The Molecular Basis of Inheritance

The Molecular Basis of Inheritance

1. Learning Objectives

  • DNA as the Genetic Material

    • Explain the significance of Griffith’s experiment.

      • Understanding the concept of transformation.

      • Avery, McLeod, and McCarty confirmed that DNA was responsible for transformation.

    • Understand the Hershey-Chase experiment.

      • Definition of bacteriophage and its lytic cycle.

    • Explore contributions of Wilkens, Franklin, Watson, and Crick to DNA structure elucidation.

    • Understand Chargaff’s rules.

    • Know the structure and components of DNA.

      • Review key topics from Chapter 5: double helix, antiparallel strands, nucleotide structure, base pairs (A, T, C, G), purines and pyrimidines, deoxyribose, hydrogen bonding.

2. DNA Replication, Repair, and Packing

  • DNA Replication

    • Describe the semiconservative model of replication.

    • Explain the process of DNA replication, including origins, replication forks.

    • Discuss the role of DNA polymerases.

    • Illustrate the antiparallel arrangement of DNA strands.

    • Distinguish between leading and lagging strands.

      • Explain synthesis of the lagging strand in relation to DNA polymerase's limitation of adding nucleotides only at the 3′ end, including Okazaki fragments.

    • Explain the roles of:

      • DNA ligase

      • Primer

      • Primase

      • Helicase

      • Topoisomerase

      • Single-strand binding proteins.

    • Discuss limitations in continuous synthesis of DNA strands.

    • Analyze roles of DNA proofreading as a repair mechanism.

    • Describe telomere structure/function and role of telomerase in cancer.

    • Understand DNA packaging in terms of chromatin, histones, and nucleosomes.

3. Genetic Information and Heredity

  • Griffith’s Experiment

    • Demonstrated transformation using two strains of Streptococcus pneumoniae: smooth (capsule) and rough (no capsule).

    • Result: Transformation of the rough strain into smooth via the uptake of killed smooth strain components.

4. Hershey-Chase Experiment**

  • A pivotal experiment proving DNA as hereditary material using bacteriophages, specifically T2.

  • Importance of confirming that DNA, not protein, acts as genetic information in viruses.

5. Chargaff’s Rules

  • Base composition variations across species.

  • Equal ratios of bases: A = T and G = C, leading to the understanding of complementary base pairing in the double helix.

6. Structure of DNA

  • X-ray Crystallography by Wilkins and Franklin showed DNA's helical structure.

    • Key observations included double helix formation, sugar-phosphate backbone configuration, base spacing.

  • Watson and Crick built DNA models conforming to observed data leading to understanding specific base pairings:

    • Purines (A, G) pair with pyrimidines (C, T).

7. Mechanism of DNA Replication

  • Description of Semi-conservative Replication

    • Each original strand serves as a template for a new strand.

  • Process of DNA Replication

    • Begins at origins where strands separate, forming replication bubbles.

    • Replication proceeds bidirectionally until the entire DNA is copied.

    • Multiple enzymes coordinate synthesis ensuring efficiency and accuracy.

8. Enzymes Role in DNA Replication

  • Helicases: unwind DNA.

  • Single-strand binding proteins: stabilize unwound strands.

  • Topoisomerases: relieve tension in helices.

  • DNA polymerases: synthesize new DNA strands requiring RNA primers.

9. Antiparallel Elongation and Mechanism Differences

  • Leading Strand: synthesized continuously towards replication fork.

  • Lagging Strand: synthesized in fragments (Okazaki fragments) away from the fork, requiring ligase to join segments.

10. DNA Proofreading

  • DNA polymerases proofread newly added nucleotides, resulting in a very low error rate, eliminating incorrect pairings efficiently.

11. Replicating Telomeres

  • Telomeres: protect gene ends with repetitive sequences, preventing gene erosion through cell divisions.

  • Shortening leads to “cellular aging” and may prevent cancer cell growth, although some cancer cells utilize telomerase to maintain telomere length.

12. DNA Packing in Eukaryotes

  • DNA wrapped around histones forms nucleosomes, creating the chromatin structure crucial for fitting DNA into nuclei.

  • Chromatin resembles beads on a string when in a less compact state.