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Questions and Answers
What is the process called when two DNA molecules with nearly identical sequences exchange genetic information?
What is the process called when two DNA molecules with nearly identical sequences exchange genetic information?
What is the role of RecA in homologous recombination?
What is the role of RecA in homologous recombination?
Which type of recombination involves homologous sequences and requires specific proteins?
Which type of recombination involves homologous sequences and requires specific proteins?
What distinguishes specialized transduction from generalized transduction?
What distinguishes specialized transduction from generalized transduction?
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Which of the following is NOT a mechanism of genetic recombination?
Which of the following is NOT a mechanism of genetic recombination?
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What is the primary function of the RecBCD complex in E. coli?
What is the primary function of the RecBCD complex in E. coli?
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Which recombination type occurs in the absence of RecA?
Which recombination type occurs in the absence of RecA?
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What is required for the stable maintenance of DNA in bacterial cells?
What is required for the stable maintenance of DNA in bacterial cells?
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What is one requirement for conjugation to occur?
What is one requirement for conjugation to occur?
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What occurs during natural competence induction?
What occurs during natural competence induction?
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Which protein processes the modified precursor comX?
Which protein processes the modified precursor comX?
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Which component greatly reduces the transfer of multiple F factors into the same cell?
Which component greatly reduces the transfer of multiple F factors into the same cell?
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In which situation are cells signaled to induce com genes?
In which situation are cells signaled to induce com genes?
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What is the function of the Relaxasome during conjugation?
What is the function of the Relaxasome during conjugation?
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What mechanism is used by F plasmid for replication?
What mechanism is used by F plasmid for replication?
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What is the role of TraM in the Relaxasome complex?
What is the role of TraM in the Relaxasome complex?
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Study Notes
Genetic Recombination
- Genetic recombination is the exchange of genetic information between DNA molecules
- This process occurs through an intricate series of enzymatic reactions
- It involves pairing of DNA molecules and breaking/rejoining phosphodiester bonds
Types of Genetic Recombination
- Homologous recombination: Involves regions of nearly identical DNA sequences
- Site-specific recombination: Occurs at specific DNA sequences
- Illegitimate recombination: Doesn't require specific DNA sequences or homology
Homologous Recombination Substrates
- DNA sequences that will be recombined are called homology
- Proteins involve:
- Identical or similar DNA sequences
- Ability to form hydrogen bonds
- Enzymes like endonucleases, exonucleases, RecA, and ligase
- Products involved are 50-100 base pairs or the entire genome
Chi Sites
- Crossover hotspot instigator (Chi)
- About 1000 Chi sites in E. coli
- RecBCD complex: 330 kDa, containing exonuclease V, helicase, and ATPase
- 10 to 50 RecBCD molecules per cell
- RecA binds per 5 base pairs of DNA
- It is a DNA-dependent ATPase
- Facilitates strand unwinding by breaking hydrogen bonds, thereby promoting recombination with other DNA molecules
RecA Loading and Reciprocal/Non-reciprocal Break-Join
- RecA loading facilitates strand exchange
- Reciprocal break-join is a type of homologous recombination. Non-reciprocal break-copy is another type of homologous recombination.
Earlier Models of Recombination
- Breakage and rejoining: Two DNA molecules break at homolgous positions and rejoin.
- Copy choice: DNA polymerase switches between homologous DNA molecules, acting as replication templates.
Holliday Model
- Start with two homologous dsDNA molecules
- Single-strand breaks occur at the same site in both strands of same polarity.
- Dissociation and assimilation
- Ligation forms Holliday junction
- Branch migration
- Isomerization
Single-Strand Invasion Model (Meselson and Radding)
- Single-strand breaks occur in one strand
- Dissociation and assimilation
- DNA polymerase I fills in gap
- D-loop formation (due to displaced strand)
- D-loop digestion
- Ligation
- Resolution by nicking to separate joined DNA molecules
Site-Specific Recombination
- Occurs between specific DNA sites
- Cutting catalyzed by integrase, then exchange and ligation.
- Site is a target sequence that can be recognized by integrase, and the excision and integration of DNA requires the participation of integrase and site-specific proteins.
Illegitimate Recombination
- Occurs in the absence of RecA and DNA homology or specific sites
- Leads to spontaneous DNA rearrangements, involving deletions and duplications
- Associated with Short Sequence Repeats (SSRs)
Significance of Genetic Recombination in Bacteria
- Genetic diversity: Primary source of diversity in bacterial populations, crucial for adaptation to varied environments (temperature, nutrient availability, toxins).
- Evolutionary innovation: Allows bacteria to acquire new genes for traits like antibiotic resistance and virulence factors.
- Horizontal gene transfer (HGT): Movement of genetic material between organisms, not via parent-offspring transmission.
Mechanisms of Gene Transfer Among Bacteria
- Vertical transmission: Plasmids are passed from parent to offspring during replication.
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Horizontal transmission: Three key mechanisms:
- Transformation: Direct uptake of free DNA from the environment.
- Transduction: Transfer of DNA via bacteriophages (viruses that infect bacteria).
- Conjugation: Transfer of genetic material between bacteria through a pilus.
Transduction (specific components)
- Generalized transduction: Bacteriophage can package random pieces of bacterial DNA instead of just its own.
- Specialized transduction: Bacteriophage integrates into bacterial chromosome, then excision occurs causing phage to only pick specific host genes during the process.
Transformation
- Bacteria take up free DNA from environment
- DNA binding proteins, nucleases, free nucleotides, and RecA proteins are involved.
Natural Competence
- Induced when cell density increases, or optimal to poor growth conditions are shifted
- Cell pathways monitor nutrient depletion.
- Signals cells to induce competency genes
Quorum Sensing System
- ComX and ComQ proteins modify precursor ComX into smaller active peptides.
- Active ComX is excreted, affecting cell density, and is involved in cell-cell communication
Conjugation
- Requires donor and recipient cells
- Donors must contain a conjugative plasmid encoding conjugation genes.
- One conjugative E. coli plasmid is called an F plasmid (conjugative plasmid).
Map of Drug Resistance Plasmid
- Shows various genes on the drug-resistance plasmid (e.g., transfer region, origin, resistance genes)
F cell and Hfr cell
- F+ cells donate to F-cells via rolling circle replication
- Hfr cell is a high-frequency recombination cell
- F’ cells donate F-plasmid which also contains chromosomal DNA
Notes on specific sections as needed (e.g., from certain pages).
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Description
This quiz covers the fundamental concepts of genetic recombination, including the processes and types involved. You will learn about homologous, site-specific, and illegitimate recombination, as well as important proteins and substrates involved in these mechanisms. Test your knowledge on Chi sites and the enzymatic reactions that facilitate genetic exchange.
