Answer:
Base excision repair (BER) and nucleotide excision repair (NER) are two different mechanisms that cells use to repair DNA damage. One key difference between these two mechanisms is the type of DNA damage that they repair.
BER is a mechanism that primarily repairs single base lesions or small base modifications, such as the removal of a damaged base or the addition of an incorrect base. BER begins with the recognition and removal of the damaged base by a DNA glycosylase enzyme, which cleaves the glycosidic bond between the damaged base and the sugar-phosphate backbone. The resulting abasic site is then processed by a series of enzymes to remove the damaged base and replace it with a correct base.
In contrast, NER is a mechanism that primarily repairs bulky lesions or damage that distorts the DNA helix, such as those caused by UV radiation or chemical carcinogens. NER begins with the recognition of the damaged DNA by a complex of proteins that scans the DNA for distortions. The damaged region is then excised, along with several nucleotides on either side of the lesion, creating a gap in the DNA. The gap is then filled in by a DNA polymerase enzyme and sealed by a DNA ligase enzyme.
Therefore, the key difference between BER and NER is the type of DNA damage they repair, with BER repairing single base lesions, and NER repairing bulky lesions that distort the DNA helix.