13.1 Nucleotide Structure and DNA/RNA Architecture

Nucleotides are the building blocks of DNA and RNA, crucial for storing and transmitting genetic information. They consist of a nitrogenous base, a pentose sugar, and a phosphate group, each playing a vital role in the molecule's structure and function.

DNA and RNA have distinct architectures that reflect their different roles in cells. DNA's double-stranded helix provides stability for long-term genetic storage, while RNA's single-stranded nature allows for diverse functions like protein synthesis and gene regulation.

Nucleotide Structure

Components of nucleotides

• Nucleotides comprise three main components forming building blocks of DNA and RNA
  • Nitrogenous base determines genetic code
    • Purines: larger, double-ring structures (adenine, guanine)
    • Pyrimidines: smaller, single-ring structures (cytosine, thymine, uracil)
  • Pentose sugar provides structural backbone
    • Deoxyribose in DNA lacks 2' oxygen, enhancing stability
    • Ribose in RNA contains 2' hydroxyl group, increasing reactivity
  • Phosphate group contributes to nucleic acid backbone and overall negative charge
• Nucleoside forms when nitrogenous base bonds with pentose sugar via glycosidic linkage
• Numbering convention orients molecule
  • Carbon atoms in sugar ring labeled 1' to 5' (prime notation distinguishes from base numbering)
  • Phosphate group attaches to 5' carbon, creating 5' end of nucleotide chain
  • Nitrogenous base connects to 1' carbon through N-glycosidic bond

DNA and RNA Architecture

DNA vs RNA structure

• Sugar component differentiates DNA and RNA
  • DNA: 2'-deoxyribose increases stability by reducing hydrolysis susceptibility
  • RNA: ribose with 2'-OH group enables catalytic activities (ribozymes)
• Nitrogenous bases vary slightly
  • DNA: adenine, guanine, cytosine, thymine (A, G, C, T)
  • RNA: adenine, guanine, cytosine, uracil (A, G, C, U)
• Structure differs significantly
  • DNA: double-stranded helix stores genetic information long-term
  • RNA: typically single-stranded, forms various secondary structures (hairpins, loops)
• Base pairing follows complementarity rules
  • DNA: A-T (2 hydrogen bonds) and G-C (3 hydrogen bonds)
  • RNA: A-U (2 hydrogen bonds) and G-C (3 hydrogen bonds)
• Stability varies due to structural differences
  • DNA: more stable from double-stranded nature and deoxyribose sugar
  • RNA: less stable, prone to hydrolysis due to 2'-OH group and single-stranded structure

Role of phosphodiester bonds

• Phosphodiester bonds form polynucleotide chain backbone
  • Connects 3' carbon of one nucleotide to 5' carbon of adjacent nucleotide
  • Forms through condensation reaction between hydroxyl and phosphate groups
• Crucial for nucleic acid structure and function
  • Creates sugar-phosphate backbone, providing structural support
  • Establishes 5' to 3' directionality, important for replication and transcription
  • Contributes to overall negative charge, influencing interactions with proteins and other molecules
• Exhibits remarkable stability under physiological conditions
  • Resistant to spontaneous hydrolysis, enabling long-term genetic information storage
  • Requires specialized enzymes (nucleases) for cleavage during biological processes

DNA double helix stability

• Double helix structure characterized by specific features
  • Two antiparallel strands wind around common axis
  • Right-handed spiral forms major and minor grooves, important for protein interactions
  • Dimensions: ~2 nm diameter, 10 base pairs per turn, 0.34 nm rise per base pair
• Multiple chemical interactions stabilize structure
  • Hydrogen bonding between complementary base pairs
    • A-T: two hydrogen bonds
    • G-C: three hydrogen bonds, contributing to GC-rich regions' higher stability
  • Base stacking interactions (π-π interactions) between adjacent bases
    • Contributes significantly to overall stability
    • Influenced by base sequence and local structure
  • Hydrophobic effects drive structure formation
    • Bases oriented towards helix center, minimizing water exposure
    • Hydrophilic sugar-phosphate backbone faces outward, interacting with aqueous environment
• Environmental factors influence stability
  • Salt concentration affects electrostatic interactions between phosphate groups
  • pH impacts hydrogen bonding and base stacking (extreme pH can denature DNA)
  • Temperature increase leads to denaturation (melting), separating strands