Chromosomal Analogues: Long-range Order in  -Condensed DNA

It seems fair to say that the secondary and tertiary structures of DNA are reasonably well understood under the restricted conditions that generally prevail in the laboratory. The DNA is pure, only alkali metal cations are present and in moderate concentration, and the DNA is examined in either a very dilute aqueous solution or in a somewhat hydrated fiber. In both the dilute solution and the fiber, at least one dimension of the space within which the DNA molecule is enclosed is exceedingly large compared to the maximum dimension of enclosure of the corresponding amount of DNA in a cell nucleus, a condensed chromosome, or the head of a virus. The fibers of the DNA-protein complex that appear to constitute the main substance of chromosomal material contain manyfold more DNA per unit length than can be accounted for an extended double helix. Thus, one concludes that DNA in vivo is...