Lecture 10 - Recombination

Question 1

What is genetic recombination?

Answer 1

A process of breaking and re-joining DNA strands to create new sequences, leading to genetic diversity and evolution.

Question 2

Name the four types of recombination.

Answer 2

Homologous (extended homology),
Site-specific (limited homology),
Illegitimate (no homology), and Replicative (via transposition).

Question 3

What is homologous recombination?

Answer 3

Requires extended homology; used for genome stability, repairing replication forks and DSBs, and increasing genetic diversity.

Question 4

What is site-specific recombination?

Answer 4

Relies on short homologous sequences and involves enzymes like recombinase to perform recombination at specific DNA sites.

Question 5

What is illegitimate recombination?

Answer 5

No sequence homology is required, as seen when HIV integrates into the human genome using viral integrase.

Question 6

How does transposition occur?

Answer 6

By transposable elements jumping between locations or creating and moving their copies.

Question 7

How does HIV integrate into the human genome?

Answer 7

Via illegitimate recombination using viral integrase, often in actively transcribed regions.

Question 8

What happens in HIV infection progression?

Answer 8

Silent integration → activation → production of viral particles → immune system compromise (AIDS).

Question 9

What is the lysogenic cycle in bacteriophages?

Answer 9

Phage integrates as a prophage into bacterial DNA, replicating passively. Upon stress, it enters the lytic cycle to produce viral particles.

Question 10

What is the role of site-specific recombination in bacteriophage λ?

Answer 10

Uses attP and attB sites for precise integration into the bacterial genome, facilitated by recombinase.

Question 11

What is the role of homologous recombination?

Answer 11

Repair of DSBs, chromosome segregation, genetic diversity, and horizontal gene transfer.

Question 12

Describe the process of homologous recombination.

Answer 12

1. DSB resection → 2. Strand invasion → 3. Repair synthesis → 4. Holliday junction formation → 5. Resolution (crossover or non-crossover).

Question 13

What is the Holliday junction?

Answer 13

A key structure in homologous recombination, resolved to restore DNA integrity.

Question 14

What does Rad51 recombinase do?

Answer 14

Catalyzes strand invasion and repair during homologous recombination, supported by BRCA2 in humans.

Question 15

What does recombination frequency (RF) indicate?

Answer 15

Reflects the physical distance between genes; RF < 50% indicates linkage, RF = 50% means genes assort independently.

Question 16

How are linkage maps constructed?

Answer 16

By analyzing RF between multiple genes to determine their relative positions.

Question 17

Why is homologous recombination important in meiosis?

Answer 17

Ensures accurate homolog alignment and segregation, creating genetic diversity through crossovers.

Question 18

What causes replication fork stalling?

Answer 18

RNA barriers, secondary DNA structures, tightly bound proteins, or DNA lesions.

Question 19

How are stalled forks resolved?

Answer 19

By regression into a Holliday junction, repair via homologous recombination, or other specialized repair enzymes.

Question 20

What is the significance of Rad51 in replication fork repair?

Answer 20

Rad51 repairs broken replication forks by facilitating homologous recombination.

Question 21

What is bacterial conjugation?

Answer 21

Bacterial conjugation is a process where a donor cell transfers genetic material to a recipient cell via direct contact, using a pilus.

Question 22

What role does the fertility plasmid play in conjugation?

Answer 22

The fertility plasmid enables the donor cell to form a pilus and initiate plasmid transfer, allowing both cells to eventually have a plasmid copy.

Question 23

What happens when a plasmid integrates into the donor chromosome?

Answer 23

The plasmid initiates its own transfer, but the donor chromosome follows because the plasmid and chromosome are a single replicon.

Question 24

How is gene order determined in conjugation experiments?

Answer 24

By screening recombinants for the presence of specific genes, such as A, B, or C, after mating experiments.

Question 25

What is a Wooclap integration?

Answer 25

A hypothetical experiment analyzing gene order by determining the least frequent gene combinations among recombinants.

Question 26

Why are circular chromosomes incompatible with meiotic recombination?

Answer 26

Circular chromosomes can create dicentric chromosomes during meiosis, leading to double-stranded breaks and chromosome instability.

Question 27

How does recombination contribute to genome maintenance in prokaryotes?

Answer 27

It helps repair broken replication forks and maintains genome stability.

Question 28

What are the key outcomes of recombination in eukaryotes?

Answer 28

Recombination increases genetic diversity, ensures chromosome segregation in meiosis, stabilizes linear chromosomes, and prevents cancer in large organisms.

Question 29

What is bacterial conjugation?

Answer 29

A process where a donor bacterial cell transfers genetic material (plasmid or part of its chromosome) to a recipient bacterial cell via a pilus.

Question 30

What are the steps in bacterial conjugation?

Answer 30

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.

Question 31

How can plasmids integrate into the chromosome during conjugation?

Answer 31

Plasmids integrate by homologous recombination.
The donor cell can transfer both plasmid and parts of its chromosome.

Question 32

What do the WooClap experiments show?

Answer 32

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.

Question 33

What are the issues with circular chromosomes during recombination?

Answer 33

Circular chromosomes can form dicentric chromosomes during crossover in meiosis.

This creates random double-strand breaks, which can disrupt genome integrity.

Question 34

How does recombination contribute to genome maintenance in prokaryotes and eukaryotes?

Answer 34

In prokaryotes: Stabilises genomes by repairing broken replication forks.

In eukaryotes: Maintains linear chromosomes, assists in meiosis, and ensures proper chromosome segregation.