<P> DNA diameter is about 2 nm, while the length of a stretched single molecule may be up to several dozens of centimetres depending on the organism . Many features of the DNA double helix contribute to its large stiffness, including the mechanical properties of the sugar - phosphate backbone, electrostatic repulsion between phosphates (DNA bears on average one elementary negative charge per each 0.17 nm of the double helix), stacking interactions between the bases of each individual strand, and strand - strand interactions . DNA is one of the stiffest natural polymers, yet it is also one of the longest molecules . This means that at large distances DNA can be considered as a flexible rope, and on a short scale as a stiff rod . Like a garden hose, unpacked DNA would randomly occupy a much larger volume than when it is orderly packed . Mathematically, for a non-interacting flexible chain randomly diffusing in 3D, the end - to - end distance would scale as a square root of the polymer length . For real polymers such as DNA, this gives only a very rough estimate; what is important, is that the space available for the DNA in vivo is much smaller than the space that it would occupy in the case of a free diffusion in the solution . To cope with volume constraints, DNA can pack itself in the appropriate solution conditions with the help of ions and other molecules . Usually, DNA condensation is defined as "the collapse of extended DNA chains into compact, orderly particles containing only one or a few molecules". This definition applies to many situations in vitro and is also close to the definition of DNA condensation in bacteria as "adoption of relatively concentrated, compact state occupying a fraction of the volume available". In eukaryotes, the DNA size and the number of other participating players are much larger, and a DNA molecule forms millions of ordered nucleoprotein particles, the nucleosomes, which is just the first of many levels of DNA packing . </P> <P> In viruses and bacteriophages, the DNA or RNA is surrounded by a protein capsid, sometimes further enveloped by a lipid membrane . Double - stranded DNA is stored inside the capsid in the form of a spool, which can have different types of coiling leading to different types of liquid - crystalline packing . This packing can change from hexagonal to cholesteric to isotropic at different stages of the phage functioning . Although the double helices are always locally aligned, the DNA inside viruses does not represent real liquid crystals, because it lacks fluidity . On the other hand, DNA condensed in vitro, e.g., with the help of polyamines also present in viruses, is both locally ordered and fluid . </P> <P> Bacterial DNA is packed with the help of polyamines and proteins . Protein - associated DNA occupies about 1 / 4 of the intracellular volume forming a concentrated viscous phase with liquid crystalline properties, called the nucleoid . Similar DNA packaging exists also in chloroplasts and mitochondria . Bacterial DNA is sometimes referred to as the bacterial chromosome . Bacterial nucleoid evolutionary represents an intermediate engineering solution between the protein - free DNA packing in viruses and protein - determined packing in eukaryotes . </P> <P> Sister chromosomes in the bacterium Escherichia coli are induced by stressful conditions to condense and undergo pairing . Stress - induced condensation occurs by a non-random, zipper - like convergence of sister chromosomes . This convergence appears to depend on the ability of identical double - stranded DNA molecules to specifically identify each other, a process that culminates in the proximity of homologous sites along the paired chromosomes . Diverse stress conditions appear to prime bacteria to effectively cope with severe DNA damages such as double - strand breaks . The apposition of homologous sites associated with stress - induced chromosome condensation helps explain how repair of double - strand breaks and other damages occurs . </P>

When does dna condense and form visible structures