Which structure forms during prophase and is composed of microtubules?

Which structure forms during prophase and is composed of microtubules?

• A. Nucleolus
• B. Golgi apparatus
• C. Spindle fibers
• D. Endoplasmic reticulum

Answer: (C)

During mitosis, the mitotic spindle forms in prophase from microtubules. This spindle apparatus is built from tubulin-based fibers that emanate from the centrosomes (the poles) and attach to chromosomes at kinetochores. These spindle fibers organize and move the chromosomes, guiding them toward alignment and eventual separation as cell division progresses. The other structures listed are not formed from microtubule spindle fibers during prophase: the nucleolus is a nuclear region involved in ribosome production and it dissolves as chromatin condenses; the Golgi apparatus and endoplasmic reticulum are membrane-bound organelles involved in trafficking and synthesis, not the mitotic spindle. So the structure composed of microtubules that forms during prophase is the spindle fibers.

During mitosis, the mitotic spindle forms in prophase from microtubules. This spindle apparatus is built from tubulin-based fibers that emanate from the centrosomes (the poles) and attach to chromosomes at kinetochores. These spindle fibers organize and move the chromosomes, guiding them toward alignment and eventual separation as cell division progresses. The other structures listed are not formed from microtubule spindle fibers during prophase: the nucleolus is a nuclear region involved in ribosome production and it dissolves as chromatin condenses; the Golgi apparatus and endoplasmic reticulum are membrane-bound organelles involved in trafficking and synthesis, not the mitotic spindle. So the structure composed of microtubules that forms during prophase is the spindle fibers.