Final answer:
In a centrosome, γ-tubulin rings serve as nucleation sites for the formation of microtubules, which are essential for cell structure and division.
Step-by-step explanation:
In a centrosome, the structures that serve as nucleation sites for the formation of microtubules are γ-tubulin rings (gamma-tubulin). Microtubules play a crucial role in maintaining the cell's structure, facilitating intracellular transport, and organizing the mitotic spindle during cell division. The centrosome, positioned near the nucleus in animal cells, acts as the microtubule-organizing center and contains a pair of centrioles, from which the growth of microtubules occurs.
These microtubules extend from the centrosomes much like fingers extending from both hands as they organize into the mitotic spindle. Although microtubules are made up of α and β-tubulin dimers, it is the γ-tubulin within the centrosome that facilitates the nucleation process, serving as the starting point for the polymerization of these dimers into the larger structure of the microtubule.