Prokaryotes typically have a single, circular chromosome. This chromosome is located in the nucleoid region of the cell, which is not surrounded by a nuclear membrane. The chromosome contains the genetic material of the prokaryote, including the genes that code for proteins, RNA molecules, and other essential molecules. In addition to the circular chromosome, some prokaryotes may also have smaller, circular DNA molecules called plasmids. Plasmids are not essential for the survival of the prokaryote, but they can carry genes that provide the prokaryote with additional functions, such as antibiotic resistance or the ability to metabolize certain compounds.