Protein Synthesis - Transcription Unit and the Gene

Estimated time: 6 minutes

CBSE: Class 12
CISCE: Class 12

Key Points: Transcription Unit

Component	Location	Function
Promoter	At the 5′ end of the structural gene	Provides binding site for RNA polymerase and initiates transcription
Structural Gene	Between promoter and terminator	Contains genetic information to be transcribed
Template Strand	DNA strand with 3′ → 5′ polarity	Serves as template for RNA synthesis
Coding Strand	DNA strand with 5′ → 3′ polarity	Does not code directly; used as reference strand
Terminator	At the 3′ end of the coding strand	Signals the end of transcription

Maharashtra State Board: Class 12

A transcription unit is a segment of DNA consisting of a promoter (start site), a structural gene, and a terminator (end site).
The template strand (3′→5′) is used for RNA synthesis, while the coding strand (5′→3′) has the same sequence as mRNA (except U replaces T).
RNA polymerase binds to the promoter, synthesises RNA in 5′→3′ direction, and stops at the terminator.
A gene is a DNA sequence that codes for RNA or protein; a cistron is a unit coding for a polypeptide.
Monocistronic mRNA (in eukaryotes) has one gene per transcript, while polycistronic mRNA (in bacteria) has multiple genes in one transcript.
In eukaryotes, three RNA polymerases are present: RNA polymerase I (rRNA), RNA polymerase II (mRNA/hnRNA), and RNA polymerase III (tRNA and snRNA).