DNA Repair And Cancer Flashcards
What 2 ways can DNA strands be damaged?
Single strand damage
Double strand damage - DNA completely separates
What happens to damaged DNA?
It is either repaired by a DNA repair mechanism or if not it becomes muatated and causes problems for the cells
What parts of DNA can be damaged?
The backbone (one side or both) and any of the bases
What are some exogenous causes of DNA damage?
Ionising radiation Alkylation agents Mutagenic chemicals Anti-cancer drugs Free radicals
What are some endogenous causes of DNA damage??
Replication errors
Free radicals
Give some specific examples of the way DNA can be damaged.
Deamination (switch C to U) Mismatching bases Double strand breaks Pyrimidine diners (bases Bond within strand) Inter strand cross links (DNA joins up) Single strand break Apurinic site (something binds instead of base) Intercalating agent (fixes between bases, DNA cant unzip)
Define DNA replication stress.
Inefficient replication that leads to replication fork slowing, staling and/or breakage
How do we try to stop DNA replication stress?
We use proofreading methods
What can cause DNA replication stress?
Replication machinery defects
Replication for progression hinderance (fork cant go further)
Repetitive DNA can lead to fork slippage
Defects in response pathways (the fixer cant fix the problem)
How is DNA proofread?
The 5” to 3” DNA polymerase joins bases onto strand
If a wrong base is added its removed by a 3” to 5” DNA exonuclease protein (runs in opposite direction)
Then 5” to 3” DNA polymerase fixes he gap
Describe replication fork progression hinderance.
This is when the replication fork cant go any further (unzip)
Could be for a number of reasons;
DNA lesions
RNA-DNA hybrids
What is fork slippage?
Repetitive DNA sequences (base sequence repeated several times) can lead o two types of slipping
Scenario 1= new strand loops our, new strand has one extra base (forward slippage/longer strand)
Scenario 2=template strand loops out, new strand is missing a base (backward slippage/shorter strand)
How can “slippage” be fixed?
In backward slippage (new strand has extra base), then anther base can be inserted on the next replication to make both strands the same length
In forward slippage (new strand too short), a base can be deleted from the other strand to make it correct length
What are trinucleotide repeat disorders?
When fork slippage leads to trinucleotide expansion
(This is backward slippage making the DNA longer by 3bp)
Diseases including HUntingtons
What is the pathology of Huntington’s disease?
The HTT gene
CAG is repeated and so polyglutamine protein is repeatedly made
A healthy person can have 2-39 repeats
A disease person has 35-121 repeats (slippage on more replications)
These protiens aggregate in the neurones (mainly Basal ganglia)
C
Is a progressive, late onset disease
What is the response pathways to damaged DNA?
Signals, then sensors, then transducers in a signal cascade
Leads to effectors which begin either;
Senescence (stop cycle), apoptosis, transcription of more DNA or DNA repair.
What are the 3 outcomes of DNA damage response?
Senescence (cell cycle stopped)
Damaged cels proliferate (damage spreads)
Or apoptosis of the cell
If DNA damage is very high or persistent what happens?
Either senescence (cell can still perform its job but damage needs to be stopped from spreading so cell woes job until death)
Cell is apotised
Where are the cell cycle checkpoints?
Temporary arrests in the cycle allow DNA to be checked.
Before entering S phase cell chicks that cell enviro is favourable for DNA, this is the G1 checkpoint
The G2 checkpoint checks that everything has been replicated and there is no damage before meiosis.
The mitosis checkpoint checks that chromosomes are attached to spindle properly before they are pulled apart.
How is damaged DNA repaired?
Diff types of damage require different types of repair
Describe base excision repair.
Deamination error has converted a C to a U
The U is detected and removed, leaving a baseless nucleotide
The base-less nucleotide is removed, leaving a small hole in DNA backbone.
The hole is filled with the right base by a DNA polymerase and the gap is sealed by a ligase.
Describe nucleotide excision repair.
UV radiation may have produced a Thymine dimer (2 sequential bases are attached)
Once detected, the surrounding DNA is opened to form a bubble
Enzymes cut the damaged region out of the bubble
A DNA polymerase replaces the excised DNA and a ligase seals the gap
Describe a Mismatch repair.
A mismatch of bases is detected in new DNA
The new strand is cut and the mispaired nucleotide and its neighbours are removed
The correct nucleotides are replaced by DNA polymerase
DNA ligase seals the gap
How can a double strand break in DNA be repaired?
1) Non-homologous end joining. The ends of each DNA is glued back together this is prone to mistake but is used in emergency’s
2) Homologous-directed repair. Using the homologous of a correct non-broken strand the cel works out which bases should fill the gap. The gap is then filled with the correct bases.
What is the multi-step cancer model?
Mutations accumulate over time turning a cell into a premalignant then malignant cell. It is not instant there are several steps before malignancy
How can alignancy be stopped/prevented?
DNA damage response can prevent carcinogenisis by repairing DNA or stopping the cell at a certain stage or malignancy and preventing further damage.
What is intra-tumour heterogeneity?
It means that a cancerous mass may have multiple cell types , is not just made from one clone.
Heterogeneity promotes tumour evolution
How does heterogeneity of a tumour effect the response to chemo?
They cells that are vulnerable to chemo are destroyed by it. However, the cells that aren’t continue to grow and then the mass is made out of resistant cells only.
Or the chemo can damage certain cells further also changing th tumour and possibly its resistance.
Explain the synthetic lethality strategies.
If a cell has a mutation in one gene its non-mutated gene may keep it alive, but as it keeps reproducing there are more and more damaged cells present.
So if we block/damage the healthy gene the cell recognises this damage and performs apoptosis
