During the S phase of the cell cycle, chromosomes are converted to their duplicated form.

Here's what happens:

* Before S phase: Each chromosome exists as a single, unreplicated structure.

* During S phase: DNA replication occurs, creating an identical copy of each chromosome.

* After S phase: Each chromosome now consists of two identical sister chromatids, joined together at the centromere.

So, the chromosomes are not converted to a different *form* per se, but rather they are duplicated. This duplication is crucial for ensuring that each daughter cell receives a complete set of chromosomes during cell division.