The part of mitosis when the chromosomes replicate and become organized is called interphase, specifically the S phase.

Here's why:

* Interphase is the period between cell divisions. It's not part of mitosis itself, but it's crucial for preparing the cell for division.

* S phase (synthesis phase) is the part of interphase when DNA replication occurs. This is where the chromosomes are duplicated, creating two identical copies of each chromosome.

* Mitosis is the process of dividing the cell's nucleus and its replicated chromosomes into two identical daughter nuclei. While mitosis is when the chromosomes become organized into distinct structures, the actual replication happens in the S phase of interphase.

So, to summarize:

1. Interphase (S phase): Chromosome replication occurs.

2. Mitosis: Chromosomes become organized and are separated into two new nuclei.