Question

(a) Construct a complete transcription unit with promoter and terminator on the basis of the hypothetical coding strand given below :

A T G C A T G C A T A G

(b) Write the RNA strand transcribed from the above transcription unit along with its polarity.

Hint:

Complement the coding strand to get the template strand. Put the promoter upstream and the terminator downstream. The RNA matches the coding strand with U in place of T.

Answer 1

Step 1: Recall the parts of a transcription unit:
A transcription unit has three parts: a promoter, the structural gene and a terminator.
The promoter lies at the \( 3^{\prime} \) end of the template strand, which is the \( 5^{\prime} \) (upstream) side of the coding strand.
The terminator lies at the \( 5^{\prime} \) end of the template strand, which is the \( 3^{\prime} \) end of the coding strand.

Step 2: Find the template strand:
The given strand is the coding strand. We read it as \( 5^{\prime} \to 3^{\prime} \) from left to right.
The template strand is complementary and antiparallel: A pairs with T and G pairs with C.
Coding strand: \( 5^{\prime} \)-ATGCATGCATAG-\( 3^{\prime} \).
Template strand: \( 3^{\prime} \)-TACGTACGTATC-\( 5^{\prime} \).

Step 3: Part (a): Draw the transcription unit:
Coding strand: \( 5^{\prime} \)-[Promoter]-ATGCATGCATAG-[Terminator]-\( 3^{\prime} \)
Template strand: \( 3^{\prime} \)-[Promoter]-TACGTACGTATC-[Terminator]-\( 5^{\prime} \)
The promoter is the site where RNA polymerase binds. The terminator is the site where transcription stops.

Step 4: Part (b): Write the RNA:
RNA polymerase reads the template strand in the \( 3^{\prime} \to 5^{\prime} \) direction and builds RNA in the \( 5^{\prime} \to 3^{\prime} \) direction.
RNA has uracil (U) in place of thymine (T). So the RNA has the same sequence as the coding strand, with U for T.
\[ 5^{\prime}\text{-AUGCAUGCAUAG-}3^{\prime} \]

Final Answer:
(a) The promoter lies on the upstream side of the given gene and the terminator on the downstream side. (b) The RNA formed is \( 5^{\prime} \)-AUGCAUGCAUAG-\( 3^{\prime} \).