Which cytological event primarily occurs during prophase?

During prophase, the critical cytological event is the condensation of chromatin into visible chromosomes—this is when the genetic material becomes more tightly packed and organized. As the chromatin condenses, the nucleolus, which is the site of ribosome production within the nucleus, starts to disappear, marking an important transition towards mitosis.

This condensation ensures that chromosomes can be accurately segregated later in the process of cell division, which is crucial for the distribution of genetic material to daughter cells. As prophase progresses, the nuclear envelope begins to break down, allowing the mitotic spindle to interact with the chromosomes.

In contrast, the alignment of chromosomes at the equator occurs during metaphase, the separation of sister chromatids happens during anaphase, and the formation of two nuclei is observed in telophase. Each of these processes is distinct and occurs at specific stages of cell division, highlighting the importance of prophase as a preparatory phase for mitosis.