Cell - The Unit of Life — NEET UG practice

1. Recall the structure and function of the nucleolus. The nucleolus is a dense, non-membrane-bound spherical structure present inside the nucleus, and it is closely associated with specific regions of chromatin called nucleolar organizer regions.

2. Connect this structure to its main role. Since these regions carry the genes for ribosomal RNA (rRNA), the nucleolus is the site of active rRNA synthesis and ribosome subunit assembly.

3. Eliminate the other options. Centrosome organizes microtubules and spindle fibers, chromatin is the DNA-protein complex forming chromosomes, and kinetochore is the protein structure where spindle fibers attach during cell division — none synthesize rRNA.

Hence, the answer is C. Nucleolus.

1. Recall which structures lack a surrounding membrane. Ribosomes are granular structures made of RNA and protein, and they are not enclosed by any membrane.

2. Check their presence across cell types. Ribosomes are found in both prokaryotic cells (as 70S ribosomes) and eukaryotic cells (as 80S ribosomes in the cytoplasm, and 70S in mitochondria/chloroplasts), making them universal to all cells.

3. Eliminate the other options. Lysosomes and mitochondria are membrane-bound organelles found only in eukaryotic cells, and centrosomes, while non-membrane bound, are found only in animal eukaryotic cells, not in prokaryotes.

Hence, the answer is A. Ribosomes.

1. Check statement A about the endomembrane system. Mitochondria function independently and are not functionally coordinated with the ER-Golgi network, so mitochondria are not part of the endomembrane system — this statement is incorrect.

2. Check statement B about Rough ER. By definition, Rough Endoplasmic Reticulum (RER) has ribosomes studded on its outer surface, giving it a rough appearance — this statement is correct.

3. Check statement C about circular DNA. Both mitochondria and plastids are semi-autonomous organelles that contain their own circular DNA, similar to bacterial DNA — this statement is correct.

4. Check statement D about the cytoskeleton. The network formed by microtubules, microfilaments, and intermediate filaments in the cytoplasm is indeed called the cytoskeleton, providing mechanical support and shape — this statement is correct.

5. Check statement E about mitochondrial membrane number. A mitochondrion is bounded by two membranes (an outer and an inner membrane), not a single membrane — this statement is incorrect.

Hence, since B, C, and D are correct, the answer is D. B, C and D only.