During _________ chromosomes assemble at opposite poles and start forming two cells.

During Telophase, chromosomes assemble at opposite poles and start forming two cells.
In the given sentence, "Telophase" is the correct term that fills the blank. Telophase is a phase of cell division, specifically in mitosis or meiosis, where the chromosomes reach the opposite poles of the cell, and a new nuclear membrane begins to form around each set of chromosomes, ultimately resulting in the formation of two separate cells.