When RNA polymerase binds to DNA, the double helix does not completely unwind. Instead, it undergoes a localized unwinding in a specific region called the promoter.

Here's what happens:

1. Binding: RNA polymerase binds to the promoter region of the DNA.

2. Unwinding: RNA polymerase separates the two DNA strands in the promoter region, creating a "bubble" of single-stranded DNA.

3. Transcription: The separated strands serve as templates for the synthesis of a new RNA molecule.

4. Re-winding: As RNA polymerase moves along the DNA, the unwound region rewinds behind it, maintaining the double helix structure.

The unwound region is relatively short and only lasts for a short time. This localized unwinding ensures that only a small section of DNA is exposed at any given time, which is necessary for efficient and accurate transcription.

The unwound region is crucial for transcription because it allows RNA polymerase to access the DNA template strand for RNA synthesis.