Final answer:
The Nucleotide Excision Repair Mechanism (D) is responsible for fixing pyrimidine dimers, like thymine dimers, by removing the damaged segment and replacing it with the correct nucleotides.
Step-by-step explanation:
The repair mechanism responsible for fixing pyrimidine dimers, such as thymine dimers, is the Nucleotide Excision Repair Mechanism (D). This repair process is invoked when DNA is damaged by factors such as ultraviolet (UV) light, which can cause adjacent pyrimidines on a DNA strand to become covalently bonded into a dimer. The nucleotide excision repair pathway specifically addresses these kinds of lesions in DNA.
In this process, enzymes recognize the distortion in the DNA caused by the pyrimidine dimer. They make cuts on both the 3' and 5' ends of the dimer and remove the segment of DNA containing the dimer. DNA polymerase then fills in the gap by adding the correct nucleotides, using the undamaged strand as the template. Finally, DNA ligase seals the gap with a phosphodiester bond, completing the repair and restoring the integrity of the DNA molecule.