Module 11: V1 - V5 Flashcards
How common is DNA damage?
very common
every day, each one of your cells suffers 60,000 DNA lesions
What causes DNA lesions?
spontaneous oxidative damage, hydrolytic attack and uncontrolled methylation
What is hydrolytic attack also known as?
depurination because this occurs more frequently with the purines than the pyrimidines
How does UV light cause damage to DNA?
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
How does gamma radiation cause damage to DNA?
breaks in the DNA phosphate backbone
either a single stranded or double stranded break
How do DNA polymerases cause damage to DNA?
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
What are the different repair mechanisms of DNA?
double strand break repair
nucleotide excision repair
single strand base excision repair
mismatch repair
What are the different types of DNA damage?
radio and chemotherapy, UV light, replication errors and alkylating agents
What happens if a lesion is not repaired?
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)
What is the difference between DNA damage and a mutation?
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
What is mismatch repair?
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)
What happens during the replication process of E. coli DNA?
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
What happens if there is a mismatch in E. coli DNA during DNA replication?
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
What are two similar repair system which focus on different sizes of damage?
base excision repair - for small damage
nucleotide excision repair - for larger damage
What is deamination?
damage to a single base in which an amine group is removed
What is base excision repair? What is the mechanism of base excision repair?
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
What is nucleotide excision repair? What is the mechanism of nucleotide excision repair?
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
What the causes of double stranded breaks?
ionizing radiation, errors of DNA replication, oxidising agents, and other metabolites can cause breaks across both strands of the DNA
What the causes of double stranded breaks?
ionizing radiation, errors of DNA replication, oxidising agents, and other metabolites can cause breaks across both strands of the DNA
What are the characteristics of double stranded breaks?
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
What is significant about double stranded break repair?
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
Why is double stranded break repair error prone?
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
How would you define a mutation?
an alteration in the nucleotide sequence of the genome of an organism, virus or extrachromosomal DNA
How often is the DNA in your cells damaged?
every day, each cell suffers 60,000 DNA lesions
What sort of things can cause DNA damage to your cells?
spontaneous oxidative damage, hydrolytic attack and uncontrolled methylation
