DNA synthesis and repair: Revision session topics Flashcards
Define Damethalation
Tells enzymes what the new strand is
What is the role of Dammethylase?
GATC sequence are targets fro the Dam methylase after replication. During the period before this methylation occurs, the non-methylated strand is the target for repair mismatched bases
Explain the process of repair of mismatched places
Replication error generates (A and T converted to A and C)
Non-methylated adenine targets strand for repair
Non-methylated strand is degraded (C)
DNA synthesis corrects error
What is the role of MutS?
Binds Mismatches on un-methylated DNA strands
How does MutS complete its role?
Checks methylation to work out which is the wrong or newly made strand
Recognises a mismatch and translocates to a GATC site
Cleaves the unmethylated strand at the GATC
Endonucleases degrades the strand from the GATC to the mismatch site
What is the role of MutSL
Binds to mismatches on unmethylated DNA strands
MutS dimer binds to mismatch
MutL dimer binds to MutS
MutS translocates to GATC
MutH joins complex at GATC sequence
Explain MutS/MutC repair replication slippages
The MutS/MutL system initiates repair of mismatches
produced by replication Slippage
Replication Slippage generates a single-stranded loop
Explain MutS/MutC repair replication slippages: How is a single-stranded loop formed?
MutS binds to the mismatch
MutC binds
The mismatch is removed by exonuclease, helicase, DNA polymerase and ligase
Excision repair systems: What does the LevR System do?
Makes incisions, 12 base pairs apart on both sides of damaged DNA, removes the DNA between them and re-synthesises new DNA
What is the general role of an Excision repair system?
Each system removes mispaired or damaged bases from DNA and then synthesizes an new stretch of DNA to replace them
Excision repair system: Explain Incision
An endonuclease recognises the damaged area in the DNA and isolates it by cutting the DNA strand on both sides of the damage
Excision repair system: Explain Excision
A 5-3 endonuclease removes a stretch of the damaged strand
Excision repair system: Explain synthesis
Polymerase synthesizes replacement DNA
Excision repair system: Explain Ligase
Seal nick
Excision-repair removes and replaces a stretch of DNA that includes the damaged bases
How does the Uvr system remove damaged materials?
Deformation of DNA
UvrA recognises damage and binds with UvrB
UvrA released, UvrC binds
UvrC nicks DNA or both sides of the damage
UvrD unwinds regions, releasing damaged strands
How is Mammalian excision repair triggered?
Directly removing of damage base from DNA
Triggers the removal and replacement of stretch and polynucleotides
The nature of the base removal reaction determines which of two pathways for excision repair is activated
What does the PolS/6 pathway do?
Replaces a long polynucelotide stretch
What does the Pol Beta pathway do?
replaces a short stretch
Mammalian excision repair: What are the enzymes that remove DNA called?
Glycosylases and lyases
Mammalian excision repair: How is uracil removed from DNA?
By Glycosylates, cleaving the bond to deoxyribose bising H2O but lyase takes the reaction further by opening the sugar ring using NH2
explain non-homologous end-joining
Ligates blunt ends
Major mechanism to repair the double strand breaks
Non-homologous end joining requires recognition of the broken ends, trimming of overhanging ends and/or filling , followed by ligation
