Chromosomes and genes are replicated during S phase, which stands for synthesis phase.

This phase occurs during interphase, the period between cell divisions. Here's a breakdown:

* Interphase: This is the longest part of the cell cycle where the cell grows and copies its chromosomes. It has three sub-phases:

* G1 phase (Gap 1): The cell grows and prepares for DNA replication.

* S phase (Synthesis): The cell replicates its DNA. Each chromosome is duplicated, resulting in two identical sister chromatids joined at the centromere.

* G2 phase (Gap 2): The cell continues to grow and prepare for cell division.

* M phase (Mitosis or Meiosis): This is the actual cell division phase where the duplicated chromosomes are separated and distributed into two new daughter cells.

So, in summary, DNA replication and chromosome duplication happen specifically during the S phase of interphase.