Prokaryotes can have small, self-replicating circular pieces of DNA known as within their cells.

https://www.ck12.org/book/ck-12-biology-advanced-concepts/section/11.5/

Explanation:

Prokaryotes can also have small, self-replicating circular pieces of DNA called plasmids within their cells, as shown in Figure below. The DNA of prokaryotes is concentrated in an area called the nucleoid, where it is associated with ribosomes and other molecules.

when it is at an intermediate population density