Flashcards in EXAM2_L15_DNA_REPAIR Deck (22)
Mutations in germ (sex) cells result in?
Mutation in somatic cells result in?
germ cell mutations - inherited diseases
somatic cell mutations- Cancer
How do mutations become a permanent part of the genome?
ie- methylation of guanine would change its hydrogen bonding to bind with T better than C.
-Once DNA replicates the daughter strands its permanent
Single base substitution-
Ie sickle cell anemia
GTG= Glutamine sickle cell
DNA pol Proofreading- reduces errors to 1 in 10^7
What used if missed?
-DNApol cleaves wrong nucleotide before adding another
-if missed; Strand-directed Mismatch Repair used (MMR)
MMR 4 steps. when used?
detects distortions in DNA helix from errors missed by proofreading-occurs during replication
1. MSH protein binds the buldge in dsDNA
2. MLH binds to MSH and finds closest nick
3. MLH triggers degradation from error to nick
4. gap filled by DNApol & sealed by DNAligase
MSH -SITS MLH-LOOKS
HNPCC- What -where-how
Hereditary non-polyposis Colon Cancer
-tendency to develop cancer is inherited
-(occurs without polyps present)
-due to mutation in Msh1 (60%) and Mlh1 (30%) genes
-b/c dna repair is broken adenoma-->carcinoma in only 2-3 years vs usual 8-10years seen in non-HNPCC patients
What 3 ways for SS break repair?
What 2 ways for DS break repair?
SS break repair:
1. Site-directed MMR
2. Base Excision Repair (BER)
3. Nucleotide excision repair (NER)
DS break repair
1. Non-homologous end joining (NHEJ)
2. Homologous Recombination (HR)
-DNA glycosylases remove all unnatural bases created by spontaneous deamination
-leison specific (~8 types in humans)
-deamination cytosine to uracil 100 bases per cell/day
BER 3 steps
1. Glycosylase cleaves base leaving abasic sugar
2. AP endonuclease removes sugar-phosphate backbone
3. DNA pol adds new nucleotides DNA ligase seals nick
NER 4 steps.
ie- pyrimidine dimers-- 2 adjacent thymine or cytosine residues link together covalently
1. Multi-enzyme complex find distortion of helix
2. nuclease cuts backbone on both sides of distortion
3. helicase separates damage oligonucleotide
4. DNApol uses undamaged strand as template
NHEJ- function- problems
Each strand uses each other as template for DNA replication
- DNA ligase seals the gap
-prone to mutagenic and error b/c it removes several base pairs at the break site
Homologous Recombination HR
Uses sister chromatid or homologous chromosome as a template
-ds breaks during cell division (not replication) when duplicated chromosomes haven't yet separated.
-best for post replication repair
Familial breast cancer- What proteins? Chance of cancer if inherited?
BRCA1 and BRCA2 proteins normally involved in dsDNA break repair
Women are 40-80% more likely to develop breast/ovarian cancer if mutated/deficient BRCA1 and BRCA2 genes are inherited
4 Endogenous Sources of DNA damage
1. DNA replication error
3. Base Hydrolysis
2 exogenous sources of DNA damage
1. UV damage
2. Carcinogens: DNA adducts
Damage by Deamination- what- how repaired
removal of amino group (cytosine>Uracil)
-Uracil DNA glycosylase enzyme scans and removes uracil bases in DNA
-Alters what a base will pair with
_repaired by BER
Base Hydrolysis- what - how repaired?
Depurination- removal of a purine base from DNA strand
-glycosidic bond of sugar/base can spontaneously hydrolyze leaving sugar without a base (apurinic site)
-if not repaired>deletion mutation
-BER used begins w. AP endonuclease
Oxidation of Base- what is? repair type?
Reactive oxygen species (O2,H2O2,OH*)
-complete loss of base from ribose
-induce single and double-strand breaks
UV irradiation damage- What is? Repair type?
UV can induce pyrimidine dimer formation
-adjacent thymine bases can link forming cyclobutane ring
Uses NER because of double bond between adjacent bases
Xeroderma Pigmentosum (XP)- What is?
-Inability to repair DNA damage from UV b/c defective NER
-XP patients 1000x greater risk of developing skin cancer and 100,000x greater risk of developing carcinoma of tip of the tongue
DNA Adducts- Two types and examples of each? Type of repair used?
Chemical carcinogens- natural and synthetic chemicals
-add bulky adducts to DNA
1. benzopyrene (coffee; coal tar)
2. Acetaldehyde (tobacco) & occurs naturally when converting alcohol in liver.
-USES NER b/c lesion is too bulky for BER to be effective