DNA DAMAGE AND REPAIR

Question 1

DNA DAMAGE lesion

Answer 1

DNA damage is also known as DNA lesions, which disrupt the normal structure.

These occur during normal cell life, but repair mechanisms work to prevent mutations or poor cell function.

Question 2

source of DNA damage

Answer 2

External: UV light (thymine dimers), ionizing radiation (ssDNA/dsDNA breaks), alcohol (interstrand crosslinks).

Internal: Oxidative damage (ROS), mechanical stress during replication/mitosis, alcohol, drugs.

replication is the biggest source of DNA lesions
- unpacking chromatin to expose DNA to UV and IR radiation ect.

Question 3

DNA lesions vs. Mutations

Answer 3

Lesions: Can be repaired and involve structural integrity changes (backbone breaks, base mismatches).

Mutations: Permanent genetic changes that cannot be reversed.

Question 4

types of DNA lesions

Answer 4

Base pairing errors

ssDNA accumulation

Backbone breaks

Bases could be uracil or ribose-containing

base alkylation

Icl

DBS

Question 5

most dangerous DNA damage

Answer 5

Double-strand breaks (DSBs): Caused by broken forks, unseparated chromatids, radiation, or chemicals.

Interstrand crosslinks (ICLs): Caused by metabolites or chemo drugs.

Question 6

cut and patch repair system

Answer 6

Used for repairing small lesions (e.g., mismatches, added nucleotides).

CUT: The damaged strand is cut out.

PATCH: DNA polymerase fills the gap, and ligase seals the nick.

Question 7

thymine dimer repair

Answer 7

Photolyase: Cleaves thymine dimers caused by UV light, reversing the damage.

Question 8

transition from lesions to mutations

Answer 8

Unrepaired lesions lead to mutations during replication, causing incorrect base pairs (insertions or transitions).

Question 9

DBS repair pathways

Answer 9

Ligation: Error-prone method that can cause insertions/deletions.

Homologous Recombination: Uses a homologous chromosome for error-free repair.

Question 10

SOS response in bacteria

Answer 10

based on transcription activation - transcriptional response by excess ssDNA for DNA repair to occur

Triggered by excess ssDNA from degraded DNA ends due to damange by UV radiation, chemicals or stressors

Activates gene expression for DNA repair, halts replication, and delays cell division.

Question 11

erroneous repair in DNA breaks

Answer 11

Telomere addition: DBS occurs near end of chromosome, may heal the break bby adding telomeres, Causes terminal deletion, genes located there are lost

Reciprocal translocations: two non homologous chromosomes segments are exchanged or incorrectly attached.

nonreciprocal translocations - segments of one chromosomes is transferred without an exchange, leading to imbalance and loss of genetic material

Dicentric chromosomes: two centromeres formed due to 2 broken chromosomes being incorrectly fused Result in additional breaks during mitosis.

Question 12

gross chromosomal rearrangements GCR

Answer 12

Found in cancer cells and caused by erroneous DSB repair.

Includes large deletions, inversions, insertions, and translocations.

Question 13

Checkpoints in DNA damage

Answer 13

G2 Checkpoint: Monitors for DNA damage before entering mitosis.

Intra-S checkpoint: Prevents replication if there are problems, especially with DSBs.

Question 14

TLS polymerases and mutations

Answer 14

Translesion synthesis (TLS) polymerases: Synthesize DNA across lesions
prevent replication fork stalling and potential DBS

but often insert incorrect nucleotides, increasing mutation rates.

Question 15

interstrand crosslink repair ICL

Answer 15

links two strands of DNA together
ICLs block replication and transcription if not repaired.

ICLs are converted to DSBs, then repaired through homologous recombination.

Question 16

ICL and cancer

Answer 16

ICLs are highly problematic for dividing cells, making them a target in chemotherapy to kill fast-dividing tumor cells.

Question 17

acquiring a mutation - 2 step process

Answer 17

Acquiring a lesion: Damage to the DNA structure.

Escaping correct repair: Leads to a mutation if the lesion is not repaired properly.

Probability A: Lesion developing.
Probability B: Lesion escaping repair.

The overall probability of acquiring a mutation is M = A × B.

Question 18

somatic mosaicism

Answer 18

Definition: Genetic variation in cells within an individual caused by DNA changes after fertilization, leading to significant phenotypic changes.

Occurs during early mitosis: Affects a large number of cells in the body.

Examples: Mosaic Down syndrome, not actual Down syndrome, sickle cell anemia, cancer, autoinflammatory disorders.

Question 19

somatic mosaicism and aging

Answer 19

Increases with age as cells continue to replicate.

Each replication cycle introduces new mutations, passed on to daughter cells.

Mutations in somatic cells are not passed to offspring.

Question 20

mutations in carcinogenesis

Answer 20

Mutation 1 significantly increases the mutation rate.

If this mutation increases the rate 10-100 fold, the chances of acquiring further mutations rise dramatically.

Carcinogenesis: The process where normal cells transform into cancer cells.

Question 21

factors increasing MR

Answer 21

1. Increasing the probability of acquiring a lesion:

• Loss of proofreading activity in DNA polymerases.
• Insufficient nucleotide synthesis (leads to wrong nucleotides).
• Loss of function in replication auxiliary helicase (causes breaks).
• Leaky mitochondria releasing more reactive oxygen species (ROS) into the nucleus.

2. Escaping correct repair:

• Mutations in DNA repair genes.
• Mutations in DNA damage checkpoint genes.
• Mutations in chromosome segregation genes.

Question 22

external factors influencing MR

Answer 22

UV radiation, pollution, toxins: Increase the probability of acquiring DNA lesions, which in turn increases the mutation rate.

Question 23

BRCA1/BRCA2 EXAMPLE

Answer 23

BRCA1/BRCA2 mutations are common in cancers.

Individuals born BRCA +/- have a normal mutation rate.

A first mutation may cause the loss of the BRCA+ gene in breast tissue.

This increases the mutation rate, leading to an inability to repair double-strand breaks (DBs), characteristic of BRCA -/- cells.

Additional mutations follow, leading to breast cancer.

Question 24

G1 CHECKPOINT

Answer 24

cell growth stage

if DSBs in G1, then entry to S phase, where replication occurs, is blocked

because if it was allowed to continue, mutations arise

Question 25

intra s checkpoint

Answer 25

if problem in replication forks in earlier origins, later origins are not fired

Question 26

g2 checkpoint

Answer 26

if there are ssDNAA in g2, then entry to mitosis is blocked

Question 27

Post translations modifications

Answer 27

DNA damage response based mainly on PTMs with transcription activation not playing a massive role at all.

Question 28

transition mutation

Answer 28

due to mismatch mutation, a base substation where a purine replaces a purine (A-> G OR G-> A) or a pyridine replaces another (C->T OR T-> C)

Question 29

SHELTERIN

Answer 29

binds to telomeric repeats to form a protective cap and stops it form being recognised as a DNA break