Which of the following begins when pairs of sister chromatids align in the center of the cell

When pairs of sister chromatids align in the center of the cell, it marks the beginning of the metaphase stage of mitosis or meiosis. During metaphase, the chromosomes are maximally condensed and align along the metaphase plate, an imaginary plane located at the center of the cell. 

At this stage, the sister chromatids are attached to microtubules from opposite poles of the cell via protein structures called kinetochores, which are located at the centromeres of the chromosomes. This alignment ensures that each sister chromatid will be pulled toward opposite poles of the cell during the subsequent stage, anaphase.

Metaphase is a critical checkpoint in the cell cycle, as it ensures that all chromosomes are properly aligned and attached to the spindle apparatus before they are segregated during anaphase. Any errors in chromosome alignment or attachment to the spindle can lead to chromosomal abnormalities or aneuploidy in the daughter cells. Therefore, the metaphase stage is tightly regulated to maintain genomic stability during cell division.