Question

With reference to the mechanisms of mismatch repair and nucleotide-excision repair in E. coli cell, which mechanism is more energy costing? Explain your answer.

Answer 1

Mechanisms involved in E.coli cell, are nucleotide-excision repair and mismatch repair. Mismatch repair which is used to repair errors that occur during DNA synthesis, in contrast to nucleotide excision repair , mismatch repair does not operate on bulky adducts or major distortions to the DNA helix. Nucleotide excision repair is a DNA repair mechanism (NER)- is particularly important excision mechanism that removes DNA damage induced by ultraviolet light(UV).

Nucleotide excision in E.coli, proteins UvrA, UvrB, and UvrC are involved in removing the damaged nucleotides i.e the dimer induced by UV light. The gap is then filled by DNA polymerase I and DNA ligase. To repair mismatched the bases, the system has to know which base is correct one. In E.coli, this is achieved by a special methylase called the dam methylase, which can methylate all adenines that occur within (5') GATC sequences. Immediately after DNA replication, the template strand has been methylates, but the newly synthesized is not methylated yet. Thus, the template strand and the new strand can be distinguished. The distance between the GATC site and the mismatch could as long as 1,000 base pairs. Therefore, mismatch repair is very expensive and inefficient.

Nucleotide excision repairs damaged DNA which commonly UvrA2 then dissociates, in a step that requires ATP hydrolysis. This is an autocatalytic reaction, which is itself an ATPase. Like all helicase reactions , the unwinding requires ATP hydrolysis to disrupt the base pairs. Thus ATP hydrolysis is required at three steps of this series of reactions. Mismatch repair is highly conserved, and which can distinguish normal base pairs from those resulting from misincorporation . It does this by determining which strand was more recently synthesized , and repairing the mismatch on the nascent strand.