A Levels Biology (9700)•9700/12/M/J/23
Explanation
mRNA Localization in Eukaryotic Cells
Steps:
- mRNA is transcribed from DNA in the nucleus (structure 1).
- Mature mRNA exits the nucleus into the cytoplasm.
- mRNA binds to ribosomes (structure 3) for translation into proteins.
- Structures 2 (e.g., rough ER) and 4 (e.g., mitochondria) lack free or bound mRNA.
Why B is correct:
- mRNA resides in the nucleus during synthesis and processing, and associates directly with ribosomes per the central dogma of molecular biology.
Why the others are wrong:
- A includes 2 and 4, which do not synthesize or bind mRNA.
- C omits 1 and 3, the primary sites for mRNA presence.
- D omits 1, ignoring nuclear transcription.
Final answer: B
Topic: Protein synthesis
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