Cell Biology

... from active transport to vesicles

spindle

The spindle apparatus choreographs chromosome segregation during mitosis.

The spindle begins to assemble from microtubules during mitotic prophase at the centrosome, and the apparatus persists until telophase. During interphase, cells have a single centrosome, that duplicates just before prophase, producing a second centrosome. During prophase, microtubules radiate from each centrosome, with kinetochore microtubules extending from one centrosome toward the other. Polar microtubules extend only halfway along the spindle and short aster microtubules are confined to the centrosomes. Kinetochore microtubules attach to chromosomal kinetochores and translocate the chromosomes to the center of the cell at the metaphase plate. The growth and shortening of microtubules is dependent upon the rate of tubulin polymerization relative to the rate of GTP hydrolysis. The kinetochore microtubules next shorten rapidly during anaphase, dividing and dragging the attached chromosomes to opposite poles of the spindle. Simultaneously, the polar microtubules extend, pushing the chromosomes toward the poles. Telophase commences when the chromosomes reach the spindle's opposite poles. The processes of prophase are now reversed as the spindle dissociates, chromosomes uncoil, the nuclear membrane reassembles upon the nuclear lamina, and nucleoli re-develop in the forming daughter nuclei.

The inherent dynamic instability of microtubules can be modified by the interactions with microtubule-associated proteins (MAPs) and microtubule-regulatory proteins. The best-characterized MAPs are MAP-1, MAP-2, and tau proteins. MAPs can bind to microtubules, increasing their stability. The activity of MAPs is tightly regulated by their phosphorylation state. Growth factor signals activate protein kinases that catalyze phosphorylation of tubulin-binding domains of MAPs, causing them to detach from microtubules. XMAP215 is a highly conserved 215 kDa MAP, which plays an important role in controlling microtubular dynamics during the cell cycle. XMAP215 stabilizes the plus ends of microtubules, promoting elongation and preventing catastrophic shrinkage. At the onset of mitosis, higher phosphorylation of XMAP215 increases microtubular instability, causing disassembly. At termination of mitosis, protein phosphatase activity predominates as the microtubule array characteristic of interphase is re-established.

CELL BIOLOGY: ON THE ORCHESTRATION OF THE MITOTIC SPINDLE: "New work provides the clearest evidence yet that spindle assembly is coordinated by the generation, at chromosomes, of an intracellular gradient of the active guanosine triphosphate (GTP)-bound form of Ran, a small GTPase of the Ras super-family present in all eukaryotic cells."

Џ beautiful Flash 8 animation - Inner Life of the Cell, which shows centriole pair and radiating microtubules, and Interpretation: Inner Life of the Cell [] spindle(green) chromosomes(orange) peroxisomes(blue) [] actin(red) tubulin(blue) [] spindle apparatus [] centrosomes & mitotic spindle [] metaphase spindle [] kinetochore fibers [] confocal mitotic spindle []

• A • adhesion • C • cell cyclecell membranescellular adhesion moleculescellular signal transductioncentrioleschemotaxischloroplastciliacommunicationconcentration gradientscytokine receptorscytoplasmcytoskeleton • E • energy transducersendoplasmic reticulumendosomesexosome • G • Golgi apparatusGPCRs • H • hormones • I • ion channelsintermediate filaments • L • lysosome • M • meiosismicrofilamentsmicrotubulesmitosismitochondrion • N • Nitric Oxideneurotransmissionneuronal interconnectionsnuclear membranenuclear pore • P • pinocytosisproteasomepumps • R • receptor proteinsreceptor-mediated endocytosis • S • second messengerssignaling gradientssignal transductionspindlestructure • T • transporttwo-component systems • V • vacuolevesicle

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