7 - DNA Damage and Repair Flashcards
Name 5 causes of DNA damage
- 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
Name 6 types of DNA lesions
- ss break
- ds break
- bulky adduct
- interstrand crosslink
- base missmatch
- base alkylation
How does DNA replication increase chances of a lesion?
- 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
When is DNA structural integrity altered?
- DNA free ends signal backbone breaks
- ssDNA accumulates
- base pairing is incorrect
- sugar is not deoxyribose
- bases are not only ATCG
When do DNA lesions become mutations?
- when DNA lesions are repaired incorrectly or not at all
can mutations be repaired?
no as they dont alter the structural integrity of the DNA
Explain DNA repair of small damage with backbone intact and which repair mechanisms does this work for?
- cut - a small gap from 1-100 nucleotides
- patch - DNA polymerase fills gap. Ligase seals the nick
- works for:
- mismatch
- nucleotide excision
- base excision
Explain the repair of thymine dimers
- photolyase - light activated enzyme that will repair T-T dimers
- light creates damage but also activates repair mechanisms
How do double stranded breaks happen?
- broken replication forks
- unseparated sister chromatids in mitosis
- ionising radiation
- some chemicals (chemo drugs)
What are causes of interstrand cross-links?
- endogenous metabolites (acetaldehyde)
- some chemicals (chemo drugs)
How are double stranded breaks repaired?
- ligation of broken ends - erroneous (insertions or deletions)
- homologous recombination
give examples of erroneous repair of ds breaks
- 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)
How are inter stand cross links repaired?
- they are not a problem unless the cell is replicating
- they are then turned into ds breaks and repaired this way
how do interstrand cross links work in cancer drugs?
- cancer cells much more likely to be replicating
- drugs causing interstrand cross links used
- can kill health cells also
Give 4 examples of gross chromosomal arrangements
- large deletions
- large inversions
- large insertions
- translocations
How can gross chromosomal rearrangements happen?
erroneous DSB repair
What is the DNA damage response in bacteria?
- SOS response
- cell division inhibitor genes activated - replications continues
- homologous recombination genes activated
- translesion synthesis (TLS) DNA polymerase genes activated
When is the SOS response triggered in bacteria?
- extensive DNA damage
what is the tole of TLS DNA polymerase?
- highly prone to errors
- used as ds DNA much safer than ss DNA
- increases mutagenesis
what is the role of halting division but continuing replication in SOS response?
- 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
What is pathway of G2 checkpoint activation in yeast?
- 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
explain the DNA damage checkpoints in the cell cycle
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
When are mutations transmitted to offspring?
when they are in the germline, not only in somatic cells
What is the effect of a mutation that increases mutation rate in carcinogenesis?
- 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
