Step 1: Understand bacterial cell structure.
Bacterial cells are prokaryotic, meaning they lack a true nucleus. Instead of a defined nucleus, bacterial cells have a region called the nucleoid, where the genetic material (DNA) is located.
Step 2: Nucleoid in bacterial cells.
The nucleoid is not surrounded by a membrane, unlike the eukaryotic nucleus, and it contains a single, circular DNA molecule.
Final Answer:
\[
\boxed{\text{Nucleoid}}
\]