Explain how transcription produces a messenger RNA (mRNA) molecule from a DNA template strand.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
RNA polymerase binds to the promoter region of the DNA and unwinds the double helix by breaking hydrogen bonds between complementary base pairs.
RNA polymerase moves along the template (antisense) strand in the 3′→5′ direction, reading the exposed bases.
Free RNA nucleotides are added by complementary base pairing to the template strand, with uracil (U) pairing with adenine (A) instead of thymine.
RNA polymerase joins the RNA nucleotides by forming phosphodiester bonds, elongating the mRNA in the 5′→3′ direction until a termination sequence is reached and the mRNA is released.
RNA polymerase moves along the template (antisense) strand in the 3′→5′ direction, reading the exposed bases.
Free RNA nucleotides are added by complementary base pairing to the template strand, with uracil (U) pairing with adenine (A) instead of thymine.
RNA polymerase joins the RNA nucleotides by forming phosphodiester bonds, elongating the mRNA in the 5′→3′ direction until a termination sequence is reached and the mRNA is released.
Examiner tips
- Use the exact sequence of steps: binding, unwinding, reading, base‑pairing, elongation, termination.
- Mention the 3′→5′ direction of the template and the 5′→3′ direction of the mRNA.
- Include the special base pair U‑A instead of T‑A.
- Show the role of the promoter and termination sequences.
Common mistakes
- Confusing the direction of the template strand (writing 5′→3′ instead of 3′→5′).
- Forgetting that uracil replaces thymine in RNA.
- Omitting the promoter or termination sequence from the explanation.
Mark scheme (4 marks)
- RNA polymerase binds to the promoter region of the DNA and unwinds / unzips the double helix by breaking hydrogen bonds between complementary base pairs
- RNA polymerase moves along the template (antisense) strand in the 3′ to 5′ direction, reading the exposed bases
- Free RNA nucleotides are added by complementary base pairing to the template strand, with uracil (U) pairing with adenine (A) instead of thymine
- RNA polymerase joins the RNA nucleotides by forming phosphodiester bonds, elongating the mRNA in the 5′ to 3′ direction, until a termination sequence is reached and the mRNA is released
Key terms in this question
transcription · template strand · mRNA
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →