Genetics- Chapter 11 D N A Replication and Recombination

Chapter 11: DNA Replication and Recombination

Learning Objectives

• 11.1 DNA is reproduced by semiconservative replication

• 11.2 DNA synthesis in bacteria involves five polymerases and other enzymes

• 11.3 Complex issues that must be resolved during DNA replication

• 11.4 A coherent model summarizes DNA replication

• 11.5 Replication control by various genes

• 11.6 Eukaryotic DNA replication is similar to bacterial but more complex

• 11.7 Telomeres address stability and replication problems at eukaryotic chromosome ends

• 11.8 Recombination is crucial for genetic exchange and DNA repair

11.1 DNA is Reproduced by Semiconservative Replication

• Template Mechanism

  • DNA strands act as templates for replication.

  • Complementarity of nitrogenous bases allows for this process.

• Modes of DNA Replication

  • Semiconservative: Each new DNA molecule consists of one old and one new strand.

  • Conservative: Two new strands form, and the original helix remains intact.

  • Dispersive: Parental strands are dispersed into two new double helices.

• Meselson-Stahl Experiment (1958):

  • E. coli grown in 15N medium showed that DNA replication is semiconservative.

• Taylor-Woods-Hughes Experiment (1957):

  • Used Vicia faba to demonstrate semiconservative replication in eukaryotes.

11.2 DNA Synthesis in Bacteria

• Polymerases Utilized:

  • DNA synthesis in bacteria involves five polymerases (DNA Pol).

• DNA Polymerase I:

  • Isolated from E. coli, directs DNA synthesis, and requires a DNA template and dNTPs.

• Chain Elongation:

  • Occurs in the 5' to 3' direction by adding nucleotides one at a time.

• Exonuclease Activity:

  • Proofreading and correction of errors during DNA synthesis.

11.3 Issues in DNA Replication

• Key Issues to Resolve:

  • Unwinding of the helix, reducing coiling, synthesizing primers, and joining gaps.

• Helicase Activity:

  • DnaA and DNA helicase help to unwind the DNA strands at the origin of replication.

• Single-Stranded Binding Proteins (SSBPs):

  • Stabilize unwound DNA strands during replication.

• Okazaki Fragments:

  • Lagging strand synthesized in segments, requiring RNA primers and later being joined by ligase.

11.4 Coherent Model of DNA Replication

• Critical Enzymes/Proteins:

  • DNA polymerases, SSBPs, helicase, and DNA gyrase are all vital for DNA synthesis.

11.6 Eukaryotic DNA Replication

• Similarity to Bacteria:

  • Shared features include unwinding DNA at ORI and bidirectional synthesis.

• Complexity in Eukaryotes:

  • More DNA, linear chromosomes, and associated structural challenges due to chromatin.

• Multiple ORIs:

  • Eukaryotic chromosomes have multiple origins of replication for efficiency.

• Eukaryotic DNA Polymerases:

  • Various polymerases perform distinct roles in nuclear genome replication and DNA repair.

11.7 Telomeres and Telomerase

• Telomeres:

  • Protect chromosome ends from degradation; consist of repetitive DNA sequences.

• Telomerase Function:

  • Ribonucleoprotein complex that adds DNA sequences to telomeres, addressing replication problems.

• Telomerase Activity:

  • Active mainly in stem cells and cancer cells; linked to cellular aging and stability.

11.8 Importance of Recombination

• Role in Genetics:

  • Essential for genetic exchange, diversity, and DNA repair processes.