Answer
Two tubulin dimers have a lower affinity for each other in comparison to what a tubular dimer has for the end of a microtubule. In order to start a microtubule, tublin dimers come together and have to be bound long enough for the other tubulin molecules to come. This is known as the formation of initial tubulating sites
Work Step by Step
The γ-tubulin rings in the centrosome serve as permanently preassembled nucleation sites.
