Chiolo Lecture 2 Flashcards
What repair mechanism is used to repair Radiation, Oxygen radicals, Alkylating agents, and Spontaneous hydrolysis?
base excision repair and damage reversal
What health issue is caused by Base Excision Repair mutations?
familial adenomatous polyposis
What is adenomatous polyposis?
a genetic predisposition to colon cancer caused from mutations in MYH DNA Glycosilase
What results from Familial Adenomatous Polyposis?
- formation of hundreds to thousands of colon polyps
- cells are less able to repair digestive damage: acids produced during digestion induce high oxidative damage in the colon
What are three types of base mutagens?
- oxidation
- alkylation
- deamination
What is causes oxidation?
oxygen radicals - by chemicals, radiation
What is causes alkylation?
chemotherapy, lab chemicals, industrial chemicals
What causes deamination?
spontaneous hydrolysis
What is OxoG?
a mis-pair between guanine and adenine
What is O6-methylG?
a mis-pair between guanine and thymine
What causes guanine to mis-pair to adenine?
ionizing radiation
What causes guanine to mis-pair to thymine?
chemicals (nitrosamines)/ methylation
What type of repair can repair OxoG?
base excision repair
What type of repair can repair O6-methylG?
- base excision repair
- damage reversal
What is an example of deamination (3)?
- adenine to hypoxanthine
- guanine to xanthine
- cytosine to uracil
What is depurination?
leaves an abasic site
What are the three steps to base excision repair?
- base removed by a Uracil-DNA Glycosilase
- Apurinic (AP) site removed by endo- and exo- nucleases
- polymerase and ligase fill the GAP
How many DNA glycosylases are found in humans?
11 different forms
What can Base Excision Repair also fix?
abasic sites
What does the crystal structure look like of a damaged base?
the DNA glycosylase surround the ‘flipped out’ damaged base
What is the function of DNA glycosylase?
the DNA glycosylase scans the DNA and flips the damaged base to expose the damage site that will be cleaved
What are two examples of damage reversal?
- damage reversal with methyltransferase
- damage reversal with DNA photolyase (in prokaryotes)
What is the process of damage reversal with methyltransferase?
- methyltransferase recognizes the methyl group in O6-methylG
- methyltransferase binds to the O6-methylG methyl group site (guanine-cytosine)
- methytransferase transfers the methyl group from guanine to the enzyme
- the methyl group degrades the methytransferase
Why is the process of damage reversal with methyltransferase expensive?
degrades the methyltransferase and requires reproduction of the enzyme
What does UV radiation cause?
crosslinks between adjacent pyrimidine bases (TT dimers)
What is the process of photoreactivation?
- UV light causes TT dimer formation
- In the dark, DNA photolyase recognizes the TT dimer and binds to the phosphodiester backbone
- visible light catalyzes the reaction
- DNA photolyase splits the TT dimer, reversing the damage, and restoring the DNA to its original condition
What organisms undergo photoreactivation?
only present in prokaryotes
What does nucleotide excision repair mutation result in?
Xeroderma Pigmentosum
What are the symptoms of Xeroderma Pigmentosum?
extreme sensitivity to UV light, skin cancer
What is the process of nucleotide excision repair?
- UvrA/UvrB scans DNA for distortions
- UvrB opens the helix
- UVrC cleaves on both sides of the lesion
- UvrD helicase, polymerase, and ligase
How many proteins are involved in nucleotide excision repair in human cells?
25 proteins (XPA, C, D, F, G…)
What does transcription-coupled repair use?
TFIIH to couple transcription with nucleotide excision repair in eukaryotes
What is the process of transcription-coupled repair?
- initiated by recognition of RNA polymerase molecules that have stalled as a result of DNA damage
- CSA/CSB proteins mediate TCR via. direct CSB-RNA polymerase reaction, which leads to the recruitment of TFIIH (XPD,XPB)
- the stalled polymerase is removed along with the damaged DNA and the strand is restored to its original state by excision and DNA resynthesis
What is the function of CSA/CSB?
assures high efficiency of NER in transcribed genes
What do mutations in transcription-coupled repair result in?
Cockayne Syndrome
What are the symptoms of Cockayne Syndrome?
- mental retardation
- developmental defects
- UV sensitivity, skin cancer
- premature aging
- patients typically die by the age of 6-7
