Nucleic Acids Research, Vol 26, Issue 13 3228-3234, Copyright © 1998 by Oxford University Press
Z Lin, C Wang, X Feng, M Liu, J Li and C Bai
Condensation of DNA by multivalent cations can provide useful insights into
the physical factors governing the folding and packaging of DNA in vivo. In
this work, local ordered structures of spermidine-DNA complexes prepared
from different DNA concentrations have been examined by using atomic force
microscopy (AFM) and polarizing microscopy (PM). Two types (I and II) of
DNA condensates, significantly different in sizes, were observed. It was
found that for extremely dilute solutions (DNA concentrations around 1
ng/microl or below), the DNA molecules would collapse into toroidal
structures with a volume equivalent to a single lambda-DNA (type I). In
relatively dilute solutions (DNA concentrations between 1 and 10
ng/microll), a significantly larger structure of multimolecular toroids
(circular and elliptical, type II) were formed, which were constructed by
many fine particles. Measurements show that the average diameter of these
fine particles was similar to the outer diameter of the monomolecular
toroids observed in extremely dilute solutions, and the thickness of the
multimolecular toroids had a distribution of multi-layers with height
increments of 11 nm, indicating that the multimolecular toroidal structures
have lamellar characteristics. Moreover, by enriching the DNA-spermidine
complexes in very diluted solution, branch-like structures constructed by
subunits were observed by using AFM. The analysis of the pellets in
polarizing microscopy reveals a liquid-crystal-like pattern. These
observations suggest that DNA-spermidine condensation could have multiple
stages, which are very sensitive to the DNA and spermidine concentrations.
ARTICLES
The observation of the local ordering characteristics of spermidine- condensed DNA: atomic force microscopy and polarizing microscopy studies
Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China.
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