Final answer:
Microtubules require nucleating sites in the centrosome to grow properly, which is essential for cell division as they form the mitotic spindle required for chromosome segregation.
Step-by-step explanation:
Microtubules require nucleating sites such as those provided by the γ-tubulin rings in the centrosome because these sites act as the foundation from which microtubules can grow. The centrosome, located near the nucleus of animal cells, contains a pair of centrioles, which are essential for the organization of microtubules. During cell division, these centrioles aid in the formation of the mitotic spindle, which is crucial for the separation of chromosomes into daughter cells. By providing a stable base from which microtubules can polymerize, nucleating sites ensure the correct assembly and function of the spindle apparatus, contributing to successful cell division.