Centrosome and Centrioles

NCERT grounding

Section 8.5.9 of NCERT Class 11 Biology defines the centrosome as "an organelle usually containing two cylindrical structures called centrioles… surrounded by amorphous pericentriolar materials." The textbook is explicit that the two centrioles "lie perpendicular to each other," that each shows "an organisation like the cartwheel," and that they are made of "nine evenly spaced peripheral fibrils of tubulin protein," with each fibril a triplet linked to its neighbours. The proximal hub is proteinaceous and connects to the peripheral triplets through radial spokes. NCERT closes the section by stating that "the centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells." NIOS Lesson 4 supplements this with the explicit length figure (0.5 µm), the 9+0 notation, and the reminder that the centriole is absent from plant cells while a basal body of the same construction sits at the foot of every cilium and flagellum.

"Centrosome is an organelle usually containing two cylindrical structures called centrioles… both lie perpendicular to each other in which each has an organisation like the cartwheel."

NCERT Class 11, Biology, Section 8.5.9

Centrosome architecture & the cartwheel 9+0 centriole

A centrosome is a small, non-membranous organelle situated next to the nucleus in the cytoplasm of an animal cell. It is not delimited by a lipid bilayer; instead, it consists of two short cylinders called centrioles embedded inside a fuzzy halo of dense protein called the pericentriolar matrix (PCM). The two centrioles of one centrosome lie perpendicular (at right angles) to each other, like the long axis of one cylinder pointing into the side of the other. This perpendicular pair is the diagnostic signature that NEET and NCERT diagrams emphasise.

Each individual centriole is about 0.5 µm in length and roughly 0.2 µm wide. Viewed end-on in an electron micrograph, the centriole shows the famous cartwheel pattern: a circular wall divided into nine evenly spaced spokes, with a small proteinaceous hub at the centre. The wall of the cylinder is built from nine peripheral fibrils of tubulin protein, and — crucially — each of these nine peripheral fibrils is not a single microtubule but a fused triplet of three microtubules (A, B and C). Adjacent triplets are also linked to one another, and the hub at the centre is connected to each peripheral triplet by a radial spoke made of protein. There is no central microtubule pair, hence the conventional shorthand "9 + 0" — nine peripheral triplets, zero central singlets.

The pericentriolar matrix

Surrounding the two centrioles is the pericentriolar matrix — a thick, amorphous cloud of dense protein that NCERT calls "pericentriolar materials." This matrix is not a passive packing; it is the actual workshop where microtubules are nucleated. Embedded in the PCM are γ-tubulin ring complexes and a large cast of accessory proteins (pericentrin, ninein and others) that template the minus ends of new microtubules. The centrosome is therefore better described as the cell's principal microtubule-organising centre (MTOC): the centrioles supply structural identity and replication template, while the PCM does most of the nucleating work.

Functions: spindle, basal body, duplication

NCERT lists two crowning functions of centrioles, and a third is implied by the cell cycle: (a) they nucleate spindle fibres that build the spindle apparatus during cell division in animal cells; (b) they form the basal body of cilia and flagella; and (c) they themselves duplicate once per cell cycle so that each daughter cell inherits one centrosome.

Centrosome duplication through the cell cycle

The centrosome cycle runs in parallel with the chromosome cycle. In G1, a non-dividing animal cell carries one centrosome containing a perpendicular pair of mother and daughter centrioles. In S phase the two centrioles separate slightly, and each templates the growth of a new daughter centriole at right angles to itself, so a brand-new centriole forms perpendicular to each old one. By the time the cell enters G2, two centrosomes are present, each bearing its own mother–daughter pair. As prophase begins, these two centrosomes migrate to opposite poles of the cell; their pericentriolar matrices nucleate the bipolar mitotic spindle.

Plant vs animal: who has centrioles?

The cleanest one-mark fact NEET extracts from this section is the cellular distribution of centrioles. NCERT Figure 8.3 contrasts a plant cell and an animal cell, and the textbook is unambiguous: "animal cells have centrioles which are absent in almost all plant cells." NIOS Lesson 4 lists the same difference at the top of its plant-versus-animal table. Centrosomes are therefore part of the standard suite of animal-cell organelles but are missing from higher (flowering) plant cells; spindle formation in plants is "anastral" — it proceeds without any centrosome-organised aster of microtubules at the poles.

The qualifier "almost all" matters. Centrioles do appear in certain lower plants and protists — particularly in the motile flagellated sperm of algae, bryophytes (mosses) and pteridophytes (ferns), where each gamete needs a basal body to anchor its flagellum. In gymnosperms (with the exception of cycads and Ginkgo, which retain motile sperm) and in angiosperms, motile gametes are gone and centrioles are absent too.

Cross-link to cell division

The centriole story does not end at the cytoplasm; it threads directly into the cell-cycle narrative. The two daughter centrosomes generated in S phase migrate to opposite poles in prophase, and their pericentriolar matrices nucleate the bipolar spindle whose microtubules attach to kinetochores and pull sister chromatids apart in anaphase. In plant cells, where centrioles are absent, the same bipolar spindle still assembles — proof that centrioles are facilitators, not absolute requirements, for mitosis. See the chapter on Cell Cycle and Cell Division for the full mitotic and meiotic sequence.

Exam-ready facts at one place

For one-mark questions, the examiner usually reaches for one of six recurring facts: the perpendicular geometry of the two centrioles, the cartwheel cross-section, the 9 peripheral triplet microtubules with no central pair, the proteinaceous hub joined to the triplets by radial spokes, the spindle-fibre and basal-body functions, and the absence of centrioles from higher plant cells. The 2024 NEET item picked the cartwheel association; the 2016 paper picked microtubule constituency; future papers are likely to probe non-membranous status, 9+0 versus 9+2 differentiation, or the basal-body identity. Holding all six facts together — and rehearsing them against the figure-1 cartwheel — converts this short NCERT sub-section into a reliable mark on the answer sheet.

Worked examples

Common confusion & NEET traps