Ribosomal RNA locations in eukaryotic cells

Steps:

• Identify structures typically labeled as 1 (ribosomes), 2 (nucleolus), and 3 (rough endoplasmic reticulum) in cell diagrams.
• Recall rRNA is a key component of ribosomes, which are free in cytoplasm or attached to rough ER.
• Note rRNA is synthesized and assembled into ribosomal subunits in the nucleolus within the nucleus.
• Confirm all three structures associate with or contain rRNA during synthesis, assembly, or function.

Why A is correct:

• rRNA is present in ribosomes (structure 1), nucleolus (structure 2 for synthesis), and rough ER (structure 3 via attached ribosomes), per standard cell biology definitions.

Why the others are wrong:

• B excludes 3, but rough ER contains rRNA-bound ribosomes for protein synthesis.
• C excludes 1, but ribosomes are the primary site of rRNA function.
• D excludes 1 and 2, ignoring rRNA's core role in ribosomes and nucleolar assembly.

Final answer: A