Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Cancer > 6. DNA damage and repair > Flashcards

6. DNA damage and repair Flashcards

1
Q

What can cause DNA damage?

A 
Chemicals (carcinogens)
• dietary
• lifestyle
• environmental
• occupational
• medical
• endogenous

Radiation
• ionising
• solar
• cosmic

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What percentage of cancer is associated with diet?

A

40-45%

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Do most chemicals damage DNA in their initial form or metabolically converted forms?

A

Metabolically converted

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Give an example of an endogenous cause of cancer

A

Mitochondria produce reactive oxygen species that may damage DNA

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Name 6 ways DNA can be damaged

A
  • Base dimers and chemical-cross-links
  • Base hydroxylations
  • Abasic sites
  • Single strand breaks
  • Double strand breaks
  • DNA adducts and alkylation
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What do base hydroxylations involve?

A
  • Oxidative reaction occurring on one of the DNA bases
  • DNA has to be repaired
  • Mutation can occur during the repair process
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What do abasic sites involve?

A
  • Entire DNA base accidentally removed during the repair process
  • Sugar backbone still maintained, but missing base causes problems during replication
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What do single strand breaks involve?

A

• Very common (can be very useful)
• Physiological enzymes usually involved
• Topoisomerase relaxes and unwinds the DNA
- done by chopping the strand of DNA so it can unwind, and gain access as it is re-annealed
• These breaks can therefore be dealt with

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What do double strand breaks involve?

A
  • After a double strand breaks, there is a tendency for the 2 bits of DNA to drift apart
  • This is intolerable
  • Number of DNA repair mechanisms, but sometimes this can go wrong
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What do DNA adducts and alkylation involve?

A
  • General type of damage caused by chemicals
  • Some chemicals are metabolically activated into electrophiles
  • DNA is very rich in electrons (because of nitrogen in bases)
  • Electrophiles bind to DNA and form a covalent bond
  • DNA polymerase can’t recognise base and work during replication due to this bulk
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is phase I in mammalian metabolism?

A
  • Addition of functional groups (introduce or unmask functional groups
  • e.g. oxidations, reductions, hydrolysis
  • Mainly cytochrome p450-mediated (oxidation)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is phase II in mammalian metabolism?

A
  • Conjugation of phase I functional groups
  • e.g. sulphation, glucuronidation, acetylation, methylation, amino acid and glutathione conjugation
  • generates polar (water soluble) metabolites by adding a polar endogenous group
  • easier to excrete
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What are polycyclic aromatic hydrocarbons?

A
  • Common environmental pollutants
  • Formed from the combustion of fossil fuels and tobacco
  • Poisonous and carcinogenic
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What is one of the most common polycyclic aromatic hydrocarbons, how is it metabolised and how does it cause damage?

A

• Benzo[a]pyrene
• Oxidised by CYP450 to produce an epoxide/oxide
- this is reactive and unstable (electrophile)
• Epoxide hydrolase metabolises this into a dihydrodiol
- this is harmless
• Second CYP450 oxidises this to form another oxide (diol epoxide)
- incredibly reactive
- rapidly forms positively derived material (electrophile)
- best source of electrons is DNA
- DNA adducts formed, usually at guanine => mutation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is aflatoxin B1 and where does it come from?

A
  • Potent human liver carcinogen
  • Formed by Aspergillus flavus mould
  • Common on poorly stored grains/peanuts
  • Especially in Africa and Far-East
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Outline the metabolism of Aflatoxin B1 and the way it causes DNA damage

A
  • Oxidised by P450 into B1-2,3-epoxide (very reactive)
  • This product reacts with the N7-position of guanine to form bulky DNA adducts
  • DNA is now read as damaged
  • It’s fixed inappropriately and a mutation has been introduced into the DNA
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

How can benzo[a]pyrene cause tumours all over the body?

A

P450 is in cells throughout the body

18
Q

What is 2-napthylamine and benzidine and where was it common?

A
  • Potent bladder carcinogens

* Used in the German dye industry

19
Q

Outline the metabolism of 2-napthylamine and how it causes damage

A

• Substrate for CYP450
- converts the amino group to form a hydroxylamine
• Hydroxylamines are reactive
• They are glucuronidated (detoxified) in the liver by glucuronyl transferase
• Inactive metabolite is excreted and mixes with the urine

  • Acidic urine hydrolyses the glucuronides
  • Hydroxylamine derivative is released
  • Molecule rearranges to form a positively charged nitrogen (nitrenium ion) (electrophile)
  • Nitrenium binds to the DNA and forms adducts
  • Bladder can’t detoxify the hydroxylamine derivative as the liver
20
Q

How does solar radiation cause skin cancer?

A

• UV light leads to the formation of pyrimidine dimers (T, C, U)
- if there are 2 pyrimidines next to each other, UV radiation can cause them to covalently link
• Cell tries to repair this, but introduces a mutation in the process

21
Q

How does ionising radiation cause cancer?

A
  • All ionising radiation generates free radicals in cells
  • This includes oxygen free radicals e.g. superoxide and hydroxyl - very reactive
  • Free radicals possess unpaired electrons - seek out electron-rich DNA
22
Q

Compare the superoxide radical to the hydroxyl radical

A
  • Superoxide radical - molecule of oxygen with an extra electron
  • Hydroxyl radical - hydroxyl group that has grabbed an extra electron, even more reactive than superoxide (very electrophilic and DNA is electron-rich)
23
Q

How do oxygen free radicals damage DNA? (3 ways)

A

• Single and double strand breaks
- double strand breaks have to be reannealed, can introduce mutatations during the process

  • Base can be oxidised by free radical and DNA repair enzymes cut out the base
  • Abasic site left

• Base modifications are also introduced:

  • e.g. 8-hydroxyadenine + 8-hydroxyguanine (mutagenic)
  • while replicating, the machinery has to guess where it is a guanine present, so makes mistakes
24
Q

Which protein releases p53 for activity during stress on the cell, and what happens to p53 once released?

