Lecture 4: DNA Replication, Repair and Recombination 2

Question 1

What happens in depurination?

Answer 1

base leaves sugar-base complex

Question 2

What happens in deamination?

Answer 2

cytosine changes to uracil

Question 3

What can cause DNA damage?

Answer 3

exposure to reactive forms of O2 in the cell or chemicals in the environment, UV radiation can produce a covalent linkage between two pyrimidines

Question 4

What happens when unprepared DNA is replicated?

Answer 4

deletion or a based pair subsection in the daughter strand

Question 5

What are the the types of DNA repair?

Answer 5

base excision repair
nucleotide excision repair
transcription coupled repair
double strand break repair (non homologous and homologous end joining)

Question 6

What happens in base excision repair?

Answer 6

• DNA glycoslyases recognize a specific type of altered base and catalyze its removal
• flipping out of base from helix
• if base is incorrect: cleaves glycosyl bond connecting base to sugar
• AP endonuclease and phosodiesterase cut phosphodiester backbone
• Damage is removed and gap is repaired

Question 7

What happens in nucleotide excision repair?

Answer 7

• can repair any bulky lesion
• multi enzyme complex sans DNA for distortion in double helix
• cleaves phosphodiester backbone on both sides
• large gap is repaired by DNA polymerase and ligase

Question 8

What happens in transcription coupled repair?

Answer 8

• cells preferentially direct DNA repair to actively transcribed sequences by linking RNA pol with DNA
• RNA pol stalls at lesions and directs repair machinery
• works with BER, NER and other repair gens
• specific for strand being transcribed

Question 9

What is Cockayne’s syndrome?

Answer 9

defect in transcription coupled repair
growth retardation, skeletal abnormalities, sensitive to sunlight
RNA pol permanently stalled at sties of damage in important genes

Question 10

What are the unnatural bases in deamination? 
Adenine-->
Guanine-->
Cytosine-->
Thymine-->

Answer 10

hypoxanthine
xanthine
uracil
no deamination

Question 11

What is used when the cell has extensive damage?

Answer 11

translesion polymerases

Question 12

What do translesion polymerases do?

Answer 12

• lack exonuclease proofreading ability

- adds only a couple of nucleotides before the replicative polymerase reassociates

Question 13

How are double strand breaks repaired by homologous recombination?

Answer 13

• 5’ ends degraded by exonuclease
• one 3’ end invades homologous template and primes DNA synthesis
• newly synthesized 3’ end of the invading strand is then able to annular to the other original 3’ overhang in the damaged chromosome through complementary base pairing
• gaps filled and ligated

Question 14

Compare non-homologous end joining and homologous recombination

Answer 14

Non-homologous end joining: no template required, create a mutation at repair site, can create translocations
homologous recombination: uses daughter DNA duplex as template, no loss or alteration of DNA at site, can rapt other types of DNA damage