The structure present in eukaryotes but not prokaryotes is a nucleus.

Here's why:

* Eukaryotic cells have a true nucleus, a membrane-bound organelle that contains the cell's genetic material (DNA). This DNA is organized into chromosomes.

* Prokaryotic cells lack a nucleus. Their DNA is located in a region called the nucleoid, which is not enclosed by a membrane.

Let me know if you'd like to learn more about other differences between prokaryotes and eukaryotes!