During which phase do chromosomes condense into visible structures?

The proper phase where chromosomes condense into visible structures is prophase. During prophase, which is the first stage of mitosis, chromatin fibers coil and fold, becoming more compact and organized into distinct chromosomes that can be observed under a microscope. This condensation allows the chromosomes to become visible as individual entities, each composed of two sister chromatids connected at a region known as the centromere.

As the cell transitions into prophase, the nuclear envelope begins to break down, further facilitating the visibility of the chromosomes. This is a critical step in preparing the genetic material for separation into daughter cells during subsequent stages of mitosis.

In contrast, other phases listed do not involve this specific action of chromosome condensation. The G₂ phase is part of interphase where the cell prepares for mitosis but does not condense chromosomes. During metaphase, chromosomes are aligned at the cell's equatorial plane, but they have already condensed prior to entering this phase. Anaphase is characterized by the separation of sister chromatids as they move toward opposite poles of the cell, not the condensation of their structure. Hence, the prophase is definitively the stage where chromosomes first become visible due to their condensation.