In which phase of mitosis do the chromosomes begin to separate?

During mitosis, the phase where chromosomes begin to separate is known as Anaphase. In this stage, the cohesin proteins that hold the sister chromatids together are cleaved, allowing the chromatids to be pulled apart toward opposite poles of the cell. This separation is essential for ensuring that each daughter cell will receive an identical set of chromosomes.

Anaphase follows Metaphase, where chromosomes are aligned at the metaphase plate, but they remain attached during that phase. By the time mitosis reaches Anaphase, the cell’s spindle fibers attach to the centromeres of the sister chromatids, effectively pulling them apart. The distribution of these chromatids to opposite sides of the cell is critical for successful cell division.

In Telophase, which occurs after Anaphase, the separated chromatids have already reached the opposite poles, and the cell starts the process of re-establishing the nuclear envelope around each set of chromosomes. Prophase, occurring before Metaphase, is when the chromatin condenses into visible chromosomes, but no separation occurs at that stage. Therefore, Anaphase is uniquely characterized by the actual separation of chromosomes, making it the correct answer.