MolBio6 - 24 Flashcards
List 4 forms of DNA damage
Deamination, depurination, pyrimidine dimers and DNA breaks
List 3 forms of DNA repair mechanism
Base excision repair, nucleotide excision repair, homologous recombination
What does deamination and depurination cause?
DNA mutations
How are instance of deamination and depurination repaired?
Base excision repair
Outline base excision repair
Damaged base removed by DNA glycosylase, sugar removed by apurinic/apyrimidinic endonuclease, phosphate removed by phosphodiesterase, DNA polymerase extends primer:template junction, DNA ligase seals nick
What causes thymine dimers?
UV radiation
What can thymine dimers cause?
DNA replication arrest or mis-reading by polymerase
Outline nucleotide excision repair
Excision nuclease cleaves single-stranded DNA segment containing defect, DNA helicase removes damaged segment, DNA polymerase extends primer:template junction, DNA ligase seals nick
What can defective nucleotide excision repair machinery cause?
Xeroderma pigmentosum (XP)
What is XP?
Xeroderma pigmentosum - extremely sensitive to sunlight-induced skin cancer
Where is DNA repair targetted?
The most important DNA sequences - the genes
How is DNA repair targeted?
Repair machinery is physically coupled to RNA polymerase
What is the most dangerous DNA damage?
Double stranded breaks - large fragments of DNA can be lost!
How are double stranded breaks usually caused?
Ionizing radiation
How are double stranded breaks repaired?
Non-homologous end joining, or homologous recombination
Why is non-homolougous end-joining labelled ‘quick and dirty’?
Ends are redendered flush, with loss of bases, then ligated together - DNA is lost
Outline homologous recombination
Exonuclease degrades 5’ ends, RecA promotes strand invasion of undamaged chromatid, this forms heteroduplex which facilitates templated DNA synthesis of one strand only by DNA polymerase, this strand then dissociates and re-anneals to original partner, allowing second strand synthesis, DNA ligase then seals nicks
What can mutations in the homologous recombination machinery cause?
Cancer
Give 3 examples of homologous recombination mutations causing cancer
BRCA2 - breast, ovarian, prostate; ATM - leukaemia, lymphoma; FANC - fanconi anaemia (leukaemia)
How does base excision repair relate to cancer?
Can be achilles heel of BRCA2-mutated cancers - inhibition can destroy cancer because DNA cannot be repaired at all - synthetic lethality
Outline homologous recombination during meiosis
Spo11 endonuclease cleaves, Mre11 endonuclease resets 5’ ends, resection and crossover as in repair, but DNA synthesis occurs on BOTH strands, forming a double holliday junction
How can a double holliday junction be resolved?
In one of two ways - with or without crossover
Draw how a double holliday junction can be resolved WITHOUT crossover
Draw how a double holliday junction can be resolved WITH crossover
