Question

Read the following passage and answer the questions that follow :

The process of copying genetic information from template strand of DNA into RNA is called transcription. It is mediated by RNA polymerase. Transcription takes place in the nucleus of eukaryotic cells. In transcription, only a segment of DNA and only one of the strands is copied into RNA.

(a) Why is the strand of DNA with \( 3^{\prime} \rightarrow 5^{\prime} \) polarity transcribed and not the other strand of \( 5^{\prime} \rightarrow 3^{\prime} \) polarity ?

Hint:

RNA polymerase can add nucleotides only in the 5 prime to 3 prime direction.

Answer 1

Step 1: Recall how RNA polymerase works:
RNA polymerase builds the new RNA chain by adding nucleotides to its free \( 3^{\prime} \) OH end.
So the RNA always grows in the \( 5^{\prime} \rightarrow 3^{\prime} \) direction.

Step 2: Use the antiparallel rule:
The two strands of DNA run in opposite directions. The new RNA must pair with its template in the opposite direction.
For the RNA to grow \( 5^{\prime} \rightarrow 3^{\prime} \), the template must be read from its \( 3^{\prime} \) end towards its \( 5^{\prime} \) end.

Step 3: Why not the other strand:
If the \( 5^{\prime} \rightarrow 3^{\prime} \) strand were the template, the RNA would have to grow \( 3^{\prime} \rightarrow 5^{\prime} \).
No polymerase can add nucleotides in that direction. So only the \( 3^{\prime} \rightarrow 5^{\prime} \) strand is the template strand.

Final Answer:
RNA polymerase can add nucleotides only in the \( 5^{\prime} \rightarrow 3^{\prime} \) direction, so it must read the \( 3^{\prime} \rightarrow 5^{\prime} \) template strand.