DNA undergoes a series of liquid crystalline phase transitions when it is compressed in vivo into chromosome-sized packages. The photomicrograph below depicts high molecular weight calf thymus DNA at concentrations approaching 650 milligrams/milliliter. Hoffman modulated contrast and polarized light render this image both beautiful and informative.

DNA is a very unusual molecule that is shaped like a very long piece of string. The diameter of the DNA molecule is about 25-30 Angstroms (about a billionth of an inch) while the length can exceed a thousand microns (0.025 inch) in some organisms. One of the major problems of molecular biology is how this giant molecule can be packaged into a cell or virus that is far smaller than the length of the DNA molecule. In addition, it is important to the cell to be able to access the DNA molecule for the purposes of genetic control, routine cellular maintenance, and reproduction.