Question

Describe each of the following repair mechanisms. Your descriptions should (1) differentiate between the mechanisms, (2) indicate under what circumstances each mechanism is active, (3) make clear the basic steps of each mechanism and (4) identify the key enzymes involved. You might consider creating a diagram or table to assist you with your explanation.

a.   proofreading

b.   mismatch repair

c.   excision repair (focus on nucleotide excision repair)

d.   photoreactivation       

e.   recombinational repair

Answer 1

TYPE	DAMAAGE	ENZYME
Proofreading	check correction for newly added base.	DNA Polymerase
Mismatch Repair	Replication errors	MutS,MutL,and MutH in E.coli MSH,MLH, and PMS in humans
Nucleotide excision Repair	Pyrimidine dimer Bulky adduct on base	UvrA, UvrB, UvrC and UvrD in E.coli XPC, XPA, XPD, ERCI-XPF and XPG in humans
Photoreactivation	Pyrimidine dimers	DNA photolyase
Recombinational Repair	Double strand breaks	Rec-A and RecBCD in E.coli

1) In proofreading, the DNa pol reads the newly added base before adding the next one, so a correction can be made. The polymerase checks whether the newly added base has paired correctly with the base template strand. If it is right, the next nucleotide is added.

2)Mismatch repair is a process that corrects mismatched nucleotides in the otherwise complementary paired DNA strands, arising from DNA replication errors and recombination, as well as from some types of base modifications.

3)Nucleotide excision repair is a mechanism in which a damaged region of DNA is cut out and replaced by DNA synthesized using the undamaged strand as template.

4)Photoreactivation is a type of DNA repair mechanism present in prokaryotes, archaea and in many eukaryotes. It is the recovery of ultraviolet irradiated damages of DNA by visible light. As the name suggests, it is a light dependent process.

5)Recombination Repair. Recombination repair is a mechanism for generating a functional DNA molecule from two damaged molecules. It is an essential repair process for dividing cells because a replication fork may arrive at a damaged site, such as a thymine dimer, before the excision repair system has eliminated damage.