Lect. 4 - DNA replication, repair and recomb 2

Question 1

What type of damaged DNA needs repaired?

Answer 1

DNA damaged from replication errors and also from accidental lesions that occur in the genome

Question 2

how many accidental base changes result in a permanent mutation?

Answer 2

fewer than 1/1000

Question 3

Spontaneous DNA damage

Answer 3

mutations aren’t only caused by mistakes in replication

Question 4

5000 purine bases are lost every day due to a spontaneous reaction called?

Answer 4

depurination - hydrolysis of the n-glycosyl linkage

Question 5

Deamination

Answer 5

C to U - occurs at 100 bases/day

Question 6

DNA damage

Answer 6

can also occur from exposure to reactive forms of O2 in the cell or chemicals in environment

Question 7

UV radiation damage

Answer 7

can produce a covalent linkage b/w 2 adjacent pyrimidines (T-T or C-T) ; pyrimidines dimers (will distort the backbone of the DNA)

Question 8

What happens if the DNA damage is unrepaired when the DNA is replicated?

Answer 8

The changes lead to either a deletion or base pair substitution in the daughter strand

Question 9

DNA glycosylases (in BER)

Answer 9

• at least 6 different types

- each recognizes a specific type of altered base and catalyzes its removal

Question 10

AP endonuclease and phosphodiesterase (in BER)

Answer 10

cut phosphodiester backbone; damage is removed and gap is repaired

Question 11

in BER, how are depurinations repaired?

Answer 11

directly repaired beginning w/ AP endonuclease

Question 12

Nucleotide Excision Repair (NER)

Answer 12

differs from BER in how damage is removed; can repair any bulky lesion like those chemically-induced and thymine dimers (anything that distorts the DNA)

Question 13

NER process

Answer 13

• a multienzyme complex scans DNA for distortion in double helix instead of specific base change
• cleaves phophodiester backbone on both sides; DNA helicase peels lesion-containing strand away

Question 14

In NER, what is the large gap repaired by?

Answer 14

DNA polymerase and ligase

Question 15

Transcription-coupled repair

Answer 15

works w/ BER, NER and others to repair genes that are being expressed when the damage occurs; cells can preferentially direct DNA repair to sequences that are being actively transcribed by linking RNA polymerase 2/ DNA repair; sequences that urgently need repair

Question 16

RNA polymerase in TCR

Answer 16

RNA polymerase stalls at lesions and directs repair machinery there

Question 17

TCR is specific for the strand being transcribed

Answer 17

non-transcribed strand repaired at the same rate as DNA not being transcribed