Insights on plant chromosome structure provided by the FPG technique

The high resolution afforded by the fluorescence plus Giemsa (FPG) labelling method has allowed us to visualize three main features in BrdU-substituted A. cepa chromosomes. These are (1) exchanged segments smaller than the width of the chromatid detected after BrdU substitution for two S periods; (2) late-replicated chromosome segments (LRS) of similar characteristics; (3) discontinuous or 'step-wise' exchanges (SWE). A variation in longitudinal dimension has not been observed in any case; all these features have a roughly constant longitudinal dimension of about 0.5 microns. These results suggest that arrangements of the chromatin fibres having a diameter detectable under the light microscope following a longitudinal pattern can be ignored. In addition, our observations seem to support the model of a coiled structure of metaphase chromosomes. On the other hand, the analysis of the morphological pattern shown by the SWE along a chromatid and in sister chromatids has provided us with what could be evidence of a direction of the hypothetical gyres.