Super-resolution microscopy provide direct proof of helical coiling of condensed chromosomes

March 14, 2023 | Biology

In earlier cytological studies on chromatids of metaphase, chromosome conformation capture (Hi-C) sequencing supported the assumption that chromosomes coil into a spiral called chromonema. However, the direct visualization of the chromonema advocating the helical model is lacking.

The Leibniz Institute and the Institute of Experimental Botany of the Czech Academy of Science’s team of researchers provided direct evidence of the helical coiling of condensed chromatids by super-resolution microscopy.

As commonly presented in textbooks and other media, the iconic X-shaped organization of metaphase chromosomes made it seem that we clearly understood a chromosome’s ultrastructure and function. Dr. Veit Schubert from the research group claimed how this was not the case. Higher-order structure of metaphase chromosomes and several other models, which are based on data obtained from a wide range of molecular and microscopy methods, have been proposed. These models are either further classified as helical or non-helical.

IPK and IEB investigators gave a comprehensive description of the chromosome’s ultrastructure. They used the following various approaches at a super-resolution level:, Hi-C of isolated mitotic chromosomes, polymer modeling, and microscopic observations of sister chromatid exchanges and oligo-FISH labeled regions. This multidisciplinary procedure was able to provide an independent proof of coiling of the chromonema.

Large chromosomes of barley (Hordeum vulgare) were utilized as a model in examining the higher-order structure of mitotic chromosomes. The model suggested a general mechanism for the formation of condensed mitotic chromosomes. This would be applicable to all eukaryotes across a broad spectrum of genome sizes.

Following this study, chromonema coiling will be confirmed in a huge number of plant and animal species having large chromosomes. Dr. Amanda Camara expressed that in understanding chromatin dynamics during the course of the cell cycle, the identification of the principle of chromosome condensation is the stepping stone.