Read the given passage and answer the following question.
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. However, unlike in the process of replication, which once set in, the total DNA of an organism gets duplicated, in transcription only a segment of DNA and only one of the strands is copied into RNA. 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.
What is the function of the promoter in a transcription unit?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Provides binding site for RNA polymerase
The passage states that the promoter “is a DNA sequence that provides binding site for RNA polymerase”.
A promoter is a DNA sequence placed at the start of a transcription unit, towards the 5′-end (upstream) of the structural gene. Transcription is done by the enzyme RNA polymerase, and the promoter is the place where this enzyme binds.
Option (1) Provides binding site for DNA polymerase — Incorrect. DNA polymerase copies DNA during replication, not transcription. The promoter is a binding site for the enzyme that makes RNA, not for the one that makes DNA.
Option (2) Provides binding site for RNA polymerase — Correct. RNA polymerase binds at the promoter and then starts copying the structural gene into RNA. The position of the promoter also decides which strand acts as the template strand and which is the coding strand.
Option (3) Terminates transcription — Incorrect. Ending transcription is the job of the terminator, which lies at the other end, towards the 3′-end (downstream) of the coding strand. The promoter works at the start, not at the end.
Option (4) All the above — Incorrect. Options (1) and (3) are wrong. Only Option (2) gives the function of the promoter.
Figure 5.9 Schematic structure of a transcription unit