Final answer:
Mitosis begins with the start of prophase, which is characterized by chromatin condensing into visible chromosomes and the breakdown of the nuclear envelope, marking the initial stage of mitosis.
Step-by-step explanation:
The event in the cell cycle that indicates mitosis has begun is the start of prophase, the first phase of mitosis. During prophase, several events occur such as the condensation of chromatin into chromosomes, the breakdown of the nuclear envelope, the disappearance of the nucleolus, and the formation of the mitotic spindle.
Metaphase is another critical point in mitosis where the sister chromatids align at the metaphase plate, and a checkpoint ensures that all the chromosomes are properly attached to the spindle fibers before proceeding to anaphase. The adhering of sister chromatids to the microtubules of the mitotic spindle ensures they will be correctly separated and allocated to each of the two resulting daughter cells.
The event in the cell cycle that indicates the beginning of mitosis is the alignment of the chromosomes at the metaphase plate. This occurs during the metaphase stage of mitosis. During metaphase, the sister chromatids line up in the middle of the cell, forming a structure called the metaphase plate or equatorial plate. This alignment is crucial as it ensures that each daughter cell receives the correct number of chromosomes during cell division.