Nucleus as the site of rRNA synthesis

Steps:

• Recall that the nucleus houses genetic material and controls cellular activities.
• Identify key nuclear components: chromatin for DNA, nucleolus for ribosome biogenesis.
• Link function to rRNA: nucleolus transcribes rRNA from DNA templates.
• Eliminate options by matching to known cellular processes outside the nucleus.

Why D is correct:

• The nucleolus in the nucleus synthesizes rRNA, essential for ribosome assembly, as per eukaryotic cell biology definitions.

Why the others are wrong:

• A: Nuclear envelope forms and breaks down during mitosis, not a primary nucleus function.
• B: Rough ER forms in the cytoplasm, attached to ER membranes, not the nucleus.
• C: Ribosomal proteins synthesize on cytoplasmic ribosomes, not in the nucleus.

Final answer: D