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