Here's why:

* DNA Structure: DNA is made up of two long strands that are twisted together in a double helix shape.

* Chromosomes: DNA is packaged into structures called chromosomes.

* Replication: Before cell division, each chromosome is duplicated, creating two identical copies. These copies are called sister chromatids. They are joined together at a point called the centromere.

* Separation: During cell division, the sister chromatids separate, becoming individual chromosomes that are passed on to the daughter cells.

So, while a single continuous strand of DNA is technically a single strand of a double helix, the term "chromatid" is more specific and refers to a duplicated copy of a chromosome that will ultimately be separated during cell division.