L16 - DNA Repair Flashcards
(14 cards)
What is the importance of the fact that the new strand of DNA (during DNA replication) is not methylated for a short time?
You can identify that it is the new strand as the template strand is methylated.
Depurination
When a hydrolytic attack breaks off the bond between base & the sugar. Removal of a purine.
There are four steps in Base excision repair. Name the protein that catalyses each step, and what
happens.
1) DNA glycosylase (uracil glycosylase) to cut our incorrect Uracil base
2) AP endonuclease makes a cut in the phosphate backbone
3) DNA pol 1 comes in and places a small section of new DNA
4) DNA ligase to join it back together
What is a thymine dimer? Why is it a problem for the DNA? What is the usual source of a thymine dimer? What method of repair is used?
UV light catalyzes a reaction to form covalent bonds between adjacent thymines leading to a kink affecting the whole DNA structure.
Nucleotide excision repair.
There are four steps in Nucleotide excision repair. What protein catalyses each step and what happens?
1) Excinuclease (Excision nuclease)- cuts on both sides of lesion
2) Helicase takes out that section of lesion
3) DNA pol 1 creates a new section of DNA
4) DNA ligase joins it back together
Why can’t a thymine dimer be removed by base excision repair?
Bases are no longer recognisable as they’re bound to each other covalently hence nucleotide excision repair is used to take out the whole section.
What is sunburn?
DNA damage in skin cells triggering melanin production, can even trigger cell death (peeling).
Why is a double stranded DNA break potentially dangerous? What can cause a double stranded DNA
break? What might happen if there is a double stranded DNA break during DNA replication?
Dangerous as it leads to loss of DNA and it has error prone repair where 25% of repairs contain errors.
Can be caused by ionizing radiation, errors of DNA replication, oxidising agents and other metabolites.
If there is a DS break during DNA replication, you lose a large amount of DNA as DNA pol cannot go any further if there is a break.
What is nonhomologous end joining repair, and what is the downside of this repair?
Accidental double strand break, where loss of nucleotides due to degradation from ends join together, leading to deletion of DNA sequence.
True or False: As you age, DNA damage is repaired, so when you are older your cells will not have any mutations.
False, by 70, most of your cells will contain 2000 regions of small changes due to non homologus end joining repair. However most of the time it occurs in the non coding region and doesn’t have a major impact.
What are the four phases of the cell cycle? What is a checkpoint?
M, G1, S, G2
A checkpoint is where DNA is checked for damage, before S phase and before M phase. It can pause the process or lead to cell death to prevent damage to the cell.
What is apoptosis? Where does apoptosis occur naturally during development? (there are three
examples in humans)
Programmed cell death.
Non webbed hands, Nerve cells (refine neural conditions), immune system (self-antigens killed off)
What is the Ames test used to test?
Test to recognise whether a given chemical can cause mutations in the DNA of the test organism. It will be a mutagen if it forms colonies.
Methylation of the ____ in ____ sequences is used to identify an inorrect base in ___ repair
Adenines, GATC, mismatch
