Final answer:
The mechanism responsible for fixing errors by removing modified or abnormal bases is Base Excision Repair (BER). It uses specific enzymes to replace the incorrect base with the correct one after excision. Other mechanisms, such as Mismatch Repair (MMR) and Nucleotide Excision Repair (NER), target different types of DNA repair situations.
Step-by-step explanation:
The mechanism that repairs errors through the removal of modified or abnormal bases is known as Base Excision Repair (BER). This process corrects damage to a single nucleotide caused by multiple factors, such as oxidation, alkylation, hydrolysis, or deamination. The abnormal base is first identified and removed by an enzyme called glycosylase, then the area is prepared for the new base by APE1 and DNA polymerase, which fills the gap. Finally, DNA ligase seals the repaired area, restoring the DNA strand to its correct sequence.
Mismatch Repair (MMR) operates differently, targeting errors that appear as mispaired but otherwise normal nucleotides stemming from replication mistakes. Nucleotide Excision Repair (NER) also differs as it focuses on repairing bulky helix-distorting lesions, such as thymine dimers, caused by UV radiation, affecting longer strands from 2 to 30 bases. Homologous Recombination (HR) is a complex mechanism that deals with the repair of more severe DNA strand breaks.