Here's why:

* Chromosomes: These are structures found in the nucleus of cells that carry genetic information in the form of DNA.

* DNA: This is the molecule that holds the genetic code. It's a long, double helix structure.

* Chromatid: Before cell division, a chromosome replicates, meaning the DNA is copied. The two identical copies of the DNA molecule are called sister chromatids, which are joined together at a region called the centromere.

So, each chromosome is made up of two chromatids, each containing a copy of the original DNA strand.