My book says that polymerization and depolumerization of microtubules occurs on the + end however, I've found a note that says that depolymerization occurs on the - end. I need help please :) thank you
From my Campbell's Biology textbook (Ninth Edition):
Because of the orientation of tubulin dimers, the two ends of a microtubule are slightly different. One end can accumulate or release tubulin dimers at a much higher rate than the other, thus growing and shrinking significantly during cellular activities. (This is called the "plus end", not because it can only add tubulin proteins but because it's the end where both "on" and "off" rates are much higher.)
So in summary: polymerization and depolymerization occur on both the + and - ends. The "+" and "-" merely designate the speed/efficiency of polymerization and depolymerization.
Practically speaking, microtubule nucleation occurs at the plus end. This is well established in vivo and in vitro. Notably, the minus ends within most cells are "capped", so dynamics do not occur (think of the centrosome).
However, recently there is a report of MT polymerization at the minus end in a cell; this is far from the consensus among professionals at the moment, and requires much more study. Nonetheless, the rate of any growth and shrinkage at the minus end is significantly slower than the plus end, as stated above.
So for a test question, plus end is the site of polymerization.