Chromosomes become visible in the nucleus during the cell division phase called mitosis, specifically during prophase. In this phase, the chromatin material condenses, and the DNA coils tightly to form distinct structures that we recognize as chromosomes.
The visibility of chromosomes is crucial for their proper segregation into daughter cells. Prior to prophase, during interphase, the DNA exists in a less condensed form known as chromatin, making it difficult to visualize individual chromosomes under a microscope. As the cell prepares to divide, the condensation process allows the chromosomes to become more compact and organized.
This visibility is most prominent when chromosomes align along the metaphase plate, and it is at this stage that they can be observed clearly using a light microscope.