COILING THE CHROMONEMA

We consider the chromonema to be an accumulation of chromatin loops, which might be dynamically extruded, but stabilize a thick prophase-like thread of chromatin, with physical properties of its own [1]. This is a refurbished version of the term ‘chromonema’, which is actually more than a century old. It was first based only on compound microscopes observations, which described a thick thread of chromatin, coiling and compacting during the cell cycle. It was regarded as a direct morphological connection between metaphase and interphase, in a time when chromosomes were not presumably continuous [2]. Later disbelieves decreased the popularity of the chromonema, seen as an artefact from shimmering and the coiling mechanism too elaborate. The discovery of other structural components, such as topoisomerase and condensin protein complexes, and other organizational architectures, like chromatin loops, opened the way for other models of chromosome compaction, creating a Rashomon effect in the field – with more than one explanation for the same event. While the question of how chromosomes compact endures, the latest developments of chromosome conformation capture brought new light, endorsing a model of helical organization of mitotic chromosomes [3, 4]. Yet, how do they coil into a helix, since no protein scaffold could be confirmed so far? We approach this problem with polymer models of barley chromosomes, whose Hi-C data and microscopy images suggest a helical organization with different turn lengths and width, respectively, from metaphase to G1 of root meristematic cells. We first consider a compact and dynamic thread of chromatin loops – a chromonema excerpt – and force it into a helical path. This helical path can then be indirectly modulated by changing confinement conditions while dynamically extruding chromatin loops (conditions that we assume different over the cell cycle). These computational experiments suggest that the changes in the helical turn of compacted chromosomes are a response to different constraints of its chromatin loops and biological milieu, rather than an active process. These results await more experimental evidence, but we propose, for better understanding of compaction mechanisms,