Lecture 10 - Recombination Flashcards
What is genetic recombination?
A process of breaking and re-joining DNA strands to create new sequences, leading to genetic diversity and evolution.
Name the four types of recombination.
Homologous (extended homology),
Site-specific (limited homology),
Illegitimate (no homology), and Replicative (via transposition).
What is homologous recombination?
Requires extended homology; used for genome stability, repairing replication forks and DSBs, and increasing genetic diversity.
What is site-specific recombination?
Relies on short homologous sequences and involves enzymes like recombinase to perform recombination at specific DNA sites.
What is illegitimate recombination?
No sequence homology is required, as seen when HIV integrates into the human genome using viral integrase.
How does transposition occur?
By transposable elements jumping between locations or creating and moving their copies.
How does HIV integrate into the human genome?
Via illegitimate recombination using viral integrase, often in actively transcribed regions.
What happens in HIV infection progression?
Silent integration → activation → production of viral particles → immune system compromise (AIDS).
What is the lysogenic cycle in bacteriophages?
Phage integrates as a prophage into bacterial DNA, replicating passively. Upon stress, it enters the lytic cycle to produce viral particles.
What is the role of site-specific recombination in bacteriophage λ?
Uses attP and attB sites for precise integration into the bacterial genome, facilitated by recombinase.
What is the role of homologous recombination?
Repair of DSBs, chromosome segregation, genetic diversity, and horizontal gene transfer.
Describe the process of homologous recombination.
- DSB resection → 2. Strand invasion → 3. Repair synthesis → 4. Holliday junction formation → 5. Resolution (crossover or non-crossover).
What is the Holliday junction?
A key structure in homologous recombination, resolved to restore DNA integrity.
What does Rad51 recombinase do?
Catalyzes strand invasion and repair during homologous recombination, supported by BRCA2 in humans.
What does recombination frequency (RF) indicate?
Reflects the physical distance between genes; RF < 50% indicates linkage, RF = 50% means genes assort independently.
How are linkage maps constructed?
By analyzing RF between multiple genes to determine their relative positions.
Why is homologous recombination important in meiosis?
Ensures accurate homolog alignment and segregation, creating genetic diversity through crossovers.
What causes replication fork stalling?
RNA barriers, secondary DNA structures, tightly bound proteins, or DNA lesions.
How are stalled forks resolved?
By regression into a Holliday junction, repair via homologous recombination, or other specialized repair enzymes.
What is the significance of Rad51 in replication fork repair?
Rad51 repairs broken replication forks by facilitating homologous recombination.
What is bacterial conjugation?
Bacterial conjugation is a process where a donor cell transfers genetic material to a recipient cell via direct contact, using a pilus.
What role does the fertility plasmid play in conjugation?
The fertility plasmid enables the donor cell to form a pilus and initiate plasmid transfer, allowing both cells to eventually have a plasmid copy.
What happens when a plasmid integrates into the donor chromosome?
The plasmid initiates its own transfer, but the donor chromosome follows because the plasmid and chromosome are a single replicon.
How is gene order determined in conjugation experiments?
By screening recombinants for the presence of specific genes, such as A, B, or C, after mating experiments.
What is a Wooclap integration?
A hypothetical experiment analyzing gene order by determining the least frequent gene combinations among recombinants.
Why are circular chromosomes incompatible with meiotic recombination?
Circular chromosomes can create dicentric chromosomes during meiosis, leading to double-stranded breaks and chromosome instability.
How does recombination contribute to genome maintenance in prokaryotes?
It helps repair broken replication forks and maintains genome stability.
What are the key outcomes of recombination in eukaryotes?
Recombination increases genetic diversity, ensures chromosome segregation in meiosis, stabilizes linear chromosomes, and prevents cancer in large organisms.
What is bacterial conjugation?
A process where a donor bacterial cell transfers genetic material (plasmid or part of its chromosome) to a recipient bacterial cell via a pilus.
What are the steps in bacterial conjugation?
Donor cell forms a pilus to connect to the recipient cell.
Plasmid DNA is nicked and transferred to the recipient.
Both cells replicate the plasmid DNA, resulting in two cells with the plasmid.
How can plasmids integrate into the chromosome during conjugation?
Plasmids integrate by homologous recombination.
The donor cell can transfer both plasmid and parts of its chromosome.
What do the WooClap experiments show?
They illustrate recombination frequencies and crossover events in conjugation. Specific genes like A, B, and C are transferred, and the frequency of recombinants (e.g., ABC, AbC) can help determine gene order and distance.
What are the issues with circular chromosomes during recombination?
Circular chromosomes can form dicentric chromosomes during crossover in meiosis.
This creates random double-strand breaks, which can disrupt genome integrity.
How does recombination contribute to genome maintenance in prokaryotes and eukaryotes?
In prokaryotes: Stabilises genomes by repairing broken replication forks.
In eukaryotes: Maintains linear chromosomes, assists in meiosis, and ensures proper chromosome segregation.
