Lecture 49: DNA Repair Flashcards
DNA Damage Consequences
Short term/long term
Short term:
1) Reduced proliferation
2) Altered gene expression
30 Cell death = apoptosis
Long Term:
1) Aging
2) Diseases especially cancer
DNA Mutation
Types
Process:
Dna damage –> Replication before repair –> repair = mutation
-Induced or spontaneous mutations can occur
Spontaneous mutations:
1) Errors of replication (S-phase):
- Wrong base incorporated by DNA polymerase
- Tautomerism = chemicals can exist as a mixture of two
2) Spontaneous lesions: Chemical changes (Resting cells)
DNA Polymerase
- 5’-3’ polymerase activity
- 3’ to 5’ exonuclease activity
Bloom syndrome
-Defect in BLM gene (a DNA helicase enzyme)
DEFECT IN REPLICATION/REPAIR/RECOMB
Characteristics:
-smaller than average
-narrow chin, prominent nose and ears
*facial rash (pigment and dilated blood vessels) upon
exposure to sun
-often get diabetes and have neurological, lung and
immune system deficiencies
-Chromosomal instability = many
chromosomal breaks and sister chromatid exchanges
Dna Frameshift Mutation
-Form from slipping of DNA polymerase during replication
-Tend to occur at positions where there are base
repeats
-DNA kinks/loops = not all bases are copied
Spontaneous lesions
Main types
-Changes that occur in a resting cell due to the chemical nature of the DNA
Main types:
1) Depurination
2) Deamination
3) Oxidative damage
Depurination
- Spontaneous lesion
- Breaking of glycosidic bond between base and sugar in purine nucleotides = base lost, sugar-phos intact
- Can result in mutation if persists
Deamination
- Spontaneous lesion
- loss of amine group from a base
Oxidative damage
- Spontaneous lesion
- Oxidative products cause production of reactive oxidative cmpds
Effects:
- Damages cell
- Adds oxygen group to nucleotide bases
- Mis-pairing with A and potential transversion
Mutagens
-Increase the frequency of “normal” mutations
Ionizing Radiation
-High energy particles/rays = cellular damage, even death, DNA damage, heritable mutations
(Ex: X-rays, radiation, radioactive particles)
UV Light
- Generates deleterious products
- Aka primidine dimers/thymine dimers
-Interfere with normal pairing and block
replication
-Covalent linkages between bases on the
same strand
Example:
-Cyclobutane pyrimidine dimers or
6-4 photoproducts
Indirect Repair
1) Nucleotide Excision
- Removes more than a FEW bases around a damaged site
2) Base Excision
- repairs a SINGLE bases by removing it
3) Mismatch Repair
- repairs mismatched bases
Excision Repair Mechanism
-Same for all excisions, the specific method accounts for size of repair
1) Recognition of damage
2) Recruit endonucleases
3) Region removed
4) DNA Polymerase fills in gap
Repair mismatched bases
- Post-replicative repair mechanism
- A form of excision repair
- Mismatched bases are recognized and excised
Strand discrimination:
- Prokaryotes= methylation
- Eukaryotes= methylation and interactions w dna machinery
1) Mismatch missed by proofreading is
recognized by MSH proteins
2) Repair may occur during S-phase (if
missed by proof-reading) or in G2
when genome is scanned for errors
3) Excision of bases around mismatch
4) Repair by re-synthesis
- -MSH genes involved
Repair mismatched bases
Strand discrimination
- Post-replicative repair mechanism
- A form of excision repair
- Mismatched bases are recognized and excised
- Can remove small repeats that tend to expand
Strand discrimination: Figuring out “right” strand to remove
- Prokaryotes= methylation
- Eukaryotes= methylation and interactions w dna machinery
1) Mismatch missed by proofreading is
recognized by MSH proteins
2) Repair may occur during S-phase (if
missed by proof-reading) or in G2
when genome is scanned for errors
3) Excision of bases around mismatch
4) Repair by re-synthesis
- -MSH genes involved
Hereditary Nonpolyposis Colon Cancer
- May occur when there are mutations in genes encoding mismatch repair proteins MSH2, MLH1, PMS1, PMS2, or MSH6
- microsatellite instability in tumors = tandem repeats
Double stranded breaks
- Type of mutation
- Dangerous bc loss of genetic info/chrom abnormalities
Fixes:
1) Non-homologous end joining
2) Recombination repair (uses homolog chroms)
What happens when errors get through:
Somatic errors
Germline errors
Somatic errors (bad for us)
- Cancer
- Aging
Germline errors (bad for our kids) -Genetic disease passed on to offspring
How are DNA repair genes vulnerable to mutation?
1) Increased error rate
- Bases not repaired correctly
2) Genomic Instability
-Mutations in genes involving resting DNA repair/chrom breaks =destabilization of genome
(Ex: Blooms, XP)
Ataxia Telangiectasia
- Defect in ATM gene (11q22-23)
- Problem w DNA Double-stranded break repair
- AR disorder
- Serine-threonine protein kinase with the following:
1) detecting dna damage
2) Activating cell cycle arrest and DNA repair proteins (Ex: p53)
Symptoms:
- Blood shot eye (Ocular telangiectasia)
- Increased incidence of cancer
- Affects cerebellum and immune system
Triggers of Apoptosis
1) Extrinsic Pathway
- Lack of growth factor supply outside the cell
- Caspase 8 and 3
2) Intrinsic Pathway
- Response to genotoxic stress: Radiation, toxins, hypoxia
- Caspase 9 and 3
3) Perforin/Granzyme Pathway
- Mediated by immune system cells: Cytotoxic T cells
- Caspase 10 and 3
What does role does p53 play with apoptosis
- Communicates to the cell cycle and helps mediate cell cycle arrest instead of apoptosis
- Cells harbor mutations in p53 may continue proliferating w DNA damages = accumulation of mutations = cancer progression
Intrinsic Apoptotic Pathway
DNA damaged –> BAX produced + goes into mitochondria –> BAX releases cytochrome C from mitochondria –> Cleavage activates from a cascade –> Apoptosis
Cancer cells exhibit high levels of what proteins?
- High levels of anti-apoptotic proteins (Ex: BCL-2)
- Low levels of pro-apoptotic proteins (p53, BAX)
