A.
Inside cells, microtubules grow from specialized organizing centers that control the location, number, and orientation of the microtubules.
B.
In most animal cells, for example, the centrosome—which is typically close to the cell nucleus when the cell is not in mitosis—organizes an array of microtubules that radiates outward through the cytoplasm. The centrosome consists of a pair of centrioles, surrounded by a matrix of proteins.
C.
The centrosome matrix includes hundreds of ringshaped structures formed from a special type of tubulin called γ-tubulin, and each γ-tubulin ring complex serves as the starting point, or nucleation site, for the growth of one microtubule.
D.
The αβ-tubulin dimers add to each γ-tubulin ring complex in a specific orientation, with the result that the minus end of each microtubule is embedded in the centrosome, and growth occurs only at the plus end that extends into the cytoplasm.