DNA Replication, Repair 2

Question 1

What is a demination?

Answer 1

Occurs when the amine group of a base is hydrolyzed.

Adenine to Hypoxanthine

Guanine to Xanthine

Cytosine to Uracil

Thymine does not deminate.

Question 2

What are dimers caused by in DNA damage?

Answer 2

UV radiation

Produce covalent linkage between Pyrimidines.

What are those?

Question 3

What are the 4 types of DNA repair?

Answer 3

Base excision repair for deamination or depurination

Nucleotide Excision Repair

Transcription-coupled repair

Double-strand break repair

1. Non-homologous end joining
2. Homologous recombination

Question 4

Explain the steps in Base Excision Repair?

Answer 4

Deamination here…
1. DNA glycosyloase cut glycosyl bond

Depurination here…
2. Endonuclease and phosphodiesterase cut backbone.
Remove damage.

3. DNA Poly and then DNA ligase.

Question 5

Explain the steps in Nucleotide Excision Repair?

Answer 5

Can repair long stretches of bulky section by Thymine dimers.

Scanning the entire helix for bulk not one base.

Cleaves front/back phosphodiester bonds on the same DNA strand and DNA helicase takes away

DNA Poly and Ligase

Question 6

Explain the steps in transcription-coupled repair?

Answer 6

RNA Poly stalls at lesions

Works with both BER and NER to correct

Specific for the strand being transcribed

RNA transcripts being expressed are first in repair process

Question 7

Explain why methylated cytosine is so problematic?

Answer 7

Associated with inactive C-G sequences/inactive.

Deaminate methyl-C it make thymine.
Forms mismatched pair with G

Accounts for 1/3 of point mutations with disease.

Question 8

What are translesion Polymerases?

Answer 8

Pair for extensive damage.

Not very effective/better than nothing.

Lack exonuclease proofreading activity

Question 9

Explain double stranded break repair?

Answer 9

No homologous end join through Ligase

Homologous Recombination
Repair via sister chromatid as template and is accurate.

Question 10

What is the function of ATM protein kinase?

Answer 10

Generates intracellular signal cascade to up-regulate repair genes

Checkpoints

G1-S

S in slowed

G2-M

Question 11

What does Homologous recombination cause?

Answer 11

Crossing over or gene conversion

Heteroduplex DNA

Question 12

Explain double stranded DNA repair?

Answer 12

Exonuclease degrades 5’ ends

3’ ends invades homologous template and DNA poly extends it

It then releases and goes back to it complementary strand

Question 13

Wha is Rad51?

Answer 13

Hybridization requires single-stranded DNA

Binds to ssDNA hold it with backbones stretched out

Homologous sequence identified and invasion occurs

Question 14

Explain HR at a broken replication fork.

Answer 14

Approaching the “nick” 5’ exonuclease chew back the parental strand

Strand invasion from the other complementary strand to breakage.

DNA synthesis continues as normal.

Question 15

How does loss of heterozygosity?

Answer 15

Via repair by a non-functional homolog.

Question 16

Regulation of Homologous recombination?

Answer 16

Via Cell cycle

Nucleases are only active during S and G2/ ensures a likely template

Question 17

Mutations in Brca1 have what effect?

Answer 17

Brca1 regulates processing of broken ends

Mutations lead to use of non-homologous end-joining process

Question 18

Mutations in Brca2 have what effect?

Answer 18

Brca maintains Radt51 until it is in at site damage

It will not bind to DNA to from invading strand

SSDNA protein

Question 19

What is depurination?

Answer 19

Pure As Gold

When a purine base in lost

Hydrolysis of N-glycolysis linkage.

The sugar backbone and phosphodiester bond remain.