Chapter 18: Cell Organization & Movement II: Microtubules & Filaments

Cell Organization & Movement II: Microtubules & Filaments begins by defining the structure of microtubules as polarized hollow tubes assembled from alpha-tubulin and beta-tubulin heterodimers, which polymerize into protofilaments nucleated from microtubule-organizing centers (MTOCs) like centrosomes and basal bodies. A central concept explored is dynamic instability, a process driven by GTP hydrolysis where microtubule ends undergo rapid phases of growth and shrinkage (catastrophe and rescue), a phenomenon regulated by microtubule-associated proteins (MAPs) for stabilization and disassembly. The text elucidates the mechanics of intracellular transport, detailing how ATP-dependent motor proteins—kinesins (mostly anterograde) and dyneins (retrograde)—traverse microtubule tracks to move organelles and vesicles. Significant attention is given to the structure of motile cilia and flagella, describing the characteristic 9+2 axoneme arrangement, the role of axonemal dynein in generating bending forces, and the intraflagellar transport (IFT) system required for assembly and maintenance. The chapter also provides a granular breakdown of mitosis, explaining how the mitotic spindle assembles, how kinetochores capture chromosomes via mechanisms involving Ran-GTP gradients and the Chromosomal Passenger Complex (regulating attachment via Aurora B kinase), and the distinct mechanical forces driving Anaphase A and B. Furthermore, the text contrasts these dynamic systems with intermediate filaments, which lack intrinsic polarity and motor proteins but provide essential tensile strength and tissue integrity. This includes a classification of intermediate filaments into tissue-specific groups such as keratins in epithelia, desmin in muscle, neurofilaments in neurons, and nuclear lamins which support the nuclear envelope. The discussion concludes by highlighting the coordination between cytoskeletal elements, illustrating how microtubules, microfilaments, and intermediate filaments cooperate during complex processes like cell migration and neuronal growth cone advancement.