Module 1a Notes: DNA and RNA Structure (DNA/RNA Structure)

Module overview and scheduling
  • Module 1a: DNA and RNA structure.
  • Module 1b: DNA replication.
  • Quiz coverage: Both DNA/RNA structure and DNA replication.
  • Schedule note: A full weekend should be between the last lecture and the quiz.
  • Material: Chapter 6 is the primary reference.
  • Purpose: To establish a common foundational understanding for students from diverse backgrounds.
  • Instructor note: The lecturer is a molecular biologist, so biochemically deep questions might require peer input.
  • Presentation: PPT-focused, emphasizing basics and correct terminology.
DNA and RNA structure: foundations and components
  • Nucleic Acid Composition: DNA and RNA are nucleic acids, polymers made of nucleotides.
  • Deoxyribonucleotide (DNA): Composed of a heterocyclic base, a pentose sugar (deoxyribose), and a phosphate group.
    • Pentose Sugar Numbering: Carbons are numbered 1', 2', 3', 4', and 5'.
    • 5' carbon: Outside the ring, has a phosphate group (5' end).
    • 1' carbon: Site of base attachment.
    • 3' carbon: Bears a hydroxyl group (-OH) that forms phosphodiester bonds (3' end).
    • Canonical Orientation: DNA strands are directional, read 5' to 3'. The 5' end has a free phosphate; the 3' end has a free hydroxyl.
  • Bases: Nitrogen-containing heterocycles, planar, and hydrophobic, contributing to DNA stability via stacking.
    • Purines: Two-ring structures (larger bases).
    • Adenine (A) and Guanine (G).
    • Linkage to sugar: via the N9 atom.
    • Pyrimidines: Single-ring structures (smaller bases).
    • Cytosine (C), Thymine (T) (in DNA), and Uracil (U) (in RNA, replacing T).
    • T has a methyl group at C5; U lacks it.
    • Linkage to sugar: via the N1 atom.
    • Base Numbering: Nitrogens are typically at positions 1 and 3 in the ring.
  • Glycosidic Bond: The bond between the base and the 1' carbon of the pentose sugar.
    • Forms via a condensation reaction, releasing water.
    • Specific attachment points: N9 for purines; N1 for pyrimidines.
The phosphodiester backbone and DNA polarity
  • Phosphodiester Bond: Forms the sugar–phosphate backbone of DNA.
    • Connects the 3' carbon of one sugar to the 5' phosphate of the next nucleotide.
    • During DNA synthesis, an incoming nucleoside triphosphate undergoes this bond formation, releasing two phosphates (pyrophosphate, PPi).
    • Note on lecture vs. biochemistry: Lecture describes water release for phosphodiester bond formation; standard biochemistry emphasizes PPi release from dNTP hydrolysis driving the reaction.
  • Directionality: DNA strands are synthesized and read in the 5' to 3' direction.
    • 5' end: Free phosphate group at the 5' carbon.
    • 3' end: Free hydroxyl group at the 3' carbon.
  • Key Bonds Summary:
    • Glycosidic bond: Base to 1' sugar carbon.
    • Phosphodiester bond: 3' OH of one sugar to 5' phosphate of the next.
Base pairing, stability, and DNA properties
  • Base Planarity and Stacking: Bases are planar and hydrophobic; their stacking minimizes water contact, stabilizing the double helix.
  • Hydrogen Bonding: Specific pairing between complementary bases.
    • A-T pairing: Two hydrogen bonds.
    • G-C pairing: Three hydrogen bonds (stronger).
    • Non-covalent bonds, disrupted by heat (denaturation).
  • Hybridization: Complementary strands align via hydrogen bonds and base stacking.
  • Nomenclature and Energy Carriers:
    • Nucleoside: extSugar+extBaseext{Sugar} + ext{Base} (no phosphate).
    • Nucleotide: extNucleoside+extPhosphate(s)ext{Nucleoside} + ext{Phosphate(s)}
    • NMP (Nucleoside Monophosphate): 1 phosphate.
    • NDP (Nucleoside Diphosphate): 2 phosphates.
    • NTP (Nucleoside Triphosphate): 3 phosphates (e.g., ATP, GTP, CTP, UTP).
    • DNA Synthesis Substrates: Deoxyribonucleoside triphosphates (dNTPs).
    • During incorporation into DNA, two phosphates (pyrophosphate, ext{PP}\*i) are released, and the remaining phosphate becomes part of the backbone.
    • Polymerization: ext{dNTP} + ext{DNA}\n \rightarrow ext{DNA}\n+1 + ext{PP}\*i.
Connections, implications, and study cues
  • Foundational Significance: Understanding DNA/RNA structure, polarity, and bond types is crucial for future topics like replication, transcription, and repair.
  • Relevance: DNA structural features dictate enzyme recognition and processing in biological pathways.
  • Study Tips:
    • Master naming conventions (nucleoside vs. nucleotide).
    • Practice identifying base structures (A, G, C, T/U) and their glycosidic linkages (N9 for purines, N1 for pyrimidines).
    • Be able to trace and explain the 5' to 3' directionality.
  • Quick Reference Facts:
    • Purines: A, G; two rings; N9 to C1' link.
    • Pyrimidines: C, T, U; one ring; N1 to C1' link.
    • A-T: 2 H-bonds; G-C: 3 H-bonds.
    • DNA backbone: Phosphodiester bonds (3' OH to 5' phosphate), creating 5' to 3' polarity.
    • DNA monomers in cells: dNTPs; incorporation releases PPi and energy.