Molecular Basis of Inheritance

Chapter 16: The Molecular Basis of Inheritance

Nucleotide Structure
  • Components of DNA Nucleotide: Includes a sugar (deoxyribose), phosphate group, and nitrogenous bases.
  • Nitrogenous Bases:
    • Thymine (T)
    • Adenine (A)
    • Cytosine (C)
    • Guanine (G)
  • Structure:
    • Sugar-phosphate backbone
    • 3' end and 5' end to indicate directionality of the strand.
Discovery of DNA Structure
  • Key Scientists: James Watson, Francis Crick, and Maurice Wilkins.
  • Techniques: Used X-ray crystallography to deduce the structure of DNA.
  • Recognition: Shared Nobel Prize; Rosalind Franklin was ineligible due to her early death in 1958.
Structural Features of DNA
  • DNA Model: Consists of bases that are 0.34 nm apart.
  • Full Turn: Completed every 10 base pairs (3.4 nm).
  • Diameter of DNA: Approximately 2 nm.
Base Pairing Rules
  • Watson-Crick Pairing:
    • Adenine (A) pairs exclusively with Thymine (T)
    • Guanine (G) pairs exclusively with Cytosine (C)
  • Chargaff’s Rules: In any organism, the amount of A = T and the amount of G = C.
DNA Replication
  • Basic Principle: Each strand serves as a template for building new strands during replication.
  • Model: Semi-conservative model where each new DNA double helix contains one parent strand and one newly synthesized strand.
Initiation of Replication
  • Origins of Replication: DNA replication begins at specific sites called origins, creating a "replication bubble."
  • Parental vs Daughter Strands: The parental DNA unwinds and two new daughter strands are formed.
Key Enzymes and Proteins in DNA Replication
  • Helicase: Unwinds the DNA double helix at the replication fork.
  • Single-strand Binding Proteins: Stabilize single-stranded DNA.
  • Topoisomerase: Alleviates strain ahead of the replication fork.
  • Primase: Synthesizes RNA primer.
  • DNA Polymerases: Enzymes that add nucleotides to the growing DNA chain; requires a template strand and primer.
    • DNA Pol III: Extends DNA nucleotides on leading strand and lagging strand.
    • DNA Pol I: Replaces RNA primers with DNA nucleotides.
Mechanism of Elongation
  • Antiparallel Elongation: DNA strands must be synthesized in the 5' to 3' direction.
  • Leading Strand: Synthesized continuously toward the replication fork.
  • Lagging Strand: Synthesized discontinuously in segments (Okazaki fragments) that are later joined by DNA ligase.
Important Concepts in Lagging Strand Synthesis
  • Okazaki Fragments: Short segments of DNA synthesized on the lagging strand.
  • Joinage: DNA ligase connects Okazaki fragments to form a continuous strand.
Proofreading and DNA Repair
  • DNA Polymerases: Proofread the newly synthesized DNA for errors and perform mismatch repair.
  • Nucleotide Excision Repair: Specific repair mechanism that replaces damaged stretches of DNA.
Telomeres and Aging
  • Telomeres: Special nucleotide sequences at the ends of eukaryotic chromosomal DNA, which help postpone erosion of genes during replication and are connected with aging.
Concluding Thoughts
  • The processes associated with DNA structure and replication highlight the intricate relationship between form and function in biological inheritance.