Final answer:
The phase of mitosis described by the beginning of spindle fiber formation is prophase. This phase is marked by the condensation of chromosomes and the initial formation of the spindle apparatus, leading into prometaphase where spindle fibers attach to kinetochores.
Step-by-step explanation:
The phase of mitosis that is described by the formation of spindle fibers is prophase. During prophase, the chromatin condenses into chromosomes, the nuclear envelope breaks down, and the spindle apparatus starts to form. Spindle fibers, made of microtubules, extend between the centrosomes as they move to opposite poles of the cell
These spindles begin the process of attaching to the kinetochores on the centromeres of sister chromatids. This process is indicative of the transition into prometaphase, where the spindle fibers that have formed continue to develop and attach to kinetochores, effectively promoting the alignment of chromosomes for subsequent separation.