C₁₀H₁₄N₅O₈P — RNA (Repeat Unit)

Ribonucleic acid (RNA) is a vital biological macromolecule involved in the expression and regulation of genetic information. The repeating structural unit of RNA is a ribonucleotide, which has the molecular formula \(C_{10}H_{14}N_{5}O_{8}P\). Each nucleotide in RNA consists of three main components: a nitrogenous base, a ribose sugar, and a phosphate group. RNA plays multiple roles within cells — from acting as a messenger carrying genetic codes from DNA to participating in catalysis and protein synthesis through different types of RNA molecules such as mRNA, tRNA, and rRNA.

Unlike DNA, which is double-stranded, RNA is typically single-stranded but may fold into complex three-dimensional structures through intra-strand hydrogen bonding. This structural versatility enables RNA to perform both informational and enzymatic functions, a key aspect of molecular biology and the origin of life hypotheses.

The ribonucleotide unit is the building block of RNA. Each ribonucleotide is made up of:

• Nitrogenous base: Adenine (A), Guanine (G), Cytosine (C), or Uracil (U). Uracil replaces thymine, which is found in DNA.
• Ribose sugar: A five-carbon sugar with a hydroxyl (–OH) group attached to both the 2′ and 3′ carbon atoms, distinguishing RNA from DNA (which lacks the 2′–OH group).
• Phosphate group: Attached to the 5′ carbon of the sugar, forming a negatively charged backbone that imparts polarity and solubility.

The general structural formula of a ribonucleotide can be represented as:

This extra hydroxyl group on the ribose sugar increases RNA’s reactivity, making it less stable than DNA under alkaline conditions but more versatile in function. The ribose and phosphate groups alternate to form the sugar-phosphate backbone, connected by phosphodiester bonds.

RNA strands are formed when ribonucleotides polymerize through the formation of phosphodiester bonds. This covalent bond joins the 3′ hydroxyl group of one ribose molecule to the 5′ phosphate group of another, releasing a molecule of water. The reaction can be represented as:

This process is catalyzed by RNA polymerase enzymes during transcription, where the sequence of a DNA template is copied into a complementary RNA strand. The directionality of the chain is defined by the 5′ to 3′ orientation, which is essential for accurate replication and expression of genetic information.

The presence of uracil and the additional hydroxyl group in ribose make RNA more prone to hydrolysis, allowing dynamic regulation and rapid turnover within cells.

• Appearance: Colorless to white fibrous solid when purified.
• Solubility: Highly soluble in water and salt solutions due to its charged phosphate backbone.
• pH Stability: RNA is stable at slightly acidic to neutral pH but degrades in basic conditions because the 2′–OH group acts as a nucleophile.
• UV Absorption: Strongly absorbs UV light at 260 nm due to aromatic base rings; used to quantify RNA concentration.
• Thermal Stability: RNA molecules denature at high temperatures (60–80°C) but can refold upon cooling if intramolecular base pairing occurs.

The phosphodiester linkages and hydrogen bonding between bases help maintain secondary structures such as hairpins, stems, and loops.

RNA molecules perform diverse roles in cellular biochemistry. Major types include:

• Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes, where proteins are synthesized.
• Transfer RNA (tRNA): Brings specific amino acids to the ribosome during translation, recognizing codons on the mRNA via its anticodon region.
• Ribosomal RNA (rRNA): Forms structural and catalytic components of ribosomes, facilitating peptide bond formation.
• MicroRNA (miRNA) and siRNA: Regulate gene expression by silencing specific mRNA molecules post-transcriptionally.

These different types of RNA collectively ensure that the genetic code stored in DNA is accurately converted into functional proteins and regulated appropriately.

Although RNA is primarily single-stranded, it can fold upon itself due to complementary base pairing within the same molecule. Common secondary structures include:

• Hairpin Loops: Regions where the strand folds back on itself, forming base-paired stems and unpaired loops.
• Bulges and Internal Loops: Unpaired bases that cause local distortions in the helix.
• Pseudoknots: Complex tertiary interactions where bases from one loop pair with complementary bases outside that loop.

These structures are critical for RNA’s catalytic activity (ribozymes) and its role in splicing, translation, and regulation.

RNA is central to gene expression and regulation. In biological systems, it acts as both an information carrier and a biocatalyst. The discovery of ribozymes demonstrated that RNA can catalyze chemical reactions, supporting the RNA World Hypothesis, which suggests that early life may have relied solely on RNA for both genetic storage and catalysis.

In biotechnology and medicine, RNA has revolutionary applications:

• RNA Vaccines: mRNA-based vaccines, such as those developed for COVID-19, use synthetic RNA to direct cells to produce antigenic proteins.
• Gene Therapy: RNA interference (RNAi) techniques target specific gene transcripts to suppress disease-causing genes.
• Diagnostics: Reverse transcription PCR (RT-PCR) detects RNA viruses and measures gene expression levels.

These applications highlight the versatility and power of RNA in modern molecular science.