Which is not a part of a transcription unit in DNA?

The correct answer is Option (2) → Inducer

A transcription unit is the stretch of DNA that is copied into one RNA. It has three regions: a promoter, the structural gene and a terminator. Anything else is not part of the unit.

Option (1) Promoter — A part of a transcription unit. It lies at the start of the unit and gives RNA polymerase a place to bind.

Option (2) Inducer — Not a part of a transcription unit. An inducer is a molecule, not a stretch of DNA. In the lac operon, lactose is the inducer: it inactivates the repressor so that the operon is switched on.

Option (3) Terminator — A part of a transcription unit. It lies at the end of the unit and marks where transcription stops.

Option (4) Structural gene — A part of a transcription unit. It lies between the promoter and the terminator and is the part copied into RNA.

NCERT Extract:
Transcription Unit – “A transcription unit in DNA is defined primarily by the three regions in the DNA: (i) A Promoter (ii) The Structural gene (iii) A Terminator”
The Lac Operon – “… Lactose is the substrate for the enzyme beta-galactosidase and it regulates switching on and off of the operon. Hence, it is termed as inducer.”

Figure 5.9 Schematic structure of a transcription unit