Lec13_DNA Repair systems regulons SOS Regulon Flashcards
DNA Repair Systems
damage primary DNA structure due to exposure to chemical and physical factors.
NB that genetic material remain intact and unchanged =allow replication
Mechanisms (managed by DNA repair systems) were developed to prevent and repair DNA damage
there are four different forms of DNA repair
Different forms of DNA repair (just the name not in detail)
- Photo-reactivation
- Nucleotide Excision Repair
- Base Excision Repair
- Translesion DNA Synthesis
Photoreactivation
DNA gets damaged by UV light that results in the formation of pyrimidine dimers
microbes are resistant to UV rays when they are present in the light (and not in the dark).
this resistance is due to PHOTOLYASE
PHOTOLYASE
- repairs UV damaged DNA in presence of light
- reverse formation of pyrimidine dimers (breaking dimer bond)
Nucleotide Excision Repair
UvrABCD complex (UvrA, UvrB, UvrC, UvrD) binds across section that contains damaged/incorrect DNA bases
and cuts away section relative to damaged base of single-stranded DNA
DNA Polymerase
- fills gaps usinng opposite strand as template
DNA Ligase
- Attaches two ends of chain to one another
Excision Repair
A process whereby cells remove part of a damaged DNA strand and replace it through DNA synthesis using the undamaged strand as a template.
DNA Polymerase and DNA ligase
Base Excision Repair (4 enzymes involved)
- Glycosylase
- cuts through damaged/incorrect DNA bases at N bond between base and deoxyribose residue
- forms a baseless (apurine/apyrimidine) site - AP endonuclease
- recognize baseless sites
- cuts single strand at baseless site - DNA polymerase
- replaces section by using opposite strand as template - DNA ligase
- attaches two ends of chain to one another
Translesion DNA synthesis
- replication machinery reaches damaged DNA (before DNA has been repaired)
there is a gap formed opposite to damage DNA - DNA synthesis cannot take place - complimentary strand of second daughter helix is used to replace gap opposite damaged base
- DNA gap on second daughter DNA helix is filled with DNA polymerase 1
- DNA ligase
attaches two ends of chain in both daughter helixes
Which Strand should be repaired?
dam methylation sites are used to determine which strand must be used as template for DNA repair
Partially methylated DNA means that the mother strand is methylated but not the daughter strand
Because of that the mother strand is use as template for DNA repair
resulting:
error on daughter strand - repaired
error on mother strand - built in, future progeny will contain DNA error
Regulon
A single regulatory protein regulates transcription of a group of genes that are not contained in one operon
- process require products of excessive genes for successful regulation
- independent regulation of genes, as well as overall co-ordination of these genes
SOS Regulon
Inductive negative control
- of proteins involved in SOS DNA repair system
Two key proteins
- LexA
- RecA
consist of several other genes that code for proteins with DNA repair functions
LexA Protein
regulatory protein - repressor
binds to operators of several SOS regulon genes
- including operators of recA and lec A genes
prevents transcription of SOS related genes in SOS regulon
DNA consensus sequence
- non identical at different genes
- not in the same position
RecA
watchdog for SOS DNA repair system
- lookout for DNA damage
different functions”
- NB: Recombination
- binds to single- and double-strand DNA
- ATPase activity
- Protease activity (specific for LexA) = not functional under normal circumstances
SOS DNA Repair system -SOS reaction
excessive damage (damage at several places ) = causes multiple defects
- loss of negative supercoiling
- loss of genetic material
- cell death
cell goes into SOS reaction (error-prone DNA repair)
- large scale repair of DNA without consideration to replication
- expense of proof reading
