Abstract

The higher-order organization of metaphase chromosomes has been debated for almost 140 years. Classical light and electron microscopy studies suggested that chromatids are composed of helically organized chromatin fibers (chromonemata). Non-helical models were also recently proposed. We studied chromosome organization in barley using cutting-edge approaches and obtained evidence for a helically arranged 400-nm chromatin fiber representing the chromonema within chromatid arms. The number of turns is positively correlated with arm length. Turn size and chromatin density decrease towards the telomeres. Due to their specialized functions, the helical organization of centromeres and nucleolus-organizing regions is interrupted by several thinner, straight chromatin fibers. A comparison with previously published data indicates that the helical turning of metaphase chromatid arms is a conserved feature of large eukaryotic chromosomes.