Chromosome organization revealed upon the decondensation of telophase chromosomes in Allium

Chromosomes of root tip cells of Allium cepa and Allium sativum were studied in early, middle and late telophase to examine the organization of mitotic chromosomes, taking advantage of the naturally occurring chromosome dispersion during the process of decondensation in telophase. Longitudinal and transverse sections of telophase chromosomes viewed under the transmission electron microscope showed that mitotic chromosomes in Allium were composed of helically coiled 400-550 nm chromatin fibres. In some regions of the longitudinal sections, these chromatin fibres were seen to be orientated parallel to one another but formed roughly a right angle to the long axis of the chromosome. In transverse sections, the telophase chromosome appeared to have a hollow centre encircled by the 400-550 nm chromatin fibre which in turn was a hollow tube structure formed by the coiling of a thinner fibre of 170-200 nm. In addition, cross views of chromatin fibres of 170-200 nm and 50-70 nm were also identified in telophase chromosome preparations. These two organizational levels of chromatin fibres also showed a hollow centre. The process of decondensation of telophase chromosomes is described, and some morphological characteristics associated with the activities of chromosome decondensation are analysed. Based on the observations made on Allium chromosomes in this study, various models of chromosome organization are discussed.