Two complementary single strands can hybridize to form a stable double-stranded molecule. The very long DNA molecule can be packaged into the cell because it is supercoiled. In most prokaryotes this supercoiling is brought about by enzymes called topoisomers. Eukaryotic chromosomes contain very regular structures called nucleosomes. In addition to the chromosomes, a number of other genetic elements exist in cells.
Plasmids are DNA molecules that exist separately from the chromosome of the cell.
Mitochondria and chloroplasts contain their own DNA genomes. Viruses contain a genome, either DNA or RNA, that controls their own replication. Transposable elements exist as a part of other genetic elements. Section 6.5 Restriction enzymes are cellular enzymes that recognize specific short base sequences in DNA, and make two single-stranded breaks at locations within the recognition sites. A restriction enzyme does not affect the cell that produces it because its own DNA is methylated at the recognition site by a modification enzyme specific for that site.