Final answer:
Mismatch repair differentiates the new and old DNA strands based on methylation status in bacteria, while nucleotide excision repair fixes damaged DNA segments like pyrimidine dimers by removing and replacing them with correct nucleotides.
Step-by-step explanation:
During mismatch repair, specific enzymes must determine which DNA strand contains the error to be excised. In E. coli, this determination is aided by the methylation status of the strands: the parental strand is methylated while the newly synthesized daughter strand is not. This allows the repair enzymes to identify and target the unmethylated, erroneous strand. In eukaryotes, the method is more complex and not completely understood, but it involves recognizing unsealed nicks on the new strand, possibly associated with the replication machinery.
Nucleotide excision repair is a separate mechanism that specifically addresses damage such as pyrimidine dimers caused by UV radiation. This process involves excision of a DNA segment around the damaged site, which is then replaced with the correctly paired nucleotides. The key enzymes in this intricate process include DNA polymerase for nucleotide replacement and DNA ligase for sealing the gap to restore the integrity of the DNA.