Here's why:

* Before replication, a chromosome exists as a single, long strand of DNA.

* During replication, the DNA is copied, resulting in two identical DNA molecules.

* These two identical DNA molecules are held together at the centromere, forming the sister chromatids.

* The sister chromatids are essentially clones of each other, containing the same genetic information.

So, a single replicated chromosome is essentially two identical copies of the original chromosome, joined together.