Final: DNA Repair Systems

Question 1

3 basic mechanisms of lesion repair

Answer 1

direct base repair

nucleotide excision repair

base excision repair

Question 2

how to the 3 basic mechanisms of lesion repair work in general?

Answer 2

use proteins to perform corrective chemistry on defective DNA (heal)

Question 3

what are the 1st and 2nd lines of defense against aging and cancer?

Answer 3

1st: antioxidants
2nd: DNA repair

Question 4

4 types of direct repair and # of steps

Answer 4

repair lesions in a single step

alkyl transferase

photoreactivation

nick repair

gap repair

Question 5

alkyl transferases

Answer 5

enzymes pick off alkyl (methyl/ethyl) groups from DNA bases

MGMT methylguanine methyl transferase (1 molecule per methyl..can’t reuse)

Question 6

photoreactivation

Answer 6

in bacteria, thymine dimer repaired by breaking it into two thymines again

uses a photon of blue light

Question 7

nick repair

Answer 7

ligase (same enzyme used in last step of DNA replication)

Question 8

gap repair

Answer 8

DNA polymerase fills the gap, then ligase

Question 9

what is excision repair?

Answer 9

cut out a piece of damaged ssDNA

leaves a gap, which is fixed by gap repair

Question 10

Xeroderma pigmentosum

Answer 10

genetic disease caused by defective dimer excision repair system

thymine dimers caused by short-wave UV light (sun)

leads to skin cancer

Question 11

AP site repair system

Answer 11

turn AP site into a gap (nick above missing base)

then gap repair fixes

Question 12

glycosylase repair system

Answer 12

recognize bad bases and cut it out by cutting bond between base and sugar (glycosyl bond)

this cutting leaves an AP site, AP site repair fixes

Question 13

transcription-coupled repair (TCR)

Answer 13

brings an excision repair system to a damaged site during transcription

Question 14

Cockayne Syndrome

Answer 14

genetic disease caused by defective TCR

cells get stalled when transcribing genes (leads to apoptosis)

causes stunted growth

Question 15

symptoms of Cockayne Syndrome

Answer 15

shortness, skeletal deformation, premature aging, MR

Question 16

post-replication repair

Answer 16

fixes some lesions after DNA has been replicated

Question 17

mismatch repair (MMR)

Answer 17

type of post-replication repair

replication error puts wrong base on new daughter strand

identifies parent strand by finding which has methylated adenines

changes incorrect bases on daughter strand to match parent strand

Question 18

2 types of double-strand break repair (DSBR) & why needed?

Answer 18

homologus repair (HR)

non-homologus end joining (NHEJ)

when a double strand break occurs, a small bit of chromosome is lost

Question 19

homologus repair (HR)

Answer 19

repair break in chromosome by lining up broken piece with matching chromatid of DNA (or chromosome)

restores missing sequence and prevents translocation/inversion

Question 20

non-homologus end joining

Answer 20

used when HR fails

stick two broken ends back together

could be the wrong ends, causing translocation/inversion

doesn’t replace missing info accurately leadings to deletions/insertions

Question 21

loss of heterozygosity (HR)

Answer 21

if HR uses the homologus chromosome, not chromatid then LOH could occur

other chromosome may have different alleles, making repaired chromosome have recessive alleles