Key Points: Cytoskeleton, Cilia, Flagella, and Cell Division
Cytoskeleton: Three-component network providing structural support and enabling movement:
- Microtubules (tubulin): form mitotic spindle, intracellular transport tracks, 9+2 axoneme of cilia/flagella
- Microfilaments (actin): cell motility, muscle contraction (with myosin), cytokinesis cleavage furrow
- Intermediate filaments: structural support, nuclear lamina; tissue-specific (keratin in skin, vimentin in fibroblasts)
Eukaryotic Cilia and Flagella: Core structure = axoneme: 9 peripheral microtubule doublets + 2 central singlets (9+2 arrangement). Enclosed in plasma membrane. Dynein motor proteins use ATP to drive sliding of doublets → bending motion. Cilia: short, numerous, beat in metachronal waves; function in locomotion and sweeping surface (respiratory tract). Flagella: long, few, undulating; function in locomotion (e.g., sperm).
Prokaryotic Flagella: Protein: flagellin (not tubulin). No 9+2 arrangement, no membrane enclosure. Rotates like a propeller via proton motive force. Structurally and mechanistically distinct from eukaryotic flagella.
Centrioles: Made of 9 triplets of microtubules (9+0 arrangement). Found in animal cells; absent in most higher plant cells. Organize the mitotic spindle (aster formation). Form basal bodies of cilia and flagella.
Cell Division Comparison:
- Prokaryotes: Binary fission — no spindle, no centrioles, no chromosome condensation; two equal daughters
- Eukaryotes (mitosis): Chromosome condensation → nuclear envelope breakdown → spindle formation → chromosome segregation → nuclear envelope reform → cytokinesis
- Plant cytokinesis: Cell plate (Golgi vesicles) forms centrifugally
- Animal cytokinesis: Cleavage furrow (actin-myosin ring) forms centripetally
NEET Key Points:
- 9+2 = eukaryotic cilia/flagella only; centrioles = 9+0
- Prokaryotic flagella = flagellin (no 9+2); rotate
- Plant cytokinesis = centrifugal; Animal = centripetal