Chromosomes are duplicated during S phase of the cell cycle.

Here's a breakdown:

* Interphase: The period between cell divisions, where the cell grows and copies its contents.

* G1 Phase: The cell grows and prepares for DNA replication.

* S Phase: This is where DNA replication occurs, and chromosomes are duplicated. Each chromosome now consists of two identical sister chromatids held together by a centromere.

* G2 Phase: The cell continues to grow and prepares for mitosis or meiosis.

* M Phase: The stage where the cell divides.

So, while chromosomes are present throughout the cell cycle, they are specifically duplicated during the S phase.