MCM Final - DNA Damage and Repair Flashcards
depurination
removal of a purine
- remove base from backbone - link to deoxyribose hydrolyzed
spontaneous lesions
small chemical modification (single base pair)
deamination
cytosine changed to uracil
bulky lesions
cause a large change in DNA strand
covalent linkage
bulky lesion caused by oxidative species, UV light, or carcinogens
-cause a change in DNA helix structure
copy errors
DNA polymerase incorrectly adds base pair
mutagen
agent (physical or chemical) that causes an alteration in DNA
base analog
mutagen structurally resembles purine or pyrimidine and inserts into DNA strand
intercalating agent
mutagen that insert between double helix
usually flat with multiple rings
cause stretching of double helix and DNA polymerase adds extra bases opposite to it
direct acting agent
chemically react with DNA directly
indirect acting agent
require metabolic conversion before actively altering DNA
cytochrome p-450
mechanism by which an indirect acting agent is converted into a mutagen that can alter the DNA
UV radiation
shorter wavelengths - more energy - more damage
UV-A (320-400nm) - induces oxidative damage
UV-B (280-320nm) - induces dimers between pyrimidines (C or T)
UV-C (180-290nm) - doesn’t make it through ozone
ionizing radiation
causes ionization
- loss of electron - unstable reactive radical
alpha particle
helium nuclei
beta particle
high speed electron
damage due to ionizing radiation?
single strand break
double strand break **
base damage
cross linking
base excision repair
repair spontaneous lesions (single base pair)
1 DNA glycosylase scans DNA and removes damaged bases
2 AP endonuclease cuts phosphodiester backbone where empty base spot is
3 DNA polymerase adds nucleotide correctly
4 DNA ligase seals nick
DNA glycosylase
in base excision repair
- scans DNA for damaged bases and removes them - leaves backbone in tact
AP endonuclease
in base excision repair
-cleaves phosphodiester bond where DNA glycosylase has removed a base pair
nucleotide excision repair
repairs bulky lesions (larger)
1 multienzyme complex scans for distorted DNA helix
-cleaves phosphodiester backbone on either side of distortion
2 DNA helicase removes the single strand cleaved out
3 DNA polymerase adds nucleotides
4 DNA ligase seals the nick
multienzyme complex
in nucleotide excision repair
-scans for distorted DNA helix and cleaves phosphodiester bonds on each side of distortion
mismatch repair
correct DNA mismatches of DNA polymerase
follow behind replication
-recognize strand breaks (okazaki fragments - lagging)
MutS and MutL bind to mismatched base
-direct excison of DNA between the single strand break and mismatch
3’ to 5’ exonuclease activity
proofreading during replication
-by DNA polymerase
back-up polymerases
when 3’ to 5’ exonuclease doesn’t work
less accurate - take guess at what sequence should be
transcription coupled repair
RNA polymerase II stalls at lesion
-on template strand only
directs repair machinery to the site
CSB recognizes stalled RNA polymerase II
TFIIH remodels RNA polymerase II
XPG cuts DNA
Lesion repaired and polymerase restarts
double strand break causes
radiaton, errors, and oxidation
non-homologous end joining
less accurate, ends brought together and ligated
-loss of nucleotides
homologous recombination
more accurate
-sister chromatids used as a template
Process of non-homologous end joining
1 Ku70/80 recognizes free double strane ends
2 recruits DNA-PK which acts as a kinase to recruit ligase
3 DNA ligase ligates the ends
Ku 70/80
recognize free double strand break during nonhomologous end joining
DNA-PK
recruited to double strand break by Ku70/80 during nonhomologous end joining
-act as kinase to recruit DNA ligase
Process of Homologous Recombination
1 double break strands are broken to yield 3’ single strand ends
2 Rad51 pairs with 3’ overhang
-causes it to bind with sister chromatid
3 DNA synthesis occurs using sister chromatid as a template
(holliday junction is formed, but then resolved)
4 newly synthesized DNA strand then used as template for other broken strand
5 DNA ligation
Rad51
binds free 3’ ends of double strand breaks during homologous recombination
-allows the end to bind with sister chromatid as template
DNA damage checkpoints
G1 to S
Slow S
S to M
ATM protein
generates intracellular signals in response to spontaneous DNA damage
associates with damage and phosphorylation of downstream kinases
-kinases then act on target proteins
-p53 - target usually bound by Mdm2 (destroyed)
-when phosphorylated - stimulate synthesis of p21
-bind G1/S-Cdk or S-Cdk complex and inhibit them
Chk1 and Chk2 - block activation of M-Cdk
p53
inhibited when dephosphorylated by association with Mdm2
- when phosphorylated: - stimulates transcription of p21 - which binds G1/S-Cdk or S-Cdk and inhibit them
Mdm2
bind p53 targeting it for destruction
p21
its transcription activated by p53
- binds G1/S-Cdk or S-Cdk and inhibits them - arrest of cell cycle entry
Chk1 and Chk2
block activation of M-Cdk
Telomere shortening triggers what?
activation of p53
-causes cell cycle arrest
