Genetic Analysis and Mapping in Bacteria and Bacteriophages (Ch. 6) - Study Notes

Genetic Analysis and Mapping in Bacteria and Bacteriophages

Benefits of Studying Genetics in Bacteria

Bacteria offer unique advantages for genetic studies due to their simple genome structure, rapid reproduction, and ease of manipulation. These features make them ideal for investigating fundamental genetic principles and rare genetic events.

Organism	Genome Size	Genome Ploidy	Reproduction	Ease of Propagation
Homo sapiens	3.2 × 109 bp	diploid	sexual	expensive food, long generation time, few progeny
Drosophila melanogaster	1.8 × 108 bp	diploid	sexual	cheaper food, shorter generations (weeks), more progeny
Escherichia coli	4.6 × 106 bp	haploid	asexual	cheapest food, shortest generations (minutes), billions of progeny in hours

• Relative simplicity: Bacterial genetics are less complex than eukaryotes.

• No allelic interactions: Haploid genome means no dominance or recessiveness.

• Clonal progeny: Offspring are genetically identical to the parent.

• Cost-effective and enables study of rare events: Large populations allow detection of statistically rare genetic changes.

Bacterial Culture

Bacterial growth requires specific nutrients, which can be manipulated to study genetic traits.

• Medium: The nutrient source used to grow bacteria.

• Minimal medium: Contains only essential inorganic nutrients (glucose, nitrogen source, salts); lacks organic molecules like amino acids.

• Complete medium: Contains all essential molecules, including amino acids and nucleotides.

• Prototroph: Wild-type bacterium capable of synthesizing all compounds needed for growth; grows on both minimal and complete media.

• Auxotroph: Mutant bacterium unable to synthesize a specific essential compound; cannot grow on minimal medium.

Genotyping with Microbial Growth and Selection

Bacterial genotypes can be determined by their ability to grow on selective media.

• Amino acid requirement: pro mutants cannot synthesize proline and require it for growth.

• Carbon source utilization: lac mutants cannot utilize lactose unless glucose is absent.

• Antibiotic resistance: strr mutants are resistant to streptomycin and can grow on media containing the antibiotic.

Bacterial DNA

Bacteria possess a single circular chromosome and may carry additional plasmids, which are small, circular DNA molecules.

• Binary fission: Bacterial cell division resulting in identical daughter cells.

• Plasmids: Extra-chromosomal DNA elements with their own origin of replication; can exist in multiple copies per cell.

• Examples: F (fertility) plasmid, R (resistance) plasmid.

Lateral Gene Transfer - Conjugation

Conjugation is a process where genetic material is transferred between bacterial cells via direct contact.

• Requires the F (fertility) plasmid.

• F+ cells (donors) possess the F plasmid; F- cells (recipients) do not.

• Steps: Formation of conjugation pilus, transfer of T strand, rolling circle replication, synthesis of new F plasmid in recipient.

F+ Cells vs Hfr Cells

Hfr (high-frequency recombination) cells arise when the F plasmid integrates into the bacterial chromosome.

• F plasmids contain genes for conjugation, origin of transfer (oriT), and IS elements.

• Homologous recombination between IS elements on F plasmid and chromosome forms Hfr cells.

Conjugation by Hfr Cells and Exconjugant Selection

Hfr cells can transfer chromosomal genes to recipients, but transfer is often incomplete.

• Recipient cells incorporate donor DNA via homologous recombination.

• Researchers select exconjugants by screening for donor and recipient-specific DNA.

Time-of-Entry Genome Mapping with Hfr Cells

Mapping bacterial genes is possible by measuring the time required for gene transfer during conjugation.

• Genes closer to the origin of transfer enter the recipient cell first.

• Mapping is based on the appearance of donor alleles over time.

F' Plasmids Create "Partial Diploids"

F' plasmids are formed when the F plasmid excises from the chromosome, sometimes carrying chromosomal genes with it.

• Partial diploids (merodiploids) contain two copies of certain genes: one on the chromosome, one on the F' plasmid.

Recap of Conjugation Outcomes

Conjugation Outcome	Exconjugant Converted to Donor State?	Donor Bacterial Genes Transferred to Exconjugant?
F+ × F-	Yes, F- → F+	No
Hfr × F-	No	Yes
F' × F-	Yes, F- → F'	Yes

Lateral Gene Transfer - Transformation

Transformation involves uptake of free DNA from the environment by a competent recipient cell.

• Donor cell lysis releases DNA.

• Competent cells incorporate DNA into their genome if sequence homology exists, forming a heteroduplex.

• After binary fission, one daughter cell is transformed.

Bacteriophages

Bacteriophages are viruses that infect bacteria, with two main life cycles: lytic and lysogenic.

• Lytic cycle: Phage replicates and lyses host cell.

• Lysogenic cycle: Phage DNA integrates into host genome and replicates passively.

Lateral Gene Transfer - Transduction

Transduction is the transfer of bacterial genes via bacteriophages.

• During the lytic cycle, host DNA fragments are packaged into phage particles (transducing phages).

• Injected donor DNA can recombine with recipient chromosome.

• Cotransduction frequency: Used to map bacterial genomes by measuring the likelihood of two genes being transferred together.

Key Equations and Concepts

• Gene mapping by time-of-entry: $\text{Distance (min)} = \text{Time for gene transfer}$

• Cotransduction frequency: $\text{Cotransduction frequency} = \frac{\text{Number of cotransductants}}{\text{Total transductants}}$

Example

If a bacterial colony grows on minimal medium plus proline but not on minimal medium alone, it is a pro auxotroph. If it grows on streptomycin, it is strr (resistant).

Additional info: These notes expand on the original slides by providing definitions, context, and examples for each genetic concept, ensuring a comprehensive and self-contained study guide for exam preparation.