During which stage of mitosis do chromosomes become visible?

During prophase, chromosomes become visible as they condense and shorten, making them distinguishable under a light microscope. This stage marks the beginning of mitosis, where the chromatin, a loose structure of DNA and associated proteins in the nucleus, coils tightly to form individual chromosomes. Each chromosome is made up of two sister chromatids held together at a region called the centromere.

As prophase progresses, the nuclear envelope starts to break down, and the spindle apparatus begins to form, which is essential for later stages of mitosis. The visibility of chromosomes is a key characteristic of this stage, as it allows the cell to prepare for the accurate segregation of genetic material.

In contrast, during anaphase, the sister chromatids are pulled apart and begin to move toward opposite poles of the cell, but the chromosomes are already visible since this visibility was established in prophase. Metaphase involves the alignment of chromosomes at the cell's equatorial plane, while telophase occurs at the end of mitosis when chromosomes begin to de-condense back into chromatin and a new nuclear envelope forms around each set of chromosomes.