Introduction:
During transcription, only one strand of DNA is copied to form an RNA molecule. This process is crucial for the synthesis of proteins and gene expression. The reason why both strands of DNA are not copied during transcription can be attributed to several factors.

Strand Separation:
Before transcription can occur, the two strands of DNA must be separated. This separation is facilitated by the enzyme RNA polymerase, which binds to a specific region on the DNA called the promoter. The RNA polymerase unwinds the DNA double helix, exposing the template strand for transcription.

Template Strand:
During transcription, only one of the DNA strands, known as the template strand, is used as a template for RNA synthesis. This strand is chosen because it contains the complementary sequence to the RNA molecule that will be produced. The other strand, known as the non-template or coding strand, has the same sequence as the RNA molecule, with the exception of uracil (U) replacing thymine (T).

Complementary Base Pairing:
The template strand serves as a template for RNA synthesis because of the complementary base pairing rules. Adenine (A) on the template strand pairs with uracil (U) in the RNA molecule, guanine (G) pairs with cytosine (C), and cytosine (C) pairs with guanine (G). This ensures that the RNA molecule has the same sequence as the coding strand, except with U instead of T.

Efficiency and Necessity:
Copying both strands of DNA during transcription would be inefficient and unnecessary. Since the coding strand has the same sequence as the RNA molecule, it would essentially be duplicating the same information. Additionally, transcribing both strands would require more energy and resources for the cell.

Conclusion:
In summary, during transcription, only one strand of DNA is copied to form an RNA molecule. The template strand is selected due to its complementary base pairing with the RNA molecule. Copying both strands would be redundant and inefficient. Transcription is a highly regulated process that ensures the accurate synthesis of RNA and subsequent protein production.