A
  • Mdm2
  • Suppresses activity and keeps it in check
  • p53 then forms a dimer that activates many pathways
25
Q

What does p53 do if there is mild physiological stress e.g. DNA repair or growth arrest?

A

p53 orchestrates a transcriptional of events and activates proteins that help repair the problem

26
Q

What does p53 do if there is severe stress?

A

p53 can activate an apoptotic pathway

27
Q

What are the 4 types of DNA repair?

A

1) Direct reversal of DNA damage
2) Base excision repair
3) Nucleotide excision repair
4) During- or post-replication repair

28
Q

What does direct reversal of DNA damage involve?

A
  • Photolyase splits cyclobutane pyrimidine-dimers formed from UV light to recover the pyrimidines
  • Methyltransferases + alkyltransferases remove alkyl groups from DNA bases
29
Q

What does base excision repair involve?

A

• Mainly for apurinic/apyrimidinic damage

1) DNA glycosylases hydrolyse between the sugar and affected DNA base, cutting it out
2) Apurinic/apyrimidinic endonucleases split the DNA strand so there is a gap in the SP backbone (can’t just add a base, need the whole nucleotide)
3) A repair DNA polymerase (Polβ) fills the gap formed and fixes it
4) DNA ligase seals the DNA

30
Q

What does nucleotide excision repair involve?

A
  • Mainly for bulky DNA adducts
  • Xeroderma pigmentosum (XP) proteins

1) Endonuclease makes 2 cuts either side of the site of damage
2) Helicase removes this patch
3) Repair DNA polymerases fill the gap using the complementary strand as a template
4) DNA ligase seals the DNA

• Energy-demanding and requires a lot of proteins

31
Q

What does during-/post-replication repair involve?

A
  • For mismatch repair (for wrong base pairing)
  • Also for recombinational repair (recombination occurs in cell replication and can cause big problems)
  • Proteins check the DNA to make sure that it is ok before the daughter cells bud off in mitosis
32
Q

What is the most electron-rich base?

A

• Guanine

• Adenosine is also very electron rich

33
Q

At what level of damage is incorrect repair and altered DNA sequence likely to occur?

A

Damage that is too high for repair but too low for apoptosis

34
Q

When a new drug/herbicide etc. is developed, what is the first thing checked for and how can we test if it causes mutations?

A
  • Structure of the chemical - functional groups that could cause problems
  • Introduce it to bacteria (Ames test) and see whether it causes mutations
  • If it damages the DNA of bacteria, then it has the potential to damage the DNA of mammals
  • Then test on mammalian cells (in vitro) - more sophisticated genetic material e.g. histones and chromosomes
  • Then test in vivo on mammals using bone marrow micronucleus tests and transgenic rodent mutation assays (very expensive)
35
Q

Why is bone marrow used to test for the mutagenicity of chemicals?

A
  • Contains pluripotent stem cells that give rise to the cells of the blood
  • Can then look at the formed elements of the blood as a mechanism of what’s happening in the bone marrow
  • Can also examine bone marrow cells
  • It is also a proliferative compartment - expanding the potential to cause a mutation
36
Q

Outline how the Ames test is carried out

A
  • CYP450 (rat liver enzyme preparation) is added to the bacteria (normally Salmonella typhimurium) - due to poor metabolising activity of bacteria
  • This allows the bacteria to convert the chemical into something damaging
  • Mixture of enzymes and bacteria are plated
  • Bacteria are genetically engineered to not be able to synthesise histidine
  • If the chemical causes a mutation to occur, then the bacteria will be able to synthesise histidine
  • These bacteria can grow in the absence of exogenous histidine
  • Bacteria that haven’t mutated will die without exogenous histidine
  • Therefore, the more DNA damaging capability of the chemical, the more colonies grow in histidine absence
37
Q

How can you check for chromosomal aberrations in mammalian cells when testing chemicals?

A
  • Treat the cells with the chemical in the presence of liver S9
  • Then do karyotyping or look at the chromosomal structure
  • Can look for chromatid exchange, chromatid gaps etc.
  • Difficult, slow and laborious test that requires a lot of technical skill
38
Q

How is an in vitro micronucleus assay carried out?

A
  • Mammalian cells are treated with the chemical in vitro and allowed to divide
  • Trying to measure the ability of the chemical to break up the DNA into fragments, then count the fragments

• Allow the cell to go through one replication cycle then stop it when a binucleus is formed
• Cytochalasin-B is used to block cytokinesis
• Binucleate cells assessed for the presence of micronuclei (pieces of chromosomal material that have been broken off and no longer appear in the nucleus)
• The kinetochores can be stained to determine if chemical treatment caused clastogenicity or aneuploidy
- both contribute to cancer
• Normal cell with appear to have 2 nuclei, damaged cell will appear to have an extra micronucleus

39
Q

What is clastogenicity and aneuploidy?

A
  • Clastogenicity - chromosomal breakage

* Aneuploidy - chromosomal loss / change in the number of chromosomes

40
Q

How can you tell if a rat has been affected by chemical treatment of it’s bone marrow?

A

Erythrocytes cannot remove small fragments of DNA e.g. micronucleus, so they will be present

Cancer (15 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions