Question

Ribosomes are synthesized by

• A. Nucleus
• B. Nucleolus
• C. Endoplasmic reticulum
• D. Mitochondria

Hint:

The nucleolus disappears during cell division (prophase) and reappears during telophase. It is often referred to as the "Ribosome Factory" of the cell.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Ribosomes are the cellular "protein factories" composed of ribosomal RNA (rRNA) and ribosomal proteins. Their synthesis is a complex, high-energy process that occurs in a specialized sub-compartment of the nucleus.

Step 2: Detailed Explanation:
The nucleolus is a prominent, non-membrane-bound structure within the nucleus. Its primary function is the transcription of rRNA genes and the assembly of ribosomal subunits. The process begins with the transcription of long precursor rRNA molecules by RNA polymerase I. These precursors are then processed into smaller rRNA species (18S, 5.8S, and 28S in eukaryotes). Meanwhile, ribosomal proteins, which are synthesized in the cytoplasm, are imported into the nucleus and move to the nucleolus. There, they associate with the rRNAs to form the large and small ribosomal subunits. These subunits are eventually exported back to the cytoplasm through nuclear pores to form functional ribosomes.
While the nucleolus is located inside the nucleus (Option 1), "nucleolus" is the more specific and scientifically accurate answer. The Endoplasmic Reticulum (Option 3) is a site where ribosomes may attach to synthesize membrane or secreted proteins, but it does not synthesize the ribosomes themselves. Mitochondria (Option 4) have their own ribosomes (mitoribosomes) but are not the primary site of cellular ribosome synthesis.

Step 3: Final Answer:
The nucleolus is the specific site for the assembly of ribosomal RNA and proteins into ribosomal subunits.