During which phase do chromosomes initially become visible?

During prophase, chromosomes initially become visible as distinct structures. This occurs as the chromatin, which is a complex of DNA and proteins within the nucleus, begins to condense into tightly packed structures. This condensation allows individual chromosomes, each consisting of two sister chromatids joined at a centromere, to be seen under a microscope as distinct entities.

As prophase progresses, the nuclear envelope breaks down, and the mitotic spindle starts to form, further facilitating the separation and alignment of chromosomes during the following stages of cell division. The visibility of chromosomes is a key feature of prophase and is an important aspect of the cell cycle, as it signifies the cell's readiness to proceed with mitosis.