DNA damage Flashcards
why is DNA repair critical?
Our cells needs to cope with constant endogenous and exogenous DNA
damage
what diseases can DNA damage be implicated in?
–Neurological (Parkinson’s, Alzheimer’s)
–Diabetes
–Atherosclerosis (cardiovascular diseases)
–Ageing
what are exogenous and endogenous sources of DNA damage?
exo- radiation, smoking
endo- cellular metabolism, replication stress, spontaneous
what nucletide bases are pyramidine/ purine?
pyrimidine- C,T,U
purine- A,G
what base is only in RNA/ dna?
RNA= URACIL
DNA=THYMINE
How are nucleic acids formed?
the phosphate group of one nucleotide joins with a hydroxyl group of an an adjacent nucleotide
this forms a phosphodiester bond
how many H bonds are there in a C-G and T-A dna double helix?
CG=3
TA=2
what is DNA constantly under attack from?
including active oxygen species that are by-products of metabolism.
*Many environmental agents and chemicals in food attack and modify DNA
eg hypoxia, oncogenes, mitochondrial DNA mutations, tumor suppressor genes
what are the different ways that DNA can be attacked?
oxidative attack- H
hydolysis N-H
methylation- N
What happen in aqueous solution at 37 degreese?
there is spontaneous deamination of C, A,
and G bases in DNA. C deaminates to form U, A to hypoxanthine, and G to
xanthine.
what is spontaneous depurination due to?
cleavage of the glycosyl bond connecting purines to the backbone, leaving the backbone of the DNA intact, occurs at a substantial rate.
what are depurinated sites called?
abasic (lacking a base) or AP sites
(originally meaning apurinic, lacking a purine, but since generalized to
lacking any base)
what happens in depurination?
lose a base eg guanine
what happens in deamination?
lose amonium
what can alter specific bases within DNA after replication is complete?
variety of chemical agents eg ROS- hydroxyl radical
what does the hydroxyl radical react guanine to form?
8-oxoguanine.
why is 8-oxoguaine mutagenic?
*8-Oxoguanine is mutagenic because it often pairs with adenine rather than
cytosine in DNA replication.
why does 8oxoGuanidine have serious implications?
*OxoGuanine can bind to Adenine not just Cytosine!!
*Incorrect Base Pairing
why is replication stress bad?
it can give rise to single and double stranded breaks
what are the different types of mutations?
silent- no effect on amino acid seq after single mutation
missense- changes triplet
stop codon
deletion and insertion- frame shift- multiple changes
what is the stepwise process to DNA repair?
recognise damage
stabilise region/ make it more accessible
recruit repair proteins/ remove damage
replace nucleotides
ligate
what can damage to DNA be?
–Misincorporation of a single base
–Chemical modification of bases
–Chemical cross-links between the two strands of the double helix
–Breaks in one or both of the phosphodiester backbones.
how is dna damage fixed?
DNA sensors recognise and bind to DNA
pass on message to transucers
this gets checked at chk1 and 2- mediators
and all this gets passed onto p53 responsible for cell cycle arrest if error is detected
then it will undergo repair, arrest, apoptoiss or senescence
what is the basic mechanism for which DNA repair follows?
–Recognize the offending base(s)
–Remove the offending base(s)
–Repair the resulting gap with a DNA polymerase and DNA ligase
what are the 5 basic DNA repair mechanisms?
- Base Excision Repair (BER)
- Nucleotide Excision Repair (NER)
- Mis-Match Repair (MMR)
- Non-Homologous End Joining (NHEJ)
- Homologous Recombination (HR)
what is base excision repair?
The cell uses BER to correct damaged
DNA bases or single-strand DNA
breaks
*DNA Glycolase “excises” the faulty
base
–Second strand serves as a template
*MANY binding proteins
*DNA Polymerase fills in DNA Gaps
*DNA Ligase “seals” the DNA
is base excision repair inherited?
no
what is NER?
NER acts on a variety of helix-distorting
DNA lesions
main function to remove damage caused by UV
what is XP?
autosomal recessive inherited condition
mutations in the XP genes
sensitivity to sunlight
what is MMR?
Mismatch Repair (MMR)
*Catches Damage not repaired by BER / NER
*MMR rapidly removes mis-paired nucleotides
that result from replication errors
*Mismatch-repair systems consist of at least two proteins, one for detecting the mismatch and the other for recruiting an endonuclease that cleaves
the newly synthesized DNA.
*Also repair of DNA adducts such as those
resulting from platinum-based
chemotherapeutic agents.
what is NHEJ?
Non-Homologous End Joining (NHEJ)
Double strands breaks are Critical
The cell MUST repair these as soon as
possible
*ATM Halts the cell cycle (Checkpoint)
*Rapid repair mechanism
*Repairs 85% of all ionising radiation
induced DNA damage
all stages of cell cycle
what is the disadvantage with NHEJ?
error prone
what happens in a mutation of the ATM gene?
autosomal recessive inherited condition
lack of order- poor coordination
shows up as dilated spider blood vessels
sever sensitivity to ionising radiction and higher changes of infection and developing cancers
what is HHR?
*ERROR FREE mechanism
*But only operates at S and G2 phases of cell
cycle (sister chromatid is present in cell)
*Requires the presence of sister chromatid
*Notice - BRCA1 and BRCA2
how can we use BRCA as a diagnostic and therapeutic tool?
normal cells can rely on both BRAC2 and PARP for DNA repair
BRCA mutation breast cancers are addicted to PARP
BRCA mutant breast cancers are particularly sensitive to PARP inhibition
this is known as synthetic lethality
