids

Ribonucleic acid (RNA) is a polynucleotide whose monomeric units are four different ribonucleoside monophosphates (AMP, GMP, CMP, UMP) linked together by 3', 5'-phosphodiester bonds. The sequence of NUCLEOTIDES in the polynucleotide chain determines the Introduction/19.html">Primary Structure of RNA, which contains from 70 to 10,000 or more nucleotides. The ends of an RNA molecule are asymmetrical: one end features a phosphorylated OH group on the 5' carbon atom, while the opposite end terminates with the OH group of the 3' carbon atom of the pentose.

Unlike DNA, the pentose residue in RNA to which purine or pyrimidine bases and phosphate groups are attached is ribose rather than 2'-deoxyribose. Secondly, RNA contains the nitrogenous bases adenine, guanine, and cytosine, just like DNA, but its fourth base is uracil (instead of thymine in DNA). The absence of a CH₃ group at the 5-position in uracil weakens hydrophobic interactions in the A-U pair and reduces the likelihood of forming stable double-stranded regions. Thirdly,

unlike DNA, RNA molecules are built from a single polynucleotide chain. However, if the RNA chain contains complementary sequences, the single-stranded molecule can fold back on itself to form a double helix. Hydrogen bonding drives The formation of complementary pairs between adenine and uracil, as well as guanine and cytosine. The segments of the RNA chain in these Helical structures are antiparallel, though not always entirely complementary, often containing unpaired nucleotides or even single-stranded loops. Figure 9.16 illustrates the formation of double-helical regions and loops (hairpins) in RNA.

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Fig. 9.16. Formation of hairpins in RNA

The Abundance and variety of double-helical regions largely determine the rigidity of the Tertiary Structure of RNA. At the same time, the presence of non-helical segments makes all single-stranded RNAs structurally more flexible than DNA.

Loop structures can account for up to 50% of all nucleotides (especially in transfer and Ribosomal RNAs). The presence of helical regions is characteristic of all types of RNA, enabling them to perform their specific biological Functions.

Certain nitrogenous bases in RNA undergo chemical modification, incorporating methyl, thiol, isopentenyl, and hydrogen substituents. RNA contains 2'-O-methyl nucleotides with a modified ribose residue, as well as alternative linkages between uracil and ribose (pseudouridine) (see Section 9.3). Such modified nucleotides are relatively rare in ribosomal and messenger RNAs, but quite common in Transfer RNAs. As a rule, the modification of nitrogenous bases and ribose residues takes place after RNA Synthesis is complete.