Question

Given below are the two statements:
Statement I: Only one strand in the transcription unit functions as template and is called antisense strand.
Statement II: The coding strand of transcription unit is called sense strand.
In light of the above statements, choose the correct answer from the options given below.

• A. Both statement I and statement II are correct.
• B. Both statement I and statement II are incorrect.
• C. Statement I is correct and statement II is incorrect.
• D. Statement I is incorrect and statement II is correct.

Hint:

{Memory Trick:}

• {Template strand} → Used by RNA polymerase to synthesize mRNA
• {Coding strand} → Has the same sequence as mRNA (T replaced by U)

Although it is called the {coding strand}, it is actually the {template strand that is read during transcription}.

Answer 1

The Correct Option is A

Concept:
Transcription is the biological process in which genetic information stored in DNA is copied into an RNA molecule, usually messenger RNA (mRNA). Although DNA consists of two complementary strands, only one strand of DNA serves as the template for RNA synthesis. The two strands of DNA involved in transcription are named based on their relationship with the produced mRNA:

• Template strand (Antisense strand): This strand is read by RNA polymerase in the \(3' \rightarrow 5'\) direction to synthesize RNA in the \(5' \rightarrow 3'\) direction.
• Coding strand (Sense strand): This strand has the same nucleotide sequence as the mRNA (except that thymine (T) in DNA is replaced by uracil (U) in RNA).

Step 1: Understanding the template strand.
During transcription, RNA polymerase binds to DNA and reads the template (antisense) strand. The sequence of mRNA is formed by pairing complementary bases with this strand. Therefore, the mRNA sequence is complementary to the template strand. Hence, the statement that the template strand is also called the antisense strand is correct.
Step 2: Understanding the coding strand.
The other DNA strand is known as the coding (sense) strand. This strand is not directly used by RNA polymerase during transcription. However, its nucleotide sequence is almost identical to that of the mRNA produced, except that: \[ \text{Thymine (T) in DNA is replaced by Uracil (U) in RNA. \] Because its sequence resembles the final genetic code carried by mRNA, it is referred to as the sense strand. Therefore, the second statement is also correct. Final Answer:
Both Statement I and Statement II are correct.