What does prophase signify in the cell cycle?

Prophase is a critical phase of mitosis in the cell cycle where several key developments occur. During this stage, the chromatin condenses into distinct chromosomes, making them visibly distinct under a microscope. Each chromosome at this point consists of two sister chromatids that are joined at a region known as the centromere. This visibility is crucial because it prepares the chromosomes for the subsequent stages of mitosis, ensuring that they can be accurately separated.

The other phases mentioned, such as aligning chromosomes in the middle of the cell or separating chromatids into different cells, occur during other specific stages of mitosis, mainly metaphase and anaphase, respectively. The completion of division, which occurs at the end of the entire mitotic process, is accomplished in the last stage of mitosis, called telophase, when two new nuclei form. Thus, prophase specifically marks the moment chromosomes become organized and recognizable as separate entities due to the presence of sister chromatids.