Final answer:
The two rings of microtubules often referred to collectively are called centrioles, which together form the centrosome. This centrosome acts as the microtubule-organizing center and is vital for mitotic spindle formation during cell division. Centrioles are different from flagella, which are structured differently and are made of microfilaments.
Step-by-step explanation:
The two rings of microtubules you're referring to are called centrioles, and together, they are often referred to as a centrosome. Each centriole is a cylindrical structure composed of nine groups of three microtubules. These centrioles are arranged perpendicularly to each other within the centrosome, which functions as the microtubule-organizing center in animal cells. During cell division, particularly mitosis, centrosomes play a crucial role in organizing the mitotic spindle, which is essential for separating chromosomes into daughter cells.
It is important to note that although centrioles and flagella are both made of microtubules, their structures are quite different. Flagella are primarily composed of microfilaments and have a different arrangement compared to centrioles. The spindle apparatus in plant cells, which typically lack centrioles, is formed from a structure known as the MicroTubule Organizing Center (MTOC), serving a similar purpose to the centrosomes in animal cells during mitosis and meiosis.