Mutations and Repair Flashcards
Types of Spontaneous mutations:
Point mutations, insertions and deletions, Replication Slippage and Tautomerization
Typers of DNA repair system:
- Direct Repair
- Excision Repair
a. Base excision repair (BER)
b. Nucleotide excision repair (NER)
c. Mismatch excision repair (MMR) - Homologous recombination
- Nonhomologous end-joining (NHEJ)
Long-patch pathway of base excision repair.
- Glycosylases cleaves the glycosidic bond between the damaged base and the deoxribose, leaving an AP site/baseless site.
- Endonuclease APE1 cleaves on the 5’ side of the AP site
- Replication complex with DNA polymerase synthesizes 2-10 nt in the 5-3’ direction creatin a 5’ flap
- FEN1 removes the displaced DNA
- Ligase seals the nick
Short-patch pathway of base excision repair
- Glycosylases cleave the glycosidic bond between the damaged base and the deoxyribose, leaving an AP site/baseless site.
- Lyase break the sugar ring creating a nick on the 3’ side of the AP site
- APE1 and DNA polymerase replace a single nucleotide
- Ligase seals the nick
The UvrABC system in E. coli
Nucleotide excision repair
1. Recognition step -> the UvrAB complex recognized the damage and binds to DNA
2. Incision step -> UvrA dissociates, and UvrC joins, creating the UvrCB complex that cleaved on each side of the damage
3. Excision step -> Helicase UvrD removes the damaged DNA sequence
4. Gap-filling step -> DNA polymerase synthesizes the replacement DNA and DNA ligase seals the nick
MMR system in E.coli
Consists of three proteins: MutS, MutL and MutH
1. MutS dimer recognizes and binds to the mismatch
2. MutL dimer binds to MutS
3. MutS translocated along the DNA until a GATC site is encountered, creating a loop in the DNA
4. MutH endonuclease joins to MutSL and cleaves the unmethylated strand
5. Cleaved DNA is excised by an exonulcease or/and helicase from the GATC to the mismatch site
6. DNA polymerase synthesizes a new strand and DNA ligase seal the nick
