Here's why:

* DNA Structure: DNA is a double helix, meaning two strands of nucleotides are intertwined.

* Chromosomes: Chromosomes are long, thread-like structures found in the nucleus of cells. They are composed of tightly packed DNA and associated proteins.

* Packaging: In order to fit inside the nucleus, DNA is wound around proteins called histones. This forms a structure called chromatin, which then further condenses into chromosomes during cell division.

Therefore, while DNA is actually a double helix with two strands, it's packaged into long, single strands within chromosomes.