ORDERED PHASES OF DNA INVIVO AND INVITRO

In vitro, pure DNA forms multiple liquid crystalline phases when the polymer concentration is increased: precholesteric organization, cholesteric phase and columnar hexagonal phase. Similar organizations of chromatin can be found in vivo: hexagonal packing in bacteriophages and certain sperm heads, cholesteric organization in dinoflagellate chromosomes, bacterial nucleoids and mitochondrial DNA, helical-shaped chromosomes in many species. The different forms of condensed chromatin seem to be related to different local concentrations of DNA. In the highly condensed forms, chromatin is inactive and the double stranded DNA molecule is linear with small amounts of associated proteins. A more detailed analysis is presented in the case of cholesteric structures (helical pitch, defects) in polarizing microscopy and in electron microscopy. Differences observed in vitro and in vivo are probably related to the length of the DNA molecule and to the presence of proteins associated to DNA in the chromosomes.