Final answer:
DNA polymerase can fill in the gap left by excised damaged DNA without a primer by adding nucleotides using the opposite strand as a template. This is followed by sealing the gap with DNA ligase, which creates a phosphodiester bond.
Step-by-step explanation:
After exonucleases have excised the damaged portion of DNA, DNA polymerase can fill in the gap by adding deoxyribonucleotide triphosphates (dNTPs). Even though a primer is generally required for DNA polymerase to initiate the synthesis of DNA, repair mechanisms make use of a different process. During nucleotide excision repair, the DNA around the damage is unwound and the damaged nucleotides are removed. New nucleotides are then added by DNA polymerase, which synthesizes DNA in the 5'-3' direction, using the undamaged complementary strand as a template. Once the gap has been filled, the DNA ligase enzyme seals the gap by creating a phosphodiester bond to complete the repair process. This repair mechanism is crucial for correcting errors such as thymine dimers caused by UV exposure.