In nucleotide-excision repair, the first enzyme to act is: Group of answer choices excinuclease DNA ligase DNA polymerase I DNA helicase

In nucleotide-excision repair, the first enzyme to act is DNA helicase.

Here's a step-by-step explanation:

1. DNA damage is recognized: The first step in nucleotide-excision repair is the recognition of DNA damage. This is done by various proteins that scan the DNA for abnormalities.

2. Unwinding of the DNA helix: Once the damage is recognized, the DNA around the damage site is unwound by the enzyme DNA helicase. This is to expose the damaged section of the DNA strand and make it accessible for repair.

3. Excision of the damaged section: The enzyme excinuclease then cuts out the damaged section of the DNA strand.

4. Filling in the gap: After the damaged section is removed, the gap is filled in by DNA polymerase I, which synthesizes a new, correct section of DNA using the undamaged strand as a template.

5. Sealing the new section: Finally, the new section of DNA is sealed into place by the enzyme DNA ligase, completing the repair process.

So, the first enzyme to act in nucleotide-excision repair is DNA helicase.