Fluorescence in situ hybridisation experiments on mitotic chromosome
and nucleus spreads showed a unique site of hybridization
of the probe corresponding to the NORs location, i.e., the terminal
region of the long arms of the metacentric chromosome
pair 10 (Insua and Thiriot-Quiévreux, 1992 and Xu et al., 2001,
Fig 3a), demonstrating the proper probe functioning. Comet-FISH
experiments revealed major rDNA FISH-signals to appear as both
individual dots and clouds of dots dispersed over the comets. Dot
clouds were more frequent in comet heads, whereas individual signals
were more frequently observed in comet tails (Fig 3b–i). No dot
chains were observed in comet tails. The number of signals ranged
from 1 to 4 in little-damaged nucleoids (Fig 3b–d) and from 1 to 32
in nucleoids with a comet tail (Fig 3e–i). Moreover, in cells exposed
to hydrogen peroxide, nucleoids without a comet tail also exhibited
1 to 4 signals, similarly to undamaged nucleoids from the control
group.