Chiolo Lecture 2

Question 1

What repair mechanism is used to repair Radiation, Oxygen radicals, Alkylating agents, and Spontaneous hydrolysis?

Answer 1

base excision repair and damage reversal

Question 2

What health issue is caused by Base Excision Repair mutations?

Answer 2

familial adenomatous polyposis

Question 3

What is adenomatous polyposis?

Answer 3

a genetic predisposition to colon cancer caused from mutations in MYH DNA Glycosilase

Question 4

What results from Familial Adenomatous Polyposis?

Answer 4

1. formation of hundreds to thousands of colon polyps
2. cells are less able to repair digestive damage: acids produced during digestion induce high oxidative damage in the colon

Question 5

What are three types of base mutagens?

Answer 5

1. oxidation
2. alkylation
3. deamination

Question 6

What is causes oxidation?

Answer 6

oxygen radicals - by chemicals, radiation

Question 7

What is causes alkylation?

Answer 7

chemotherapy, lab chemicals, industrial chemicals

Question 8

What causes deamination?

Answer 8

spontaneous hydrolysis

Question 9

What is OxoG?

Answer 9

a mis-pair between guanine and adenine

Question 10

What is O6-methylG?

Answer 10

a mis-pair between guanine and thymine

Question 11

What causes guanine to mis-pair to adenine?

Answer 11

ionizing radiation

Question 12

What causes guanine to mis-pair to thymine?

Answer 12

chemicals (nitrosamines)/ methylation

Question 13

What type of repair can repair OxoG?

Answer 13

base excision repair

Question 14

What type of repair can repair O6-methylG?

Answer 14

1. base excision repair
2. damage reversal

Question 15

What is an example of deamination (3)?

Answer 15

1. adenine to hypoxanthine
2. guanine to xanthine
3. cytosine to uracil

Question 16

What is depurination?

Answer 16

leaves an abasic site

Question 17

What are the three steps to base excision repair?

Answer 17

1. base removed by a Uracil-DNA Glycosilase
2. Apurinic (AP) site removed by endo- and exo- nucleases
3. polymerase and ligase fill the GAP

Question 18

How many DNA glycosylases are found in humans?

Answer 18

11 different forms

Question 19

What can Base Excision Repair also fix?

Answer 19

abasic sites

Question 20

What does the crystal structure look like of a damaged base?

Answer 20

the DNA glycosylase surround the ‘flipped out’ damaged base

Question 21

What is the function of DNA glycosylase?

Answer 21

the DNA glycosylase scans the DNA and flips the damaged base to expose the damage site that will be cleaved

Question 22

What are two examples of damage reversal?

Answer 22

1. damage reversal with methyltransferase
2. damage reversal with DNA photolyase (in prokaryotes)

Question 23

What is the process of damage reversal with methyltransferase?

Answer 23

1. methyltransferase recognizes the methyl group in O6-methylG
2. methyltransferase binds to the O6-methylG methyl group site (guanine-cytosine)
3. methytransferase transfers the methyl group from guanine to the enzyme
4. the methyl group degrades the methytransferase

Question 24

Why is the process of damage reversal with methyltransferase expensive?

Answer 24

degrades the methyltransferase and requires reproduction of the enzyme

Question 25

What does UV radiation cause?

Answer 25

crosslinks between adjacent pyrimidine bases (TT dimers)

Question 26

What is the process of photoreactivation?

Answer 26

1. UV light causes TT dimer formation
2. In the dark, DNA photolyase recognizes the TT dimer and binds to the phosphodiester backbone
3. visible light catalyzes the reaction
4. DNA photolyase splits the TT dimer, reversing the damage, and restoring the DNA to its original condition

Question 27

What organisms undergo photoreactivation?

Answer 27

only present in prokaryotes

Question 28

What does nucleotide excision repair mutation result in?

Answer 28

Xeroderma Pigmentosum

Question 29

What are the symptoms of Xeroderma Pigmentosum?

Answer 29

extreme sensitivity to UV light, skin cancer

Question 30

What is the process of nucleotide excision repair?

Answer 30

1. UvrA/UvrB scans DNA for distortions
2. UvrB opens the helix
3. UVrC cleaves on both sides of the lesion
4. UvrD helicase, polymerase, and ligase

Question 31

How many proteins are involved in nucleotide excision repair in human cells?

Answer 31

25 proteins (XPA, C, D, F, G…)

Question 32

What does transcription-coupled repair use?

Answer 32

TFIIH to couple transcription with nucleotide excision repair in eukaryotes

Question 33

What is the process of transcription-coupled repair?

Answer 33

1. initiated by recognition of RNA polymerase molecules that have stalled as a result of DNA damage
2. CSA/CSB proteins mediate TCR via. direct CSB-RNA polymerase reaction, which leads to the recruitment of TFIIH (XPD,XPB)
3. the stalled polymerase is removed along with the damaged DNA and the strand is restored to its original state by excision and DNA resynthesis

Question 34

What is the function of CSA/CSB?

Answer 34

assures high efficiency of NER in transcribed genes

Question 35

What do mutations in transcription-coupled repair result in?

Answer 35

Cockayne Syndrome

Question 36

What are the symptoms of Cockayne Syndrome?

Answer 36

1. mental retardation
2. developmental defects
3. UV sensitivity, skin cancer
4. premature aging
5. patients typically die by the age of 6-7