DNA repair Flashcards
Why are DNA repairs important ?
They are essential to the maintenance of the genetic integrity of organisms : they prevent cellular malfunction, disease and cancers.
What is proofreading ?
The DNApol III cuts out the incorrect nucleotide that was added (when needed)
Is the activity of the DNApol III endonuclease or exonuclease ?
exonuclease
Name the 4 basic steps that are in all repair mechanism
- Recognize
- Cut
- Replace
- Ligase
Name what type of DNA repairs are suitable for the following mutations:
Non bulky damage ( Alkylation, Oxidation, Deamination, Depurination)
- Base excision repair
What type of non bulky damage mutations can be repaired by direct repair ?
Alkylation
What type of repair works for pyrimidine dimer damage ?
- Nucleotide excision repair
- Direct repair
What DNA repair mechanism is suitable for a bulky adduct?
Nucleotide excision repair
What type of mutations can be restored by mismatch repair ?
- Base mismatch
- Loop
What type of DNA repair mechanism works on double strand-breaks ?
- Nonhomologous end joining
- Homologous recombination
Describe the direct repair mechanism
fix non-bulky DNA damages without removing the affected nucleotide or disrputing the DNA backbone (if dimer forms –> simple cut to repair it)
in the following example, what type of DNA repair mechanism would be used ?
UV radiation forming a thymine dimer
Direct repair OR nucleotide excision repair
Describe Base excision repair (BER)
Eliminates non-bulky damages that don’t distort the double helix and affect individual abnormal bases
Name all the steps involved in base excision repairs
- During replication, a mismatch is recognized by a DNA glycosylase
- The DNA glycosylase breaks the base-sugar bond, freeing the base
- An AP endonuclease cuts the phosphodiester bond, creating a nick
- A DNA polymerase beta excises the incorrect nucleotide and adds the correct one
- A DNA ligase links the new nucleotide by forming the phosphodiester bonds
What is the difference between an endonuclease and a exonuclease ?
Nuclease = enzyme that cuts DNA
Endo = cuts the sequence it wants (anywhere on the strand)
Exo = cuts the DNA by digesting it from one end of the strand
Between the endo and exonuclease, which one is very specific ?
Endonuclease
Describe the Nucleotide Excision Repair mechanism (NER)
Repairs bulky damages in DNA that distort the double helix
- Several nucleotides in the damaged strand are removed from the DNA, and the intact strand is used as a template
Name the steps of Nucleotide Excision repair (NER)
- A nuclease excises the lesion (detects and removes the faulty nucleotides) (they remove a lil extra)
- A DNApol I synthesizes the missing nucleotides
- A DNA ligase ligates the newly synthesized fragment by forming the phosphodiester bonds.
Describe the mismatch repair mechanism (MMR)
Repairs the newly synthesized strand (not the parental!)
(fixes a base-pair mismatch, not an abnormal nucleotide like BER or NER)
Describe the Non-Homologous End Joining (NHEJ)
Fixes double-strand breaks by simply attaching the two broken strands back together
Name all the steps to Non-Homologous End Joining (NHEJ)
- the DSB (double strand break) is recognized by end-binding proteins and kept close together thanks to proteins that form a crossbridge
- Additional proteins digest one of the two DNA strands (deletion of a few nucleotides)
- Gaps are filled in by a DNA polymerase
- DNA ends are ligated together by a ligase
True or false : Non-Homologous End Joining (NHEJ) can occur at any phase of the cell cycle
True (because it doesn’t require a sister chromatid !!)
what is Homolohous Recombination Repair (HRR) ?
Also called Homology Directed Repair (HDR)
- Fixes double-strand breaks (DSB) using a sister chromatid
Name the steps to Homologous recombination repair (HRR)
- Needs a non-damaged DNA fragment
- Digestion of short segments of both DNA strands at the break site
- Invasion and exchange of strands between the broken/unbroken sister chromatids
- The unbroken strands are then used as templates to synthesize DNA
- Finally, the crisscrossed strands are resolved (broken and rejoined)
