Tubulin Labeled with Caged Fluorescein Can Be Introduced into Dividing

Short Answer

Tubulin labeled with caged fluorescein can be introduced into dividing cells by microinjection. Various small regions in the mitotic spindle (made up of tubulin subunits) are briefly irradiated with laser light that uncages the fluorescent tubulin. Five minutes after irradiation, the highest spindle fluorescence is observed when the irradiated region is close to the chromosomes near the cell equator, and the lowest fluorescence is observed when regions near the spindle poles are irradiated. Based on this observation, do you think tubulin subunits are incorporated into the spindle mostly near the poles (P) or near the equator (E)? Write down P or E as your answer.