GENE221_LECTURES

Course Overview

  • Course: GENE221 2025 Molecular and Microbial Genetics

  • Instructor: Associate Professor Htin Lin Aung

  • Department: Microbiology and Immunology

  • Email: htin.aung@otago.ac.nz

Lecture Objectives

  • Understand the differences between:

    • Complementation

    • Recombination

  • Learn molecular mechanisms of recombination

Historical Context

  • 1950s: Key developments in genetics

  • Key Researcher: Seymour Benzer

Experimental System

  • Model Organism: Bacteriophage T4

  • **Process:

    • Uninfected cell

    • Adsorption of phage to host cell

    • Entry of phage nucleic acid

    • Assembly of phages

    • Lysis of host cell

Rationale for Using Bacteriophage

  • Advantages:

    • Easily and rapidly grown

    • Genetically simple organisms

    • Probes for genetic mechanisms

    • Large-scale analyses possible

Bacteriophage Plaques

  • Clear areas in a bacterial lawn

  • Indicate bacteriophage activity

    • Opaque: Lawn of bacteria

    • Clear: Plaques

rII Genes of Bacteriophage T4

  • Wild-type T4 can infect two E. coli strains: K and B

  • rII Mutants:

    • Large plaques on strain B

    • No plaques on strain K

Complementation

  • Definition: Wild-type phenotype from two different mutant genomes

  • Benzer's experiment with rII mutants in E. coli K12

  • Outcome:

    • Complementation occurs when mutations are in different genes

    • No complementation if mutations are in the same gene

Recombination Analysis

  • Definition: Generates new gene combinations

  • Details of Benzer's recombination analysis

  • Infect E. coli B: Pairs of mutants used for recombination

Testing Recombination

  • Progeny phage infection on E. coli K

  • Wild-type can grow; mutant cannot

Molecular Mechanisms of Recombination

  • Key Features:

    • Exchange of genetic material between chromosomes

    • Involves breakage and rejoining of DNA

  • Holliday Junction Model: Proposed by Robin Holliday

    • Supports available genetic information with refinements

Key Concepts of Holliday Model

  • Alignment of DNA sequences

  • Breakage and rejoining of DNA

  • Branch Migration: Creates heteroduplex DNA

  • Resolution: Leaves DNA molecules with new allele combinations

Recombination in E. coli

  • Key Enzymes Involved:

    • RecBCD: Nicks, unwinds, degrades DNA

    • RecA: Catalyzes base-pairing with target DNA

    • RuvA, RuvB: Cause branch migration

    • RuvC: Resolves Holliday junctions

Summary of Key Takeaways

  • Complementation indicates if mutations are in the same or different genes

  • Recombination results in genetically different progeny

Lecture Road Map

  • Lecture 1: One-gene-one-enzyme hypothesis

  • Lecture 2: Spontaneous mutations and mutagens

  • Lecture 3: Mechanisms of spontaneous mutations

  • Lecture 4: Mechanisms of mutagens

  • Lecture 5: Complementation and Recombination

Contact Information

  • Building: Microbiology Building

  • Email: htin.aung@otago.ac.nz