The two strands in the double helix are antiparallel. The strands of a double-helical DNA molecule can be separated experimentally by heat in a process called melting. 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.