Module 11: V1 - V5

Question 1

How common is DNA damage?

Answer 1

very common

every day, each one of your cells suffers 60,000 DNA lesions

Question 2

What causes DNA lesions?

Answer 2

spontaneous oxidative damage, hydrolytic attack and uncontrolled methylation

Question 3

What is hydrolytic attack also known as?

Answer 3

depurination because this occurs more frequently with the purines than the pyrimidines

Question 4

How does UV light cause damage to DNA?

Answer 4

interacts with chemical bonds in DNA
results in thymine bases binding to each other instead of base pairing with the opposite strand -> large disruptive lesion

Question 5

How does gamma radiation cause damage to DNA?

Answer 5

breaks in the DNA phosphate backbone

either a single stranded or double stranded break

Question 6

How do DNA polymerases cause damage to DNA?

Answer 6

extremely accurate, but do introduce rare errors
two mechanisms: binding of the nucleotides and proofreading
if these fail, DNA repair may be able to fix it later

Question 7

What are the different repair mechanisms of DNA?

Answer 7

double strand break repair
nucleotide excision repair
single strand base excision repair
mismatch repair

Question 8

What are the different types of DNA damage?

Answer 8

radio and chemotherapy, UV light, replication errors and alkylating agents

Question 9

What happens if a lesion is not repaired?

Answer 9

the replication fork will reach the lesion and a base will not be added on the other side -> DNA replication will fail and the ssDNA will be degraded -> this DNA is therefore lost and not replicated (similar for an unrepaired break)

Question 10

What is the difference between DNA damage and a mutation?

Answer 10

DNA damage means that the DNA cannot be used properly while mutation means that there is a heritable difference in the DNA compared to the wild type -> mutation could give rise to a tumour

Question 11

What is mismatch repair?

Answer 11

method of DNA repair that is specific to E. coli

works because E. coli DNA is methylated on the adenines in GATC sequences (NOT the same as methylation in eukaryotes)

Question 12

What happens during the replication process of E. coli DNA?

Answer 12

the GATC sequence of the new strand is not methylated (hemimethylated DNA)
marks which one is the new strand and which one is the old strand -> after a few minutes the new strand is methylated

Question 13

What happens if there is a mismatch in E. coli DNA during DNA replication?

Answer 13

for a very short time, E. coli cells can identify the new strand and the old strand and enzymatic processes will occur to repair this section of DNA -> this is what makes DNA polymerases so accurate

Question 14

What are two similar repair system which focus on different sizes of damage?

Answer 14

base excision repair - for small damage

nucleotide excision repair - for larger damage

Question 15

What is deamination?

Answer 15

damage to a single base in which an amine group is removed

Question 16

What is base excision repair? What is the mechanism of base excision repair?

Answer 16

repair of damaged or missing single bases
damage is recognised by DNA glycosylase which cuts out the base, the DNA backbone is cut by an AP endonuclease and DNA polymerase replaces this section with the correct NTs
DNA ligase then repairs the nick in the DNA backbone

Question 17

What is nucleotide excision repair? What is the mechanism of nucleotide excision repair?

Answer 17

repairs larger lesions by removing nucleotides
excinuclease cuts upstream and downstream of the lesion, DNA helicase unwinds two strands from each other allowing cut segment to leave -> DNA polymerase fills empty gap and DNA ligase repairs nick

Question 18

What the causes of double stranded breaks?

Answer 18

ionizing radiation, errors of DNA replication, oxidising agents, and other metabolites can cause breaks across both strands of the DNA

Question 19

What the causes of double stranded breaks?

Answer 19

ionizing radiation, errors of DNA replication, oxidising agents, and other metabolites can cause breaks across both strands of the DNA

Question 20

What are the characteristics of double stranded breaks?

Answer 20

DS breaks are the most biologically significant lesion caused by ionizing radiation
most DS breaks are repaired within 24 hours, but 25% of the repairs contain errors

Question 21

What is significant about double stranded break repair?

Answer 21

error prone method of repair
error prone as broken ends are prone to degradation -> loss of nucleotides
repair pathway also removes some nucleotides from the ends so that they are ready to be joined together

Question 22

Why is double stranded break repair error prone?

Answer 22

error prone as broken ends are prone to degradation -> loss of nucleotides
repair pathway also involves removal of some nucleotides from the ends so that they are ready to be joined together

Question 23

How would you define a mutation?

Answer 23

an alteration in the nucleotide sequence of the genome of an organism, virus or extrachromosomal DNA

Question 24

How often is the DNA in your cells damaged?

Answer 24

every day, each cell suffers 60,000 DNA lesions

Question 25

What sort of things can cause DNA damage to your cells?

Answer 25

spontaneous oxidative damage, hydrolytic attack and uncontrolled methylation