unit 14 - gentics

UNIT 14: Bacterial Genetics

Introduction to Bacterial Genetics

  • Focus on the study of genetic mechanisms in bacteria.

  • Exploration of methods used to study bacteria and how genetic variation occurs.

Questions to Consider

  • Advantages of Using Bacteria for Genetic Studies:

    • Bacteria have simple genetics, short generation times, and can be manipulated easily.

  • Auxotrophs:

    • Mutant strains that require an additional supplement to grow, as they cannot synthesize certain compounds.

  • Genetic Variation in Bacteria:

    • Occurs through mutations, conjugation, transformation, and transduction.

  • Plasmids:

    • Small, circular DNA molecules independent of chromosomal DNA.

  • Experiments on Genetic Exchange by Conjugation:

    • Key studies that established the process of genetic transfer through physical contact between cells.

  • Mapping Bacterial Genes via Interrupted Mating Experiments:

    • Techniques to determine the relative positions of genes based on their transfer during conjugation.

  • Hfr and F' Bacterial Strains:

    • Hfr (high-frequency recombination): Bacterial cells with integrated F factors.

    • F' (F prime): Bacteria containing the F factor along with other chromosomal genes.

  • Transformation of Bacteria:

    • Uptake of free DNA from the environment leading to genetic changes.

  • Mapping Genes by Transformation:

    • Analysis of traits in transformed bacteria to determine gene locations.

  • Transduction:

    • The process where bacterial DNA is transferred from one bacterium to another via bacteriophages.

  • Mapping Bacterial Genes by Transduction:

    • Study of gene transfer and recombination through viral vectors.

Learning Objectives

  • Understand modern methods for studying bacteria.

  • Define key terms:

    • Plasmid: Small circular DNA that replicates independently.

    • Episome: Plasmid that can integrate into the chromosome.

    • F Factor: A plasmid that enables conjugation.

  • Summarize gene exchange mechanisms:

    • Conjugation: Contact-dependent transfer of genetic material.

    • Transformation: Uptake of naked DNA.

    • Transduction: Gene transfer via bacteriophages.

  • Identify bacterial cell types and their roles in conjugation:

    • F+ cells: Donor cells with a fertility factor.

    • F- cells: Recipient cells without the fertility factor.

    • Hfr cells: High-frequency donor cells.

    • F' cells: Cells with extrachromosomal F factors containing genes.

  • Map genes based on various methods of genetic exchange.

Analyzing Bacterial Cultures

  • Cultures can be liquid or solid (media types).

  • Variability in bacterial phenotypes exists among strains of the same species.

Conjugation in Bacteria

  • Does genetic exchange occur in bacteria?

    • Experiment with combining two auxotrophic strains in complete media (CM) generates prototrophs, indicating genetic exchange.

  • Definition Reminders:

    • Auxotrophs: Require supplements to grow.

    • Prototrophs: Wild type strains that can grow on minimal media (MM).

Is Contact Necessary for Genetic Exchange?

  • Genetic exchange via conjugation requires direct contact between cells.

  • Utilization of streptomycin to assess cell division and validate the necessity of contact.

  • Unidirectional Transfer: Genes transfer from F+ donor to F- recipient.

    • Donors: F+ cells with fertility factor (F factor).

    • Recipients: F- cells without F factor.

    • F+ cells confer the ability to donate chromosomal genes during conjugation, including genes for sex pilus formation.

Mechanism of Conjugation

  • Sex Pilus Formation:

    • Conjugation is facilitated by sex pili, structures only produced by F+ strains acting as attachment sites for other bacteria.

  • Genetic Material Transfer:

    • A subset of F+ cells can become Hfr (high-frequency recombination) cells, allowing for transfer of chromosomal DNA.

Hfr Strains and Chromosome Mapping

  • Definition of Hfr Strain:

    • Hfr strains act as donors during conjugation but low rates of obtaining the F+ status in recipients.

  • In Hfr x F- conjugations, genetic exchange is high, but the recipient remains F-.

Experimental Procedure for Chromosome Mapping:

  1. Mix Hfr Donor and F- Recipient Cells: Collect a large number of cells.

  2. Disrupt Conjugation: Utilize a blender after varying time intervals to interrupt the mating process.

  3. Plating Surviving Colonies: Apply selective conditions to isolate transformed bacteria based on nutrient requirements (e.g., streptomycin).

  4. Testing Survivors: Analyze colonies for susceptibility to agents and metabolic capabilities (e.g., ability to metabolize lactose).

  5. Data Interpretation: Assess which genetic traits are present based on observed growth patterns.

Conclusion from Mapping Studies:

  • The distance between genes is inferred from the time of their entry during interrupted mating.

  • Mapping exercises reveal relative distances and possible gene locations based on tranfer times in E. coli.

Transformation in Bacteria

Types of Transformation

  1. Natural Transformation:

    • Occurs without any external assistance; bacteria can uptake DNA spontaneously.

  2. Artificial Transformation:

    • Utilizes specific techniques to facilitate DNA uptake in laboratory settings.

  • Competent cells are those that can uptake DNA.

  • Historical Discovery:

    • Transformation was first observed by Frederick Griffith in 1928 using strains of Streptococcus pneumoniae.

Mapping via Transformation (Cotransformants)

  • The process involves fragmented donor DNA entering the recipient cell, becoming integrated through crossover events.

  • Genes close together exhibit a higher cotransformation rate, meaning proximity increases the likelihood of co-incorporation during transformation.

Transduction: Viral-Mediated Gene Transfer

  • Generalized Transduction:

    • Transfers random segments of bacterial genes.

  • The process entails bacteriophages injecting their DNA into bacterial cells, leading to the formation of recombinant chromosomes.

Mapping Genes via Transduction

  • Gene proximity influences the likelihood of simultaneous transduction events, with closer genes being cotransduced more frequently than those that are distantly located.

Summary of Genetic Exchange Mechanisms

  • Cytoplasmic Conjugation: Merges genetic material from donor to recipient through physical contact.

  • Transformation: Naked DNA uptake from the environment enhances genetic diversity in bacterial populations.

  • Transduction: Viral action allows for genetic material to be transferred between bacteria, creating recombinant strains.

Review Questions

  1. What are the three mechanisms of genetic exchange in bacteria?

    • Conjugation, transformation, transduction.

  2. How is chromosomal DNA transferred between bacteria?

    • Typically transferred from Hfr cells to F- cells where recombination takes place.

  3. Identify the differences between F+, Hfr, and F' cells.

  4. Explain the significance of cotransformation and its relationship to gene mapping.

  5. Differentiate between natural and artificial transformation.

  6. Discuss the implications of transduction for genetic diversity in bacterial populations.

Clicker Questions

  1. Distinction between auxotrophs and prototrophs:

    • Auxotrophs require supplements to grow; prototrophs can survive on minimal media.