DNA Structure and Replication

DNA Structure and Replication

Presented by Dr. Kherie RoweAssistant Professor of BiochemistryEmail: krowe@rossu.eduReading: Lippincott Reviews in Biochemistry, 8th Ed., Chapter 30

Page 1: Introduction to Meier-Gorlin Syndrome

  • Genetic Basis: Caused by mutations in genes such as ORC1, ORC4, ORC6, CDT1, and CDC6.

    • Function: These genes encode proteins that form the pre-replication complex.

    • Role: Regulates DNA replication initiation before cell division.

Page 2: Learning Objectives

  1. Human Genome Structure: Describe the general structure.

  2. DNA Sequence Types: Functions and proportions of centromeric DNA, repetitive DNA, telomeres, transcribed regions, and regulatory DNA.

  3. Transposable DNA: Concept and its role in genome diversity and pseudogene formation.

  4. DNA vs. RNA: Basic structural differences.

  5. Nucleotide Structures: Roles of bases, 3’ hydroxyl, and 5’ phosphate.

  6. DNA Polymerases: Functions of α, δ, ε, helicase, beta sliding clamp, DNA ligase, and topoisomerases.

  7. Direction of DNA Synthesis: Explanation of 5’ to 3’ synthesis.

  8. Proofreading Mechanism: How DNA polymerases correct errors.

  9. Replication Initiation Complex: Its relationship to cell cycle control.

  10. Telomere Maintenance: Production and maintenance mechanisms.

Page 3: Importance of Studying DNA

  • Essential Functions: Inheritance, protein coding, life instructions.

  • Applications: Genetic disease study, gene therapy design, diagnostics, forensic science, and paternity testing.

Page 4: The Human Genome

  • Composition: 46 chromosomes (22 pairs of autosomes and 2 sex chromosomes).

  • Size: Over 6 billion bases, approximately 2 meters when unwound.

  • Packaging: Extensive packaging required to fit within the nucleus.

Page 5: DNA Packaging in the Nucleus

  • Length: ~3 billion base pairs, unwound length ~2 meters.

  • Winding: DNA is wound to fit in the nucleus.

Page 6: Histones

  • Structure: Histone octamer consists of H2A, H2B, H3, and H4.

  • Function: DNA wraps around histones to form nucleosomes.

Page 7: Supercoiling

  • Mechanism: DNA supercoiling around histone proteins allows tighter packing.

Page 8: Chromatin Structure

  • Types:

    • Euchromatin: Relaxed, transcriptionally active.

    • Heterochromatin: Condensed, transcriptionally less active.

  • Histone H1: Stabilizes higher-order chromatin structures.

Page 9: Gene Expression

  • Euchromatin vs. Heterochromatin: More gene expression in euchromatin.

  • Regulation: Histone modifications (acetylation, methylation) affect transcription.

Page 12: Genome Composition

  • Transcribed Regions: 27% of the genome is transcribed into mRNAs.

    • Introns: 26% of the genome.

    • Protein Coding: Only 1.5% encodes proteins.

  • Transposable Elements: LINEs (20%), SINEs (13%), transposons (11%), heterochromatin (8%).

Page 13: Transposable Regions

  • Impact: Can alter gene expression and create pseudogenes.

  • Clinical Example: L1 insertion in clotting factor VIII gene causing hemophilia.

Page 14: Repetitive DNA Functions

  • Gene Repair: Promotes repair using copies from other chromosomes.

  • Gene Duplication/Deletion: Misalignment during recombination.

Page 15: Centromeres

  • Function: Essential for chromosome segregation.

  • Structure: Composed of satellite DNA organized in tandem.

Page 16: VNTRs

  • Classification: Minisatellites and microsatellites.

  • DNA Fingerprinting: Variation in repeat length used for identification.

Page 22: Hydrogen Bonding

  • Chargaff’s Rule: A=T and G=C pairing in double-stranded DNA.

Page 23: Double-Stranded DNA Structure

  • Characteristics: Antiparallel strands, complementary bases, phosphodiester bonds in the backbone.

Page 24: DNA Helix Structure

  • B-DNA: Right-handed, standard form under physiological conditions.

  • Drug Interaction: Cisplatin binds to DNA, causing damage in cancer cells.

Page 28: DNA Replication

  • Semi-Conservative: New strands synthesized in 5’ to 3’ direction.

  • Leading vs. Lagging Strands: Continuous vs. discontinuous synthesis (Okazaki fragments).

Page 29: Fidelity of DNA Replication

  • Proofreading Mechanism: DNA polymerases correct errors during synthesis.

Page 32: Initiation of Replication

  • G1 Checkpoint: Assesses conditions for cell division.

  • Origin Recognition: Specific sites in the genome initiate replication.

Page 35: Regulation of CDC6

  • Role in Cell Cycle: Regulates DNA replication timing and prevents re-replication.

Page 37: Telomeres

  • Function: Protect chromosome ends, prevent DNA loss during replication.

  • Structure: Repeated sequences (TTAGGG) added by telomerase.

Page 39: Key Points on DNA Replication

  • Complex Process: Involves multiple proteins and enzymes.

  • Synthesis Direction: Continuous on leading strand, discontinuous on lagging strand.

  • Proofreading: Ensures fidelity of DNA