Where are ribosomal subunits assembled?

• A. In the cytoplasm
• B. In the mitochondria
• C. In the endoplasmic reticulum
• D. In the nucleolus inside the nucleus

Answer: (D)

Ribosomal subunits are assembled in the nucleolus, a region inside the nucleus where ribosomal RNA genes are transcribed and processed and ribosomal proteins imported from the cytoplasm come together with rRNA to form the small and large subunits. After assembly, these subunits are exported through nuclear pores into the cytoplasm, where they participate in translation as 40S and 60S units. The cytoplasm is where ribosomes function, not where the subunits are first assembled. Mitochondria have their own ribosomes and assemble those ribosomal units inside the organelle, not the cytoplasmic ones. The endoplasmic reticulum houses ribosomes that translate certain proteins, but the actual assembly of ribosomal subunits occurs in the nucleolus. Hence, the nucleolus inside the nucleus is the correct location for assembly.