Genetic Analysis and Mapping in Bacteria and Bacteriophages
Topic 5: Genetic Analysis and Mapping in Bacteria and Bacteriophages
Key Concepts and Learning Outcomes
Analyze the work of Lederberg and Tatum, as well as Davis, to explain genetic material transfer in bacteria via direct physical contact.
Outline the steps of conjugation via F factors.
Compare and contrast different types of plasmids.
Overview
Key terminology and concepts relevant to genetic transfer in bacteria include:
F+ cell: A bacterium that contains the F factor.
Bacterial Chromosome: The genetic material within bacteria.
F factor: A plasmid that enables fertility and conjugation between bacteria.
Bacteriophages: Viruses that infect bacteria, often referred to as phages.
Bacterial Growth Curve
Typical Bacterial Population Growth Curve illustrates three phases:
Lag Phase: Initial adaptation to the environment (no cell division).
Log Phase (Exponential Growth): Rapid cell division and population increase.
Stationary Phase: Nutrient exhaustion stops growth; cell death begins to equal cell division.
Mechanisms of Genetic Transfer in Bacteria
Bacteria engage in genetic exchange through several mechanisms:
Conjugation:
Requires direct contact between donor and recipient cells.
DNA, often a plasmid, is transferred from donor to recipient.
Transduction:
Occurs via bacteriophages that transfer DNA from one bacterium to another.
Involves insertion of bacterial DNA into the genome of a new host through recombination.
Transformation:
Involves uptake of free DNA from the environment by a competent bacterium.
This process also requires recombination, where the foreign DNA replaces a homologous region in the recipient's chromosome.
Genetic Recombination in Bacteria
Genetic Recombination: Refers to the exchange of genetic material where bacterial chromosomes can receive DNA from genetically distinct donors.
The result is an altered genotype, akin to meiotic crossing over in eukaryotes.
Discovery of Conjugation
The Work of Lederberg and Tatum
In 1946, researchers Lederberg and Tatum demonstrated conjugation in E. coli using two auxotrophic strains, each requiring specific nutrients:
Strain A: required methionine and biotin.
Strain B: required threonine, leucine, and thiamine.
After growing these strains separately and later mixing them, they isolated prototrophs demonstrating genetic recombination between the strains.
Conjugation Steps in Bacteria
Process of Conjugation
Contact: F+ (donor) and F- (recipient) cells connect through an F pilus (sex pilus).
DNA Transfer: One strand of the F factor is nicked and moves across to the recipient cell through the conjugation tube.
Synthesis: The recipient synthesizes the complementary strand, thus bringing it to the F+ configuration after ligation of the DNA.
The Davis Experiment
Conducted by Bernard Davis, it confirmed that cell-cell contact is essential for chromosome transfer.
A U-tube with a filter allowed media passage but not bacterial cells. No prototrophs were recovered, indicating direct contact is critical for genetic recombination.
High-Frequency Recombination (Hfr) Strains
Certain strains can retain and transfer the F factor at high rates (Hfr).
Mutagenic experiments show that these strains can recombine at significantly higher frequencies than regular F+ strains.
Interrupted Mating Technique
Developed by Wollman and Jacob, this technique differentiates between Hfr and F+ strains by timing the mating process and disrupting it to analyze gene transfer order.
Specific genes transfer sooner in the process, with a defined timeline for when individual genes can be detected in the recipient.
The experiment demonstrated the linear and time-dependent nature of gene transfer.
Conversion of F to Hfr and Gene Transfer
Gene adjacency during conjugation leads to the transfer order, affecting which genes enter the recipient.
Most cells remain F- because the entire chromosome rarely passes through before mating ends.
Plasmid Biology
Characteristics of Plasmids
Plasmids are small, double-stranded circular DNA, often responsible for traits like drug resistance.
Key types include:
Fertility (F) Factor - Enables conjugation.
R Plasmid - Contains resistance transfer factors and confers antibiotic resistance.
Col Plasmid - Produces proteins toxic to other bacteria.
Transformation Process
Steps of Transformation
Binding: Extracellular DNA binds to receptors at the cell surface.
Entry: DNA enters the cell, and one strand degrades while the other pairs with host DNA.
Recombination: The transforming strand incorporates into the host chromosome.
Cotransformation
Cotransformation refers to simultaneously transferring multiple linked genes during transformation.
Bacteriophage Lifecycle
Phage Types
Lysogenic Cycle: Viral DNA integrates into the bacterial chromosome as a prophage, replicating with it.
Lytic Cycle: Virus hijacks bacterial machinery to produce virions, lysing the host cell to release new phages.
Bacteriophage Discovery of Transduction
Transduction involves viral-mediated genetic exchange between bacteria.
Early experiments observed the recombination of auxotrophic strains in Salmonella, confirming that bacterial DNA could be transferred by phages.
Genetic Recombination Studies
Studies by Seymour Benzer
Investigated intragenic recombination using the rII locus of phage T4. - It facilitated the identification and mapping of mutations through complementation between strains and resulted in distinctive plaque formation patterns.
Deletion Testing
Benzer's deletion testing identified the localization of mutations within the rII locus and addressed how mutations could impede recombination unless fixed by pivotal complementary interactions.
References and Resources
Video links and supplementary resources on transformation, transduction, and conjugation provide additional context and clarification for these complex genetic processes.