The correct answer is 4. All of the above.
A transcription unit in DNA is defined primarily by the following regions:
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Promoter: The promoter is a region of DNA that signals the starting point for transcription. It provides the binding site for RNA polymerase, the enzyme responsible for synthesizing RNA from the DNA template.
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Structural Gene: The structural gene is the DNA sequence that encodes the information to build a functional RNA or protein product. It includes the coding region for the RNA molecule that will be transcribed.
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Terminator: The terminator is a DNA sequence that signals the end of transcription. It marks the point at which RNA polymerase should stop synthesizing the RNA molecule.
Therefore, all of the above components (Promoter, Structural Gene, and Terminator) collectively define a transcription unit in DNA.