Final answer:
MTOCs are found at the centrioles within the centrosome and at the centromeres (kinetochores when present) of chromosomes, not under or attached to the nuclear envelope, nor within the nucleolus.
Step-by-step explanation:
MTOCs, or microtubule organizing centers, are crucial structures in the cell that help in organizing the mitotic spindle during cell division. They are found in the centrosome, which is a small body located near the nucleus—often referred to as the cell's microtubule organizing centre. The centrosome features a dense center and radiates tubules. During cell division, specifically in prophase, the nuclear envelope dissolves and microtubules from the centrosomes help form the mitotic spindle. This spindle eventually attaches to the kinetochores located at the centromeres of the chromosomes, allowing for proper segregation of the chromosomes into the daughter cells.
Therefore, MTOCs are found at the centrioles within the centrosome and at the centromeres (kinetochores when present) of chromosomes. They are not located under or attached to the nuclear envelope, nor within the nucleolus.