Here's why:

* DNA Replication: During the S phase of the cell cycle, each chromosome replicates its DNA. This means that a single chromosome, which initially had one DNA molecule, now has two identical DNA molecules. These two DNA molecules are held together at a point called the centromere.

* Sister Chromatids: The two identical DNA molecules are called sister chromatids. They are attached to each other and are considered a single chromosome.

* Mitosis: During mitosis, the sister chromatids separate, each becoming a full chromosome in a new daughter cell.

So, just before mitosis, each chromosome is actually two copies of the original DNA molecule, attached together as sister chromatids.