Biology · Cell: The Unit of Life · NEET
The nucleolus is NOT membrane-bound. This is a very common NEET trap. Unlike the nucleus, mitochondria or lysosome, the nucleolus has no membrane around it. Its material is continuous with the rest of the nucleoplasm. So it is a naked, dense, spherical structure. NCERT says clearly: 'it is not a membrane bound structure.'
For NEET, the direct NCERT answer is: the nucleolus is the site of active ribosomal RNA (rRNA) synthesis. The rRNA made here later joins with proteins to help build ribosome subunits. But if the question asks 'site of active RNA synthesis' or 'which RNA', the answer is rRNA. Do not write 'protein synthesis' - that happens on ribosomes in the cytoplasm.
The nucleus is the whole large organelle covered by a double nuclear envelope with pores, and it holds the DNA (chromatin). The nucleolus is a small, round, non-membrane structure sitting INSIDE the nucleus that makes rRNA. So nucleolus is a part inside the nucleus, not the same thing. One cell can have one or more nucleoli.
NCERT states that larger and more numerous nucleoli are present in cells that are actively making protein. More protein synthesis means the cell needs more ribosomes, so it needs more rRNA. To make more rRNA, the nucleolus grows bigger and there may be more of them. Note the trap: NCERT links big nucleoli to 'protein-making cells', not simply to 'dividing cells'.
No. The nucleolus does NOT form the spindle. Spindle fibres come from centrioles (centrosome). A 2018 NEET option tried to trick students with 'It takes part in spindle formation' - that is wrong for the nucleolus. Its job is rRNA synthesis, not spindle formation.
Which of the following is true for nucleolus?
Non-membranous nucleoplasmic structures in nucleus are the site for active synthesis of:
Match List I with List II. A. Nucleolus B. Centriole C. Leucoplasts D. Golgi apparatus | I. Site of formation of glycolipid II. Organisation like the cartwheel III. Site for active ribosomal RNA synthesis IV. For storing nutrients
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Its main function is the active synthesis of ribosomal RNA (rRNA). This rRNA later helps make ribosome subunits, which then carry out protein synthesis in the cytoplasm.
No. The nucleolus has no membrane. Its contents are continuous with the nucleoplasm. This is a key NEET point that separates it from organelles like the nucleus and mitochondria.
The nucleolus is a round, dense structure found inside the nucleus, floating in the nucleoplasm. A cell may have one or more nucleoli.
No, it does not make proteins directly. It makes rRNA. Protein synthesis takes place on ribosomes in the cytoplasm or on the rough endoplasmic reticulum.
Cells that make a lot of protein need many ribosomes, and ribosomes need rRNA. So the nucleolus becomes larger and more numerous to produce more rRNA.
