Theme 4-2 replication

Theme 4, Module 2: Replication

Overview of Module

• Investigate early research on DNA replication.

• Understand the process of creating identical daughter strands from parental templates.

• Evaluate the roles of proteins in replication.

• Compare eukaryotic and prokaryotic DNA replication.

• Discuss the significance of telomere length conservation.

Unit 1: Transmission of Information

• DNA as a Macromolecule: Determines cellular characteristics.

• Influential Figures:

  • Francis Crick, James Watson, and Rosalind Franklin elucidated DNA's helical structure.

• Base Pairing:

  • Purines are paired with pyrimidines, providing stability through hydrogen bonds.

• Copying Mechanism:

  • Watson and Crick hypothesized that DNA's structure suggests a copying mechanism, concluding that the base-pairing allows for replication.

Unit 2: Evidence for the Semiconservative Model

• Scientific Revolution: Hypotheses on DNA replication methods arose.

• Hypotheses Considered:

  • Conservative: Parental strands rejoin post-replication.

  • Dispersive: Mixture of old and new strands.

• Meselson and Stahl Experiment:

  • Cultured E. coli in 15N (heavy) and switched to 14N (light).

  • Showed distinct DNA bands via centrifugation, confirming semiconservative replication.

  • Results indicated DNA consists of one parental and one new strand after replication.

Unit 3: Replicating DNA

• Origin of Replication:

  • Initiates in S-phase of the cell cycle.

  • Prokaryotes: One origin for circular DNA.

  • Eukaryotes: Multiple origins for linear chromosomes.

• Process of DNA Replication:

  • Template strand is replicated from 3' to 5', producing a daughter strand elongating in the 5' to 3' direction.

  • Requires primers for initiation, synthesized by RNA primase.

• Leading and Lagging Strands:

  • Leading strand: Continuous synthesis, one primer required.

  • Lagging strand: Discontinuous synthesis, multiple Okazaki fragments requiring separate primers.

Unit 4: Replicating Eukaryotic Chromosomes

• Issues with Linear DNA:

  • Lagging strand cannot complete 5' ends post-priming, leading to shorter DNA molecules in eukaryotic cells.

• Telomeres:

  • Protect against DNA erosion through repetitive sequences (e.g., TTAGGG in humans).

  • Shortening observed during cell division except in germ and stem cells due to telomerase activity.

• Telomerase:

  • A ribonucleoprotein that extends telomeres by adding repeats using reverse transcriptase capabilities.

  • Critical for maintaining chromosome integrity across generations, impacting aging and cancer research.

Summary of Key Points

• DNA replicates in a semiconservative manner in both prokaryotes and eukaryotes.

• Eukaryotic replication has multiple origins due to genome complexity.

• Essential protein machinery is required in both types of cells for DNA synthesis.

• Telomere length conservation is crucial for the integrity of genetic information during replication, especially in germ cells and stem cells.