Because of its applicability to biological specimens (nonconductors),
a single-molecule-imaging technique, atomic force microscopy (AFM), has
been particularly powerful for visualizing and analyzing complex
biological processes. Comparative analyses based on AFM observation
revealed that the bacterial nucleoids and human chromatin were constituted
by a detergent/salt-resistant 30–40-nm fiber that turned into
thicker fibers with beads of 70–80 nm diameter. AFM observations of
the 14-kbp plasmid and 110-kbp F plasmid purified from Escherichia
coli demonstrated that the 70–80-nm fiber did not contain a
eukaryotic nucleosome-like “beads-on-a-string” structure.
Chloroplast nucleoid (that lacks bacterial-type nucleoid proteins and
eukaryotic histones) also exhibited the 70–80-nm structural units.
Interestingly, naked DNA appeared when the nucleoids from E. coli
and chloroplast were treated with RNase, whereas only 30-nm chromatin
fiber was released from the human nucleus with the same treatment. These
observations suggest that the 30–40-nm nucleoid fiber is formed with
a help of nucleoid proteins and RNA in E. coli and chroloplast,
and that the eukaryotic 30-nm chromatin fiber is formed without RNA. On
the other hand, the 70–80-nm beaded structures in both E.
coli and human are dependent on RNA.