Transcription Unit — Promoter (5'), Structural Gene, Terminator (3')
📚 Practice ConceptThe promoter and terminator flank the structural gene in a transcription unit. The promoter is said to be located towards 5'-end (upstream) of the structural gene (the reference is made with respect to the polarity of coding strand). It is a DNA sequence that provides binding site for RNA polymerase, and it is the presence of a promoter in a transcription unit that also defines the template and coding strands. By switching its position with terminator, the definition of coding and template strands could be reversed. The terminator is located towards 3'-end (downstream) of the coding strand and it usually defines the end of the process of transcription. There are additional regulatory sequences that may be present further upstream or downstream to the promoter. Some of the properties of these sequences shall be discussed while dealing with regulation of gene expression.
The template strand has a sequence identical to the mRNA and the terminator is located towards the 5' end of the coding strand.
The CODING strand has a sequence identical to mRNA (T→U). The TEMPLATE strand is the antiparallel complement, read 3'→5' by RNA polymerase. Terminator is at the 3' (DOWNSTREAM) end.
Promoter = 5' upstream. Terminator = 3' downstream. Coding strand = mRNA-like (T→U). Template strand = read 3'→5'.
- Three components of transcription unit: promoter + structural gene + terminator.
- Promoter: 5' end (UPSTREAM) of structural gene; binds RNA polymerase; defines coding vs template strand.
- Terminator: 3' end (DOWNSTREAM) of coding strand; defines end of transcription.
- Coding strand has same sequence as mRNA (except T → U). Template strand is read by RNA polymerase 3' → 5'.
Identify which of the following statements about a transcription unit are NOT correct: S1: The promoter region is located towards the 5'-end (upstream) of the structural gene with respect to the polarity of the coding strand. S2: The template strand has a sequence identical to the mRNA and serves as the direct coding sequence for the polypeptide. S3: In prokaryotes, a single transcription unit can often be polycistronic, encoding multiple structural genes. S4: The terminator sequence is located towards the 5'-end (upstream) of the coding strand and defines the end of transcription. S5: Regulatory sequences such as enhancers are considered primary defining components of a transcription unit along with promoter, structural gene, and terminator.
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Correct answer: B — S2, S4, S5 only
S1 CORRECT: Promoter is at 5' upstream of the structural gene (NCERT). S2 WRONG: The CODING strand (not template) has a sequence identical to mRNA — template is the antiparallel complement read 3'→5'. S3 CORRECT: Prokaryotic transcription units can be polycistronic. S4 WRONG: Terminator is at the 3' DOWNSTREAM end, not 5' upstream. S5 WRONG: Regulatory sequences are additional, NOT primary components of the transcription unit — the three core components are promoter + structural gene + terminator only.
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