RECOMBINATION

Question 1

what is homologous recombination important for in genome stability

Answer 1

A: Homologous recombination is important for:

1. Repair of broken replication forks and double-strand breaks (DSBs).
2. Accurate chromosome segregation during meiosis.

Question 2

What are the two main roles of meiotic homologous recombination in genetic diversity?

Answer 2

A: 1. Creates novel gene combinations.

2. Crossovers ensure accurate segregation of homologous chromosome pairs in meiosis I.

Question 3

Q: What are the four types of genetic recombination?

Answer 3

A: 1. Homologous recombination (extended homology).

2. Site-specific recombination (limited homology).
3. Illegitimate recombination (no homology).
4. Replicative recombination (transposition, no homology).

Question 4

What is genetic recombination?

Answer 4

A: It is the process of breaking and re-joining DNA strands, often resulting in new DNA sequences, which is essential for creating genetic diversity and evolution.

Question 5

What happens if there is no recombination in meiosis?

Answer 5

Alleles will stay the same, leading to no genetic diversity, and chromosome segregation will occur randomly, possibly leading to errors.

Question 6

What determines recombination frequency (RF) between genes?

Answer 6

A: The physical distance between genes. The closer the genes, the lower the RF, making them more likely to be inherited together.

Question 7

What is a Holliday Junction (HJ)?

Answer 7

: A structure that forms during homologous recombination when DNA strands from two molecules exchange segments.

Question 8

What is the role of HJ resolvases in homologous recombination?

Answer 8

They cleave and resolve Holliday Junctions, either resulting in crossovers (exchanging DNA between homologues) or non-crossovers (original strands stay intact).

Question 9

What is Rad51 and why is it important in homologous recombination?

Answer 9

: Rad51 is a recombinase enzyme that catalyzes the homology search and strand invasion during homologous recombination

Question 10

Q: What is the function of the BRCA2 protein in homologous recombination?

Answer 10

A: BRCA2 promotes the formation of Rad51 filaments on single-stranded DNA (ssDNA). Without sufficient BRCA2, recombination fails, which can lead to genomic instability and cancer

Question 11

Q: How does site-specific recombination differ from homologous recombination?

Answer 11

A: It involves limited homology between DNA sequences and is used for integrating DNA like phage genomes into bacterial chromosomes.

Question 12

Q: How do crossover events during homologous recombination ensure proper chromosome segregation in meiosis?

Answer 12

A: Crossovers hold homologous chromosomes together, allowing them to align properly during meiosis I, ensuring accurate segregation.

Question 13

Q: What is the role of the protein cohesin in meiosis?

Answer 13

A: Cohesin holds sister chromatids together, and separase cleaves cohesin to allow chromatid separation during anaphase.

Question 14

Q: How do double Holliday junctions (double HJ) increase genetic diversity?

Answer 14

A: Double HJs can be resolved in four different ways, two of which result in crossover events that generate recombinant chromosomes, contributing to genetic diversity.

Question 15

Q: What is illegitimate recombination and give an example of where it occurs?

Answer 15

A: Illegitimate recombination occurs without homology, such as when the HIV virus integrates into the host genome randomly.

Question 16

what are the causes of replication fork pausing

Answer 16

A: Replication forks pause due to:

RNA blocking the way.

DNA secondary structures.

Tightly bound proteins.

Heterochromatin unwinding.

Telomeres.

Question 17

What can cause replication forks to become unstable?

Answer 17

: Damage to DNA, such as damaged nucleotides or crosslinked nucleotides, can cause replication forks to pause and become unstable

Question 18

What are stalled replication forks (RF)?

Answer 18

Stalled replication forks are those that cannot pass through a replication barrier due to DNA damage or other obstacles.

Question 19

What happens when a replication fork stalls?

Answer 19

A stalled replication fork can lead to a single-stranded DNA break (ssDNA), which is then processed as a double-stranded break (DSB). Rad51 filaments form on the resected 3’ end to allow homologous repair.

Question 20

How is the replication fork re-established after a break?

Answer 20

The broken end invades the unbroken sister chromatid, and the replication fork is re-established. The two sister chromatids are connected by a nicked Holliday junction, which is resolved by Holliday junction resolvase.

Question 21

What is bacterial conjugation and how does it affect the cell?

Answer 21

Bacterial conjugation is the transfer of genetic material through a fertility plasmid, changing the sex of the cell (the recipient becomes a donor with the fertility plasmid).

Question 22

How can plasmids integrate into bacterial chromosomes during conjugation?

Answer 22

If the plasmid integrates into the donor’s chromosome, the donor’s chromosome can be transferred along with the plasmid to the recipient, allowing recombination between donor and recipient chromosomes.

Question 23

Why are circular chromosomes incompatible with meiotic recombination?

Answer 23

Circular chromosomes would need an even number of crossovers to properly segregate, which is why linear chromosomes are used in eukaryotic meiosis.

Question 24

What is the role of recombination in genome maintenance?

Answer 24

A: Recombination maintains genome integrity by:
1 .Repairing broken replication forks.

2. Ensuring genome stability via homologous recombination.
3. Contributing to genetic diversity in meiosis.
4. Allowing proper segregation of linear chromosomes in meiosis.
5. Stabilizing linear chromosomes through telomeres.

6.Suppressing cancer by downregulating telomerase in large animals and humans.

Question 25

What is the significance of homologous recombination in cells?

Answer 25

Cells have evolved homologous recombination as a key mechanism to repair broken replication forks, ensuring the stability of the genome.

Question 26

What is the importance of HR recombinases across species?

Answer 26

HR recombinases, such as Rad51 in humans, are conserved from bacteria to humans, emphasizing their fundamental role in homologous recombination and genome stability.