Choose the incorrect statements from the following:
A. In a transcription unit, the promoter is a DNA sequence that provides the binding site for RNA polymerase.
B. In a transcription unit, switching the position of the promoter with the terminator could reverse the definition of the coding and template strands.
C. In a transcription unit, the terminator is located towards the 5′-end (upstream) of the coding strand.
D. In a transcription unit, the strand with polarity 3′→5′ is displaced during transcription and is referred to as the coding strand.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → C and D only
A transcription unit has three parts: a promoter, the structural gene and a terminator. RNA polymerase works only in the 5′→3′ direction, so it uses the 3′→5′ strand as the template. All positions (upstream and downstream) are stated with respect to the coding strand.
A. In a transcription unit, the promoter is a DNA sequence that provides the binding site for RNA polymerase. — True. RNA polymerase binds at the promoter, which lies upstream (towards the 5′-end) of the structural gene.
B. In a transcription unit, switching the position of the promoter with the terminator could reverse the definition of the coding and template strands. — True. The promoter decides which strand is read. If promoter and terminator change places, the enzyme would read the other strand, so the two strands would swap names.
C. In a transcription unit, the terminator is located towards the 5′-end (upstream) of the coding strand. — False. The terminator is towards the 3′-end (downstream) of the coding strand. It is the promoter that lies towards the 5′-end (upstream).
D. In a transcription unit, the strand with polarity 3′→5′ is displaced during transcription and is referred to as the coding strand. — False. The 3′→5′ strand acts as the template strand. The strand that is displaced and called the coding strand has the polarity 5′→3′, with the same sequence as the RNA (T in place of U).
So the incorrect statements are C and D.
NCERT Extract:
Transcription Unit – “… Since the two strands have opposite polarity and the DNA-dependent RNA polymerase also catalyse the polymerisation in only one direction, that is, 5′→3′, the strand that has the polarity 3′→5′ acts as a template, and is also referred to as template strand. The other strand which has the polarity (5′→3′) and the sequence same as RNA (except thymine at the place of uracil), is displaced during transcription. Strangely, this strand (which does not code for anything) is referred to as coding strand.”
Transcription Unit – “… The 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 (Figure 5.9).”
Figure 5.9 Schematic structure of a transcription unit