9 + 2 fibrillar arrangement is present in :

Question

Accepted Answer

The correct option is 1: Eukaryotic flagella

The 9 + 2 fibrillar arrangement is the hallmark ultrastructure of the central core of cilia and flagella found in all eukaryotic organisms, a structure known as the **axoneme**. This highly conserved pattern consists of a precise arrangement of microtubules: nine pairs of **microtubule doublets** are evenly distributed around the circumference of the axoneme, while two single microtubules are located at its very center, giving rise to the characteristic "9 + 2" description. These microtubules are built from tubulin protein and are responsible for the whip-like or wave-like motion of the flagellum. The mechanical motion is driven by the motor protein **dynein**, which forms arms extending from one doublet to the adjacent one, facilitating the regulated sliding of the microtubule doublets that translates into the overall bending movement necessary for motility. This complex and organized arrangement is critical for functions such as swimming in single-celled organisms and moving mucus in human respiratory tracts.

The incorrect option is 2: Bacterial flagella

Bacterial flagella belong to prokaryotes and are structurally and functionally distinct from their eukaryotic counterparts. They are much simpler, rigid, and filamentous appendages that are entirely made up of a single protein subunit called **flagellin**, completely lacking any internal microtubule arrangement. Instead of the complex 9+2 pattern, the bacterial flagellum operates like a propeller, driven by a rotary motor embedded in the cell wall and plasma membrane. Therefore, bacterial flagella do not possess the 9 + 2 fibrillar structure.

The incorrect option is 3: Basal granule

The basal granule, also known as the basal body or kinetosome, serves as the anchor point for the eukaryotic flagellum or cilium within the cell cytoplasm. While it is structurally related to the flagellum, its microtubule arrangement is fundamentally different. The basal granule exhibits a **9 + 0 arrangement**, meaning it is composed of nine peripheral **microtubule triplets** arranged in a ring, but it completely lacks the two central single microtubules. Its function is to provide the template and structural base for the growth of the axoneme and is not the site where the 9 + 2 pattern is found.

The incorrect option is 4: Centriole

Centrioles are cytoplasmic organelles primarily involved in organizing the cell's cytoskeleton and initiating the formation of the mitotic spindle during cell division. Like basal granules, centrioles also exhibit the characteristic **9 + 0 arrangement** of microtubules. They consist of nine peripheral **microtubule triplets** arranged in a cylindrical pattern, completely lacking any central pair. Although a centriole can sometimes migrate to the plasma membrane and differentiate into a basal granule, and thus indirectly lead to the growth of a 9+2 flagellum, it does not itself possess the 9 + 2 fibrillar structure.