During which phase of mitosis does the nucleus of the cell begin to divide?

The phase of mitosis during which the nucleus of the cell begins to divide is indeed an important aspect of cell division. Anaphase is characterized by the separation of sister chromatids, which are pulled apart towards opposite poles of the cell. However, the actual division of the nucleus starts at a different phase.

In prophase, the chromatin condenses into distinct chromosomes, the nuclear envelop begins to break down, and the mitotic spindle starts to form. While the nucleus does not fully divide in prophase, it is during telophase that the nuclear envelopes reform around the separated sets of chromosomes at each pole, effectively marking the end of nuclear division.

Hence, the correct answer is not anaphase. Instead, it is critical to recognize that the visible physical division of the nucleus occurs in telophase, completing the changes that began in prophase. This sequence emphasizes that while anaphase signals movement in preparation for division, the actual formation of two nuclei occurs afterward in telophase.