Final answer:
The formation of new spindle fibers and breakdown of the nuclear envelope occur during the prometaphase stage of mitosis, facilitating the attachment of spindle fibers to kinetochores.
Step-by-step explanation:
The formation of new spindle fibers and the breakdown of the nuclear envelope occur during the prometaphase stage of mitosis. In this phase, the nuclear membrane breaks down, allowing the mitotic spindle fibers to attach to the kinetochores located on the chromosomes. During Prometaphase II, a similar process occurs where the nuclear envelopes are completely broken down, and each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles.