  Real time laser induced fluorescence spectroscopy usage for microanalysis of
DNA double helix defects is shown. The method is based on Forster resonance
energy transfer (FRET) in intercalator-donor pair (acridine orange as a donor
and ethidium bromide as an acceptor). Transition metal ions such as Cu(II),
Cu(I), Ag(I), silver nanoparticles (AgNPs), photo- and thermo effects were used
to cause double helix defects in DNA.
  FRET radii were experimentally estimated in background electrolyte solution
(0.01 M NaNO3) and proved to be 3.9 +- 0.3 nm and the data are in satisfactory
agreement with the theoretically calculated value Ro = 3.5 +- 0.3 nm.
  Concentration of DNA sites, exposed to Cu(II), Cu(I), Ag(I) ions, AgNPs
impact as well as laser irradiation ({\lambda} = 457 nm) and temperature, which
are applicable for intercalation, were estimated in relative units.
  FRET method allows to estimate the concentration of double helix areas with
high quality stability applicable for intercalation in DNA after it was
subjected to stress effect. It gives the opportunity to compare DNAs of 1)
different origin; 2) with various damage degrees; 3) being in various
functional state.
