dna damage and repiar

Question 1

Which of the following are the most difficult type of damageto repair?
A Deamination of cytosine
B Depurination
C UV radiation
D Ionising radiation
E ROS from oxidative metabolism

Answer 1

OPTION D - because ionisng readiation created ROS which are highly toxic to DNA (can create ds breaks, cross links, 8oxoG, cyclopurines = prevent replication,trancription,introduce more mutations,lead to chromosome fusions, prevent transcription

depurination - around 10,000 a day but can be repaired
UV rad - CPD and 6-4PP repaired by NER pathway (tc or global genome
deamination - 100 a day
ROS from oxmetabolism is an endogenous process so easily fixed
but ROS from metabolism happens all the time

Question 2

in a stretch of DNA, which CPD would be repaired the most efficiently if found in these region
A CPD found in 5' UTR
B CPD found on exon - noncoding strand
C CPD found upstream of TSS
D CPD found on exon - coding strand

Answer 2

option D - The more efficient Transcription-coupled Nucleotide Excision Repair (TC-NER) occurs at sites where RNA polymerase II stalls due to encountering a CPD on the template/coding strand. This prioritises DNA repair to coding regions of the genome

yeah that makes sense, you want to repair what you are actively making

Question 3

describe the link between DNA damage repair machinery and mutations

Answer 3

around 99% DNA is accurately repaired
BUT when damage is not or inaccurately repaired it can lead to errors during DNA replication -> then this introduces a mutation

> we can say DNA damage is mutagenic / toxic
mutations can be carcinogenic too

Question 4

describe the link between DNA damage and aging

Answer 4

mutations can accumulate with age. if left unrpeaired can be toxic to the body and can initiate DNA damage response to either cell cycle arrest/cellular senecnce or intiate apoptosis.

Question 5

What would be the most significant source of mutation : replication error, inaccurate replication, inaccurate repair

Answer 5

studies suggest that innacurate DNA repair is the major contributor to mutagenesis.
> it seems that the accumulation of mutations is independent of the cell division and neurons and mitotic cells accumulate mutations at the same rate
> so inaccuracies during DNA replication isn’t a major factor here

however. cancers have ahigh prevelence in epithelial cells which are mitotically active and have a large turnover

Question 6

what would be the major source of mutations in DNA?

Answer 6

spontaneous mutation

from mutagens : radiation, smoking, ROS

Question 7

describe ways in which spontaneous DNA damage can occur

Answer 7

• through depurination (abasic site creation) or deamination (C -> U)
• also through the creation of ROS/free radicals from oxidative metabolism a normal cellular process

Question 8

describe in detail the problems associated with spontaneous DNA damage

Answer 8

• depurination : apurinic sites will BLOCK transcription processes and lead to inaccurate DNA replication ~ 10,000 a day
• deamination: if left unrepaired the U is replaced by a T so a C=G becomes a T=-A ~ 100 a day
• ROS: can oxidise a guanine (8-oxo-G) which can be mistaken for a T.
can also form cylopurines distorting structure of DNA.
can also create ss. and ds breaks (10,000 and 10 a day)
also can create crosslinks/covelent bonds between dna stands which prevent unwinding of DNA previenitg replication and transcription