Questions: Transcription: DNA to RNA

Question 1

A DNA template strand reads 3'-ATCGTA-5'. What is the sequence of the RNA transcript produced from this strand?

• A. 3'-ATCGTA-5'
• B. 5'-TAGCTA-3'
• C. 5'-UAGCAU-3'
• D. 3'-UAGCAU-5'

Answer: (C)

RNA polymerase reads the template strand 3'→5' and synthesizes RNA 5'→3' by complementary base pairing (A→U, T→A, C→G, G→C). Reading 3'-ATCGTA-5' gives RNA 5'-UAGCAU-3'. Option B is incorrect because it uses T instead of U. Option D has the wrong directionality. The RNA matches the coding strand sequence (with U replacing T), which is why option C is correct.

Question 2

The RNA sequence produced during transcription is identical to the coding (non-template) strand of DNA, except that uracil replaces thymine.

• T. True
• F. False

Answer: True

RNA polymerase uses the template (antisense) strand as a guide, synthesizing RNA complementary to it. Since the coding strand and template strand are complementary to each other, the RNA produced is the same sequence as the coding strand — with U in place of T. This is why the coding strand is also called the 'sense strand': it has the same sequence as the mRNA (with T→U substitution). Many students confuse which strand is read, but the product always matches the coding strand.

Question 3

Why can prokaryotes begin translating an mRNA while transcription is still in progress, but eukaryotes cannot?

Model answer: In prokaryotes, there is no nuclear membrane separating transcription from translation — both occur in the same cytoplasmic compartment simultaneously. In eukaryotes, transcription occurs in the nucleus and the primary transcript must be processed (5' cap, poly-A tail, splicing of introns) before export to the cytoplasm, where translation occurs. The physical and processing separation makes concurrent transcription-translation impossible.

This structural difference has major consequences for gene regulation. Prokaryotes can respond to environmental signals almost instantly by beginning translation as soon as RNA synthesis starts. Eukaryotes pay a time and energy cost in RNA processing but gain regulatory opportunities: alternative splicing, nuclear export control, and RNA stability mechanisms all act between transcription and translation, enabling far more complex gene regulation.