7 - DNA Damage and Repair

Question 1

Name 5 causes of DNA damage

Answer 1

• UV light - thymine dimers - covalently bonded thymines
• Ionising radiation - ss or ds break
• alcohol - adlehyde - interstrand crosslinks - covanelt bonds between bases
• oxidative damage - oxygen can leak out of mitochondria
• mechanical stress - DNA may break

Question 2

Name 6 types of DNA lesions

Answer 2

• ss break
• ds break
• bulky adduct
• interstrand crosslink
• base missmatch
• base alkylation

Question 3

How does DNA replication increase chances of a lesion?

Answer 3

• unpaired bases are exposed are more likely yo react with chemically active molecules (ROS)
• missed or added nucleotides
• missmatches
• ribonucleotides incorporated
• unpacked chromatin looses some protection to UV

Question 4

When is DNA structural integrity altered?

Answer 4

• DNA free ends signal backbone breaks
• ssDNA accumulates
• base pairing is incorrect
• sugar is not deoxyribose
• bases are not only ATCG

Question 5

When do DNA lesions become mutations?

Answer 5

• when DNA lesions are repaired incorrectly or not at all

Question 6

can mutations be repaired?

Answer 6

no as they dont alter the structural integrity of the DNA

Question 7

Explain DNA repair of small damage with backbone intact and which repair mechanisms does this work for?

Answer 7

• cut - a small gap from 1-100 nucleotides
• patch - DNA polymerase fills gap. Ligase seals the nick
• works for:
• mismatch
• nucleotide excision
• base excision

Question 8

Explain the repair of thymine dimers

Answer 8

• photolyase - light activated enzyme that will repair T-T dimers
• light creates damage but also activates repair mechanisms

Question 9

How do double stranded breaks happen?

Answer 9

• broken replication forks
• unseparated sister chromatids in mitosis
• ionising radiation
• some chemicals (chemo drugs)

Question 10

What are causes of interstrand cross-links?

Answer 10

• endogenous metabolites (acetaldehyde)
• some chemicals (chemo drugs)

Question 11

How are double stranded breaks repaired?

Answer 11

• ligation of broken ends - erroneous (insertions or deletions)
• homologous recombination

Question 11

give examples of erroneous repair of ds breaks

Answer 11

• two broken chromosomes - reciprocal translocations
• telomere added to a break - other end will be lost as it does not have a centromere - terminal deletion
• break ligated to the telomere of another chromosome - non-reciprocal translocations - dicentric chromosome (two centromeres, very unstable)

Question 12

How are inter stand cross links repaired?

Answer 12

• they are not a problem unless the cell is replicating
• they are then turned into ds breaks and repaired this way

Question 13

how do interstrand cross links work in cancer drugs?

Answer 13

• cancer cells much more likely to be replicating
• drugs causing interstrand cross links used
• can kill health cells also

Question 14

Give 4 examples of gross chromosomal arrangements

Answer 14

• large deletions
• large inversions
• large insertions
• translocations

Question 15

How can gross chromosomal rearrangements happen?

Answer 15

erroneous DSB repair

Question 16

What is the DNA damage response in bacteria?

Answer 16

• SOS response
• cell division inhibitor genes activated - replications continues
• homologous recombination genes activated
• translesion synthesis (TLS) DNA polymerase genes activated

Question 17

When is the SOS response triggered in bacteria?

Answer 17

• extensive DNA damage

Question 18

what is the tole of TLS DNA polymerase?

Answer 18

• highly prone to errors
• used as ds DNA much safer than ss DNA
• increases mutagenesis

Question 19

what is the role of halting division but continuing replication in SOS response?

Answer 19

• allows for the repair of DNA as there will be multiple copies of the same chromosome
• if some are broken, some may be fine
• these can be used as templates in DNA replication

Question 20

What is pathway of G2 checkpoint activation in yeast?

Answer 20

• Mec1 kinase binds ssDNA
• it phosphorylates RAD9 bound to histones at the damage site
• Rad53 kinase binds phosphorylated Rad9 and undergoes autophosphorylation and addition phosphorylation by Mec1.
• phosphorylated Rad53 kinase blocks mitosis and simulates DNA repair by phosphorylating other proteins

Question 21

explain the DNA damage checkpoints in the cell cycle

Answer 21

G2 - if there is ssDNA, entry to mitosis is blocked

G1 - DSBs, entry to S-phase is blocked

S - replication fork errors (ssDNA), late origins are not fired

Question 22

When are mutations transmitted to offspring?

Answer 22

when they are in the germline, not only in somatic cells

Question 23

What is the effect of a mutation that increases mutation rate in carcinogenesis?

Answer 23

• I the first mutation increases the mutation rate 10 -100 fold the probability of the next mutations will increase 1,000 - 1,000,000 times

Question 24

Name 4 mutations that would increase the probability of acquiring a mutation

Answer 24

• DNA pol looses proof reading activity
• nucleotide synthesis is insufficient
• replication helicase loss of function, forks break more often
• leaky mitochondria (ROS)

Question 25

Name 4 mutations that would increase the likelihood of escaping correct repair

Answer 25

• mutations in DNA repair genes
• mutations in DNA damage checkpoint genes
• mutations in chromosome segregation genes

Question 26

What are BRCA genes?

Answer 26

tumor supressors

Question 27

What is the effect of a mutation in the BRCA genes?

Answer 27

Increased risk of breast cancer

Question 28

How could a BRCA mutation carrier be affected?

Answer 28

• heterozygous for BRCA mutation
• DSB happens near BRCA
• other chromosome is used to repair
• could lead to homozygous mutated BRCA