Q.26 Nucleolus is involved in the synthesis of
Answer: (A) rRNA
The nucleolus is the site of ribosomal RNA (rRNA) synthesis within the eukaryotic nucleus. It transcribes rRNA genes by RNA polymerase I, processes precursor rRNAs, and assembles ribosomal subunits essential for protein synthesis.
Option Analysis
(A) rRNA
This is correct. The nucleolus forms around nucleolar organizer regions (NORs) containing rRNA genes and handles rRNA transcription, maturation, and initial ribosome assembly.
(B) tRNA
Incorrect. Transfer RNA (tRNA) is synthesized in the nucleoplasm by RNA polymerase III, not in the nucleolus.
(C) DNA
Incorrect. DNA replication and no synthesis occur in the nucleus generally during S phase, but the nucleolus has no role in DNA synthesis.
(D) mRNA
Incorrect. Messenger RNA (mRNA) is transcribed from DNA by RNA polymerase II in the nucleoplasm, outside the nucleolus.
The nucleolus plays a central role in nucleolus rRNA synthesis, making it a high-yield topic for competitive exams like GATE Life Sciences. This structure within the nucleus ensures cells produce ribosomes for protein synthesis.
What is Nucleolus rRNA Synthesis?
Nucleolus rRNA synthesis begins with RNA polymerase I transcribing ribosomal DNA (rDNA) genes into pre-rRNA in the fibrillar center of the nucleolus. The pre-rRNA then undergoes cleavage, modifications like methylation, and assembly with ribosomal proteins in the dense fibrillar and granular components to form ribosomal subunits.
These subunits (40S and 60S) export to the cytoplasm for translation. Disruptions here impact cell growth and are linked to diseases like cancer.
Why Not Other RNAs or DNA?
-
tRNA: Synthesized elsewhere by RNA pol III; nucleolus uninvolved.
-
mRNA: Handled by RNA pol II in nucleoplasm for gene expression.
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DNA: No synthesis role; nucleolus focuses on RNA processing.
| Component | Synthesis Location | Enzyme Involved |
|---|---|---|
| rRNA | Nucleolus | RNA Pol I |
| tRNA | Nucleoplasm | RNA Pol III |
| mRNA | Nucleoplasm | RNA Pol II |
| DNA | Nucleus (S phase) | DNA Pol |
Exam Relevance for GATE Life Sciences
In questions like “Nucleolus is involved in the synthesis of,” pick rRNA to score points. Understand its tripartite structure (fibrillar center, dense fibrillar, granular) for deeper PYQs. Focus on nucleolus rRNA synthesis for molecular biology sections.
