Unit 1-Lecture 5 Flashcards
Nuclease
Breaks phosphate sugar bond
-Can be DNAse or RNAse
Endonuclease
Breaks phosphate sugar bond throughout the middle of DNA strand
Exonuclease
Breaks phosphate sugar bond at the ends of DNA strand
DNA polymerase
Adds nucleotides to DNA strand gaps
Ligase
Connects phosphate sugar bonds between DNA strands (acts like glue after DNA polymerase)
How does it know which is mutated strand
Template strand is labeled with a chemical mark (methylation), so only the other strand is checked for errors
Mismatch Excision Repair
Incorrect base in daughter strand is recognized
Base Excision Repair
C base gets changed to a T base and is paired with G (T -G)
How does the base excision repair machinery know which base to remove?
It does not know, T automatically gets removed/fixed in order to be efficient
Nucleotide Excision Repair
-Caused by UV light
-Crosslink forms Thymine-Thymine dimer (TT dimer)
What is the general strategy of repair of DNA single strand or replication error?
- Damage/error is recognized
- Nucleases remove damaged/error nucleotide
- Gap is filled with DNA. polymerase and ligase
How does a double strand fix a break?
- Homologous Recombination
- End-joining
Homologous Recombination
-error free
-Undamaged siuster chromatid serves as template–>end of S phase
End-joining
-error prone
-Two separated ends rejoined or different broken ends joined. (translocations)
Which way does DNA Replication and DNA Polymerization occur from?
-5’ to 3’ of growing DNA
-New nucleotide is added to 3’ end of growing DNA b/c it has the OH group
