SS

12 A MODULE 5

  • Introduction to Central Dogma

    • Understanding DNA structure and its role as hereditary material
    • Learning goals:
    • Structural differences between RNA and DNA
    • Key experiments leading to discovery of DNA
    • Identifying donors and acceptors in hydrogen bonds
    • Applying Chargaff's rules to double-stranded DNA

  • Key Features of Hereditary Material

    • Must be localized to the nucleus and part of chromosomes
    • Must replicate accurately
    • Must store information
    • Must express information

  • Experiments Leading to DNA as Genetic Material

    • Frederic Griffith's Experiment: Demonstrated bacterial transformation
    • Avery-MacLeod-McCarty Experiment: Identified DNA as the transforming principle
    • Hershey-Chase Experiment: Confirmed DNA as the genetic material by showing DNA, not protein, is inherited
    • Watson and Crick Model (1953): Described DNA as a double helix with complementary and antiparallel strands

  • Structure of DNA

    • Double Helix: Comprised of two polynucleotide chains
    • Nucleotides Composition:
    • Deoxyribose sugar (5-carbon)
    • Phosphate group (provides negative charge)
    • Nitrogenous bases (Adenine (A), Guanine (G), Cytosine (C), Thymine (T))
    • Nitrogenous Bases:
    • Pyrimidines: Cytosine (C), Thymine (T), Uracil (U — only in RNA)
    • Purines: Adenine (A), Guanine (G)

  • Base Pairing Rules

    • Complementary pairing: A with T, G with C
    • Hydrogen bonds:
    • A & T form 2 hydrogen bonds
    • G & C form 3 hydrogen bonds
    • Antiparallel orientation:
    • 5' to 3' directionality
    • Stability increases with GC content due to more hydrogen bonds required to separate

  • Hydrogen Bonds

    • Donor and acceptor:
    • Donor: The hydrogen involved in the bond, typically from nitrogen or oxygen
    • Acceptor: The nitrogen or oxygen that accepts the hydrogen

  • Melting Temperature (Tm)

    • GC-rich DNA has a higher Tm due to stronger bonds
    • AT-rich DNA has a lower Tm due to fewer hydrogen bonds

  • Tautomeric Shifts

    • Rearrangement of atoms within a base that alters hydrogen bonding potential
    • Important in base-pairing fidelity during DNA replication

  • Polarity and Orientation

    • 5' End: Phosphate group
    • 3' End: Hydroxyl group
    • Establishes directional properties necessary for DNA function

  • DNA Structure Characteristics

    • Diameter: 20 Å (angstroms) or 0.2 nm
    • Major and minor grooves:
    • Major groove is wider; important for protein binding
    • Minor groove plays a lesser but still significant role

  • Different Forms of DNA

    • B-form: Standard form described by Watson and Crick
    • A-form: Occurs under dehydrated conditions
    • Z-form: Appears as a zigzag, potential role in pathogenicity, e.g., vaccinia virus

  • Chargaff's Rules

    • In any double-stranded DNA:
    • Amount of A = Amount of T
    • Amount of G = Amount of C
    • Used to determine the potential for a DNA sample to be double-stranded

  • Examples of Chargaff's Application

    • Calculate proportions of bases in samples to verify double-stranded DNA (Criteria based on equal ratio)
    • Example: Sample with 22% A and 28% G follows the Chargaff's rules; thus, it could be double-stranded

  • Conclusion

    • Understanding DNA structure helps in grasping broader concepts in genetic replication and expression
    • Encourage further study and questions during office hours if needed.