Condensed DNA Particles Formed in a PCR with Plasmid Templates: An Electron Microscopy Study

Suspensions of microparticles produced in a polymerase chain reaction on plasmid templates using different primers have been studied by electron microscopy. In all of the samples, microspheres of two types have been detected: of a spherical or ellipsoid form (smooth, without thorns) and of an ellipsoid form with big thorns and/or outgrowths; the diameter of the microspheres varies from 1 to 3 μm. Along with microspheres, so-called three-dimensional network structures of various shapes formed by three-dimensional nanoparticles of a particular topology have been observed. In some samples, discs a few microns in diameter and several dozen nanometers in thickness have been detected. It has been shown that the quantity of netshaped structures and discs sharply increases during asymmetric PCR. In addition, DNA nanofilaments (nanowires), electron-dense point particles (nanodots), electron-translucent nanoparticles in the form of rags, and large electron-dense spherical nanoparticles have been found. Suspensions of microparticles after their quick (5 min) heating at 94°C have been examined. The partial melting of smooth microspheres (without thorns) has been established: they lose a part of the DNA so that the details of their structure (ultrastructure) can be discerned. Simultaneously, clusters of nanowires appear after the heating. The molecular mechanisms of the formation of nanoand microparticles are discussed.