The DNA of the cell is found in a region called the nucleoid or nuclear area. Prokaryotes lack membrane-bound structures such as the nuclear membrane of eukaryotes. Their DNA is in a circular form called the bacterial chromosome. Though most bacteria have a single chromosome, some such as Vibrio cholerae have two. Some also have small, extrachromosomal rings of DNA called plasmids that carry additional genetic information such as genes for the resistance to antibiotics and to pass genetic information to other bacteria through a process called conjugation.