Lecture 4 Flashcards Preview

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Flashcards in Lecture 4 Deck (23):

At which two points can the cell cycle be stopped if genome is damaged?

Restriciton point: Blocks passage into S phase

Intra-S phase: Blocks DNA replication


What three routes can a cell take after checks for DNA damage at restriciton point?

Divide - enter S phase

Arrest - pause to rest

Die - undergo apoptosis 


Which gene product is commonly compromised in cancer cells and hence cause the cell to loss ability to deal with DNA damage 'decisions'?



Which type of mutations ofP53 are commonly seen?

Alterations in arganine248, which binds the DNA backbone, and other DNA binding residues

Mutations in the zinc finger motif, which is vital for the interaction between loop 2 and 3 of the p53 to allow DNA binding. 


Which Cdki is regulated by P53?

p21 is upregulated by P53 


What role does p21 play?

Inhibits Cdk2 (entry into S phase)

Inhibits PCNA (halts ongoing DNA replication)



What are the roles of PCNA?

DNA polymerase 'sliding clamp'

Holds polymerase onto the DNA and increases possible length of product by several orders of magnitude

Also attracts DNA repair factors to arrested DNA replication forks (not inhibited by P21)


What is the other way a cell can deal with damage if it has lost its p53?


The DNA damage response in S1


Why is the DNA damage response being targeted as a possible cancer therapy?

If cancer cell has lost p53 dependent cell cycle arrest then knocking out DNA damage response should cause cell death. But healthy cells will be unaffected due to still having p53 pathway available


What are the four broad classes of molecule involved in DNA damage response?

  1. Sensors
  2. Mediators
  3. Tranducers/Effectors
  4. Responders


What is the main role of the transducer molecules?

Amplify the once local signal to spread across the entire genome 


Outline the general cascade for recruiting repair proteins to sites of damage

A PI3 Kinase (PIK) family kinase is recruited to the site of damage by partner proteins where they phosphorylate signal proteins to recruit repair proteins 


Outline an example cascade for recruiting repair proteins to sites of DS break damage

Nbs1, part of the MRN complex, recruits ATM which phosporylates CHK2 and causes signalling.

CHK2 can then phosphorylate cdc25A leading to its destruction and inhibiting its ability to remove the inhibitory phosphate group from cdk2, causing cell cycle arrest.

Additionally AMT and the MRN complex can then cuase DNA repair through a variety of mechanisms


How will ATM 'choose' between homologous repair or non-homologous end joining?

NHEJ is the default pathway but HR will be used if in S phase - measures the level of CDK which are higher in S phase


Why does CHK2 offer a quick damage response?

Constantly available in the genome and does not need to be made when damage discovered


What is activated by replicative stress?

CHK1 via ATR


Germ line muations of what protein are found in familial breast and ovarian cancers?


50% for breast

70% for ovarian


What is the role of BRCA1?

Scaffolding protein for all proteins involved in damage repair


How is damaged flagged by ATM/ATR?

Phosphorylation of  which causes ubiquitination


What non-repair protein is attracted by this H2AX flag?

Cohesin - which acts ot hold the chromatids togther


What is the outcome of chronic activation of DDR?

Deactivation and then hyper mutation


What can cause replication forks to slow down or stall?

  • Disruption of nucleotide pools
  • DNA damage/obstructions 
  • DNA secondary structures stop unwinding of DNA


Why would a stalled replication fork be dangerous?

Transiently in single stranded form