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.