Final answer:
The final step in DNA repair on one strand of the double helix is the action of DNA ligase, which seals the repaired strand, restoring the DNA's integrity.
Step-by-step explanation:
The last step that must occur to repair damage on one strand of the DNA double helix is the sealing of the strand by an enzyme known as DNA ligase. After the process of recognizing the damaged site, removing the incorrect nucleotides, and replacing them with the correct ones using the undamaged strand as a template, DNA ligase performs the critical function of sealing the nicks in the sugar-phosphate backbone of the repaired strand.
This step is essential to restore the integrity and continuous double-stranded structure of the DNA, maintaining the stability of the genetic information.