Lecture 7 - DNA Damage and Repair

Question 1

What is DNA damage?

Answer 1

DNA damage refers to alterations in the DNA structure, known as DNA lesions, which disrupt the normal structure of DNA.

Question 2

What is the difference between a lesion and a mutation?

Answer 2

A lesion is a temporary DNA alteration that can potentially be repaired. A mutation is a permanent change to the DNA sequence.

Question 3

What is the primary source of DNA damage in cells?

Answer 3

DNA replication is the largest source of DNA damage due to mistakes by DNA polymerase and exposure to reactive oxygen species.

Question 4

How does single-stranded DNA (ssDNA) contribute to DNA damage?

Answer 4

ssDNA, exposed during replication, is vulnerable to oxidation and other damaging reactions, leading to lesions.

Question 5

What are inter-strand crosslinks (ICLs), and why are they problematic?

Answer 5

ICLs link the two strands of DNA, preventing them from separating during replication, which can cause replication fork collapse.

Question 6

What are the two major pathways for double-stranded break (DSB) repair?

Answer 6

DSBs can be repaired by:

Non-homologous end joining (NHEJ): Error-prone, ligates broken DNA ends.

Homologous recombination (HR): Accurate, uses a homologous sequence as a template.

Question 7

How does homologous recombination repair DSBs?

Answer 7

HR uses an unbroken homologous chromosome as a template to copy the missing sequence and replace the damaged region.

Question 8

What can result from erroneous repair of DSBs?

Answer 8

Erroneous DSB repair can cause large chromosomal rearrangements like translocations, deletions, and duplications, leading to genomic instability.

Question 9

What is the SOS response in bacteria?

Answer 9

It is a transcriptional response to extensive DNA damage, activating repair genes, inhibiting cell division, and increasing mutagenesis.

Question 10

What are the three DNA damage checkpoints in eukaryotic cells?

Answer 10

G1 checkpoint: Prevents S-phase entry if damage is detected.

Intra-S-phase checkpoint: Delays late-origin firing if replication problems are detected.

G2 checkpoint: Blocks mitosis entry until DNA damage is repaired.

Question 11

How is the G2 DNA damage checkpoint activated?

Answer 11

Mec1 kinase binds single-stranded DNA, phosphorylates Rad9, which activates Rad53 kinase, leading to a halt in the cell cycle until repair is complete.

Question 12

What is the probabilistic formula for acquiring mutations?

Answer 12

M=A×B, where A is the probability of acquiring a DNA lesion and B is the probability of lesion repair failure.

Question 13

How do environmental factors contribute to mutation rates?

Answer 13

UV radiation, pollution, smoking, and toxins increase DNA lesions, elevating the chances of mutations and cancer.

Question 14

What is somatic mosaicism?

Answer 14

It refers to the presence of cells with different genetic makeups within the same organism, often increasing with age.

Question 15

Which cancers are associated with BRCA1 and BRCA2 mutations?

Answer 15

Breast and ovarian cancers commonly involve mutations in BRCA1 and BRCA2 genes.

Question 16

Why are gross chromosomal rearrangements (GCRs) significant in cancer?

Answer 16

GCRs, including translocations and aneuploidy, result from erroneous DSB repair and contribute to genomic instability in tumour cells.

Question 17

What is the role of trans-lesion synthesis polymerases in bacteria?

Answer 17

These error-prone polymerases bypass DNA damage, preventing further strand breakage during replication.

Question 18

What are the primary types of DNA lesions caused by internal cellular processes?

Answer 18

DNA lesions caused by internal processes include oxidative damage (e.g., oxo-G), deamination (e.g., cytosine to uracil), and replication errors such as mismatches or additional base incorporation.

Question 19

What role does the SOS response play in bacterial survival under DNA damage?

Answer 19

The SOS response activates transcriptional programs for DNA repair, stops cell division, promotes filamentous growth, and expresses error-prone DNA polymerases to manage extensive DNA damage.

Question 20

How does homologous recombination repair double-stranded breaks?

Answer 20

Homologous recombination uses an unbroken homologous chromosome as a template to repair the break, ensuring accurate restoration of the original DNA sequence.

Question 21

What is the end-replication problem in eukaryotic cells, and how is it solved?

Answer 21

The end-replication problem arises because lagging-strand synthesis cannot replicate the very ends of linear chromosomes. It is solved by telomerase, which extends the telomeres using an RNA template.

Question 22

What are gross chromosomal rearrangements (GCRs), and how do they relate to cancer?

Answer 22

GCRs involve large-scale DNA changes such as translocations, deletions, and inversions. These mutations are common in cancer cells and often arise from erroneous double-stranded break repair.

Question 23

What is the difference between non-homologous end-joining (NHEJ) and homologous recombination (HR) in DSB repair?

Answer 23

NHEJ directly ligates broken DNA ends but is error-prone, while HR uses a homologous template for accurate repair, requiring an unbroken chromosome.

Question 24

How does somatic mosaicism increase with age?

Answer 24

Somatic mosaicism increases with age due to the accumulation of DNA mutations over time, leading to genetic variation within the cells of an individual.

Question 25

What is the role of BRCA1 and BRCA2 in preventing cancer?

Answer 25

BRCA1 and BRCA2 are tumour suppressor genes involved in DNA repair through homologous recombination. Mutations in these genes increase the risk of breast and ovarian cancers.

Question 26

Why are single-stranded DNA regions particularly prone to damage?

Answer 26

Single-stranded DNA is more exposed to reactive oxygen species and other damaging agents compared to double-stranded DNA, making it prone to lesions like strand breaks or base modifications.

Question 27

How do DNA checkpoints function in the cell cycle?

Answer 27

DNA checkpoints (e.g., G1, intra-S, G2) monitor DNA integrity and halt the cell cycle if damage is detected, ensuring that replication or mitosis does not proceed until repairs are made.