A Level Biology Practice Exam

Chromosomes become visible under a microscope when cells are preparing to divide, which typically occurs during the phases of mitosis or meiosis. As the cell enters prophase, which is the first stage of cell division, the chromatin condenses into distinct structures known as chromosomes. This condensation is crucial because it allows the chromosomes to be more easily separated and moved to opposite ends of the cell, ensuring accurate distribution of genetic material to daughter cells.

During interphase, the chromatin is in a more relaxed state, making the individual chromosomes difficult to distinguish. Similarly, when cells are not dividing, such as during G1, S, or G2 phases of interphase, chromosomes are still not sharply visible. Cytoplasmic streaming, while a process that occurs in some cells for nutrient distribution, does not influence the visibility of chromosomes. Thus, visibility of chromosomes directly correlates with the active preparation for cell division, reinforcing the idea that they are most clearly seen during mitosis or meiosis.