Final answer:
The initial slow phase of microtubule assembly is called the nucleation phase. It is crucial for the formation of the mitotic spindle during cell division, where microtubules align and separate chromosomes.
Step-by-step explanation:
The slower phase of microtubule assembly in which a small portion of the microtubule is initially formed is known as the nucleation phase. This is the initial and rate-limiting step during which alpha-tubulin and beta-tubulin monomers come together to form a stable nucleus for microtubule growth. This nucleus then serves as a foundation for the addition of more tubulin dimers at the plus end of the microtubule, allowing it to elongate during the process known as polymerization.
In cell division, during mitosis or meiosis, microtubules play a critical role. They reorganize into the mitotic spindle, a dynamic, microtubule-based structure that separates chromosomes into daughter cells.
The microtubules extend from the centrosomes, with certain microtubules, known as kinetochore microtubules, attaching to the chromosomes and pulling them apart.