Question

(a) Draw a schematic representation of the structure of a transcription unit and show the following in it :

(i) Direction in which transcription occurs

(ii) Polarity of the two strands involved

(iii) Template strand

(iv) Terminator

(b) Mention the function of promoter in transcription.

Hint:

Show promoter, structural gene and terminator; the template strand reads from the 3-end to the 5-end; the promoter is the RNA polymerase binding site.

Answer 1

Step 1: Part (a): what a transcription unit has:
A transcription unit has three regions: a promoter, the structural gene and a terminator. A labelled sketch is expected. The parts are listed below in the order they appear on the diagram.

Step 2: Part (a): labelled parts of the sketch:
1. Draw two parallel DNA strands.
2. The upper strand is the coding strand, with polarity \( 5^{\prime} \to 3^{\prime} \) from left to right.
3. The lower strand is the template strand, with polarity \( 3^{\prime} \to 5^{\prime} \) from left to right. It is the strand that RNA polymerase reads.
4. On the left (\( 5^{\prime} \) end of the coding strand) mark the promoter.
5. In the middle mark the structural gene.
6. On the right (\( 3^{\prime} \) end) mark the terminator.
7. Draw an arrow from the promoter towards the terminator, to show the direction of transcription.

Step 3: Part (b): function of the promoter:
The promoter is the DNA sequence to which RNA polymerase binds. It gives the signal to start transcription.
It is placed at the \( 5^{\prime} \) side (upstream) of the structural gene, so it decides where transcription begins.

Final Answer:
Promoter at the \( 5^{\prime} \) end, structural gene, terminator at the \( 3^{\prime} \) end; template strand is \( 3^{\prime} \to 5^{\prime} \); the promoter binds RNA polymerase to start transcription.