BSC2010_5-2_Spring_2025

X-Ray Diffraction Patterns: Generated patterns supported the helical hypothesis of DNA.
Atomic Distance: Provided critical data on the arrangement of atoms within the DNA molecule.

Helical Diameter: The data elucidated the helical structure had a specific diameter, paving the way for understanding base pairing.

Breakthrough Paper: Published in Nature, detailing the chemical structure of DNA.
Role of Franklin’s Data: Essential to piecing together the DNA puzzle, including nucleotide relationships and base pairing rules.
Significance: Ushered in a transformative era in biology, impacting the understanding of genetics, mutation, and gene expression.

Double Helix: DNA consists of two strands that wrap around each other, running antiparallel (head to tail).
Base Pairing:
- Hydrogen Bonds: Form between paired bases:
  - Guanine (G) pairs with Cytosine (C) via 3 hydrogen bonds.
  - Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds.
- Melting Process: Heat disrupts hydrogen bonds, allowing DNA strands to separate.
Base Pairing Details: Approximately 10.5 base pairs per turn of the helix.

Grooves in Structure: Space-filling models reveal:
- Major Groove: More space, where proteins bind to influence gene expression.
- Minor Groove: Less space, still allows protein binding.

Base Stacking Effects: Hydrophobic interactions and van der Waals forces stabilize DNA structure, ensuring alignment and stability through base pairing.

Temperature Effects:
- Increasing temperature causes base unstacking and breaks hydrogen bonds.
- Renaturation occurs rapidly after cooling, with zippering effect facilitating quick re-pairing of complementary regions.

Parental Strands as Templates: Each strand serves as a template for new complementary strands, facilitating correct sequencing (A-T, C-G).
Replication Models:
- Conservative: Entirely new DNA strand synthesized, keeping original intact.
- Semiconservative: Each new DNA molecule consists of one original and one new strand.
- Dispersive: Original strands interspersed with new segments.

Meselson-Stahl Experiment: Used 15N and 14N nitrogen isotopes in E. coli to demonstrate that DNA replication is semiconservative based on density differences observed in successive generations.

Origin of Replication: The starting point for DNA replication, where the double helix unwinds.
Leading vs. Lagging Strands:
- Leading Strand: Continuously synthesized in the 5' to 3' direction.
- Lagging Strand: Synthesized discontinuously in Okazaki fragments, requiring RNA primers for each fragment.

Key Functions:
- Helicase: Unwinds the DNA double helix.
- SSB Proteins: Stabilize single-stranded DNA.
- DNA Polymerase III: Synthesizes new DNA strands using the old strand as a template.
- DNA Ligase: Joins Okazaki fragments and finalizes DNA strands.

Error Rates: Exceptionally low, with rates of 10^-8 to 10^-9; human genome replication errors are minimized due to several factors:
- Hydrogen bonding stability between A-T and G-C pairs aids mismatch prevention.
- DNA polymerase's active site prevents incorrect pairing, and mismatched pairs are removed through proofreading.
- Other enzyme systems are in place for additional repair.

Complementary Template Rule: Each strand serves as a template to ensure the new strand's sequence aligns with the parent strand.
Melting Mechanism: DNA unwinds at ORI to form replication forks, where synthesis can occur in both directions.
Conclusive Characteristics: Replication is termed semi-conservative due to the incorporation of one original and one new DNA strand in each resulting molecule.