Read the passage given below and answer the following five questions.
The process of copying genetic information from one strand of the DNA into RNA is termed as transcription. Here also, the principle of complementarity governs the process of transcription, except the adenosine complements now forms base pair with uracil instead of thymine. A transcription unit in DNA is defined primarily by the three regions in the DNA. There is a convention in defining the two strands of the DNA in the structural gene of a transcription unit. Since the two strands have opposite polarity, 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. The promoter and terminator flank the structural gene in a transcription unit. The promoter 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.
The promoter is said to be located towards which site of the structural gene (reference is made with respect to the polarity of the coding strand)?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Towards 5'-end (upstream)
A transcription unit has three regions: a promoter, the structural gene and a terminator. Positions in it are always described with reference to the coding strand, which runs 5′→3′. “Upstream” means towards the 5′-end, and “downstream” means towards the 3′-end. The promoter, where RNA polymerase binds, sits before the gene, so it lies upstream, towards the 5′-end. The terminator lies after the gene, downstream.
Option (1) Towards 5'-end (upstream) — Correct. The promoter is located towards the 5′-end (upstream) of the structural gene, read along the coding strand.
Option (2) Towards 3'-end (upstream) — Incorrect. The 3′-end of the coding strand is the downstream side, not upstream. The two words in this option do not match each other.
Option (3) Towards 5'-end (downstream) — Incorrect. The 5′-end side is called upstream. Calling it downstream mixes up the two terms.
Option (4) Towards 3'-end (downstream) — Incorrect. The 3′-end (downstream) side holds the terminator, which marks the end of transcription. The promoter is at the opposite end.
NCERT Extract:
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