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why does spindle toxins affect the cyclin level?

A spindle poison, also known as a spindle toxin, is a poison that disrupts cell division by affecting the protein threads that connect the centromere regions of chromosomes, known as spindles. Spindle poisons effectively cease the production of new cells by interrupting the mitosis phase of cell division at the spindle assembly checkpoint (SAC). However, as numerous and varied as they are, spindle poisons are not yet 100% effective at ending the formation of tumors (neoplasms).[1] Although not 100% effective, substantive therapeutic efficacy has been found in these types of chemotherapeutic treatments.

b. the carbon cycle ensures living things have a source of hydrogen