AbstractMicrotubules were purified from unfertilized eggs of the sea urchinsArbacia punctulata, Lytechinus pictus, Lytechinus variegatus, andStrongylocentrotus purpuratus.Numerous densely stained particles (24 × 26 nm) are associated with microtubules isolated from each of these sea urchins. The most striking aspect of this structure is an extended, slightly curved arm that appears to attach the particles to the microtubule. Morphologically similar particles are associated with microtubules of the isolated first cleavage mitotic apparatus. The particles are attached to the microtubules by ionic interactions and contain large amounts of extractable RNA. Based upon their size and density, RNA and protein composition, and sedimentation in sucrose gradients, the microtubule‐associated particles are identified as ribosom
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