Ch 8 Genetic Analysis and Mapping in Bacteria and Bacteriophages

Ch 8 Genetic Analysis and Mapping in Bacteria and Bacteriophages

Introduction to Bacterial and Bacteriophage Genetics

Bacteria and bacteriophages are essential model organisms in genetics due to their simple structure, rapid life cycles, and ease of manipulation. Their study has provided foundational knowledge in molecular genetics, recombination, and gene structure.

• Bacteria are prokaryotes with a single, typically circular chromosome, making them haploid and ideal for genetic studies.

• Bacteriophages are viruses that infect bacteria and can mediate genetic recombination.

• Both organisms are used in pure cultures, allowing for the isolation and study of mutants and genetic variants.

• Knowledge of bacterial plasmids has enabled advances in DNA cloning and recombinant DNA technology.

Bacterial Growth and Media

Bacteria can be cultured in various media to study their growth and genetic properties.

• Minimal media contains only essential nutrients. Only bacteria capable of synthesizing all required compounds (prototrophs) can grow on it.

• Prototrophs are wild-type bacteria that can synthesize all essential organic compounds.

• Auxotrophs are mutants that have lost the ability to synthesize one or more essential compounds and require supplementation to grow.

Serial Dilutions and Colony Counting

Serial dilution is a quantitative method to estimate bacterial concentration in a culture.

• Successive dilutions are plated to obtain countable colonies.

• Each colony arises from a single bacterium; multiplying the colony count by the dilution factor gives the original concentration.

Genetic Recombination in Bacteria

Genetic recombination in bacteria forms the basis for chromosome mapping and understanding gene transfer mechanisms.

• Recombination involves the replacement of genes in one cell with those from a genetically distinct cell, altering the genotype.

• In eukaryotes, this process is analogous to crossing over.

Mechanisms of Genetic Transfer

There are three main processes by which bacteria exchange genetic material:

• Conjugation: Direct transfer of DNA from one bacterium to another via cell-to-cell contact.

• Transformation: Uptake of free DNA fragments from the environment by a bacterial cell.

• Transduction: Transfer of bacterial genes by bacteriophages.

Vertical vs. Horizontal Gene Transfer

• Vertical gene transfer: Genetic information is passed from parent to offspring within the same species.

• Horizontal gene transfer: Genetic information is transferred between unrelated cells, often across species, contributing to antibiotic resistance and speciation.

Conjugation in Bacteria

Conjugation is a process where genetic material is transferred from a donor to a recipient cell through direct contact.

• Involves F+ (donor) and F- (recipient) cells.

• Physical contact is essential, typically mediated by the F pilus (sex pilus).

• Genetic recombination can be demonstrated by mixing two auxotrophic strains to produce prototrophs.

Fertility Factor (F Factor)

The F factor is a plasmid that confers the ability to donate DNA during conjugation.

• It is a circular double-stranded DNA molecule.

• During conjugation, one strand of the F factor is transferred to the recipient, and both cells become F+.

Plasmids and Their Types

Plasmids are extrachromosomal DNA elements that replicate independently and often carry beneficial genes.

• F factor plasmids: Confer fertility and contain genes for sex pilus formation.

• R plasmids: Carry antibiotic resistance genes and have two components: RTF (Resistance Transfer Factor) and r-determinants (antibiotic resistance genes).

• Col plasmids: Encode colicins, proteins toxic to bacteria lacking the plasmid.

Transformation in Bacteria

Transformation is the process by which bacteria take up extracellular DNA, leading to genetic changes.

• Involves two steps: DNA entry into the cell and recombination with the host chromosome.

• Only competent cells (in a specific physiological state) can take up DNA.

• Transformation can be used to map bacterial genes based on cotransformation frequency.

Linked Genes and Cotransformation

• DNA fragments taken up are large enough to carry several genes.

• Genes close together can be cotransformed, indicating linkage.

Bacteriophages and Transduction

Bacteriophages (phages) are viruses that infect bacteria and can mediate genetic recombination through transduction.

• Phages can package bacterial DNA and transfer it to new host cells.

• Transduction can be generalized (random bacterial DNA is transferred) or specialized (specific genes are transferred).

The Lederberg–Zinder Experiment

This experiment demonstrated transduction in Salmonella using a U-tube apparatus, showing that genetic recombination could occur without direct cell contact, implicating phages in gene transfer.

Mechanism of Transduction

• Phage infects a donor bacterium, incorporates bacterial DNA, and transfers it to a recipient cell during subsequent infections.

• Transduction can be used for mapping bacterial genes based on cotransduction frequency.

Summary Table: Mechanisms of Genetic Exchange in Bacteria

Mechanism	Definition	Key Features
Conjugation	Direct transfer of DNA between bacteria via cell-to-cell contact	Requires F factor, sex pilus, produces recombinants
Transformation	Uptake of free DNA from the environment	Requires competence, can map linked genes by cotransformation
Transduction	Transfer of bacterial DNA by bacteriophages	Can be generalized or specialized, used for gene mapping

Key Terms and Concepts

• Prototroph: Wild-type bacterium capable of synthesizing all essential compounds.

• Auxotroph: Mutant bacterium requiring additional nutrients due to loss of biosynthetic ability.

• Plasmid: Small, circular DNA molecule independent of the bacterial chromosome.

• F factor: Fertility plasmid enabling conjugation.

• R plasmid: Plasmid conferring antibiotic resistance.

• Col plasmid: Plasmid encoding colicins, bacteriocidal proteins.

• Competence: Physiological state allowing DNA uptake during transformation.

• Cotransformation: Simultaneous transformation of two or more linked genes.

• Transduction: Phage-mediated gene transfer between bacteria.