Chromosomes form during cell division, specifically during prophase (and remain visible throughout the rest of mitosis and meiosis).

Here's why:

* DNA exists as long strands: In the nucleus of a cell, DNA exists as long, thin strands called chromatin.

* Condensation is needed for division: To ensure that each daughter cell receives a complete set of genetic material, the DNA must be condensed and organized into compact structures.

* Chromosomes are formed: During prophase, the chromatin fibers coil and condense, forming visible structures called chromosomes. Each chromosome consists of two identical sister chromatids, joined at a centromere.

So, chromosomes are formed as a way to efficiently package and distribute the genetic material during cell division.