Here's why:

* Unicellular organisms (like bacteria and amoeba) often have a single, circular DNA molecule located in a region called the nucleoid, which isn't a true nucleus with a membrane.

* Multicellular organisms (like plants and animals) have true nuclei enclosed by a membrane, containing their DNA organized into chromosomes.

So, while the structure and organization of DNA differ, the fundamental function of holding the DNA is shared by both types of organisms.