Experiments are reviewed that allow one to assign naturally occurring 
DNA-containing plasmas to either of two classes by virtue of their sensitivity 
to aggregation upon dehydration in organic solvents. The interphase nuclei of 
higher cells are relatively insensitive, while the DNA plasmas represented by 
bacterial nucleoids, vegetative bacteriophage and the chromosomes of 
dinoflagellates are sensitive. In higher cells the bulk of DNA is organised with 
histones in the form of nucleosomes. In prokaryotes and in the pool of 
vegetative phage DNA the most abundant histone-like protein HU is not associated 
with the bulk DNA, but localised in the border region with ribosomes where 
transcription and translation occur. These experimental results strongly suggest 
that the two classes of DNA plasmas are distinguishable by a low (1:10) or high 
(1:1) protein-to-DNA ratio. The hypothesis is formulated that the vegetative DNA 
(replicating and transcribing), throughout the living world, is nucleosome-free; 
during evolution, nucleosomes would have been introduced as a simple and 
adequate means for compacting the resting DNA. Condensation of DNA does not 
occur with prokaryotic nucleoids, but does take place when DNA is withdrawn from 
the vegetative phage pool to become packaged into phage heads. Dinoflagellate 
chromosomes are rather condensed although structurally different from eukaryotic 
chromosomes (e.g., those from Euglena) and are much more aggregation-sensitive.