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Researchers provide evidence for the spiral structure of condensed chromosomes
In earlier studies it was assumed that the chromatids of the metaphase chromosomes roll up into a spiral, the chromonema. This assumption was recently supported by conformational detection of chromosomes. Nevertheless, the direct, differentiating visualisation of the coiled chromonema to confirm the spiral model was missing. Now, an international research team led by the IPK Leibniz Institute and the Institute of Experimental Botany of the Czech Academy of Sciences has provided this direct evidence for the first time. The results have been published today in the journal "Nucleic Acids Research".
02/03/2023
The well-known X-shaped appearance of metaphase chromosomes is often depicted in textbooks and other media. The drawings explain in a fascinating way that the majority of genetic information is stored in chromosomes, which pass it on to the next generation. „These illustrations suggest that the ultrastructure of chromosomes is well understood. However, this is not the case," says Dr Veit Schubert from the IPK research group "Chromosome Structure and Function".
Based on data obtained using a range of molecular and microscopic methods, several models exist to describe the overall structure of metaphase chromosomes. These models are divided into helical and non-helical models. Helical models assume that the chromatin in each sister chromatid in metaphase mitosis is arranged in the form of a spiral, while non-helical models assume that the chromatin is folded within the chromatids without forming a spiral.
Researchers have now revived the term chromonema, which was first used at the beginning of the 20th century, and provided a detailed description of its ultrastructure. Various experimental approaches, including chromosome conformation capture (Hi-C) on isolated mitotic chromosomes, polymer modelling, analysis of sister chromatid exchanges and oligo-FISH-labelled regions using super-resolution microscopy, provided independent evidence for the spiral formation of chromonema.
„The coiled chromatid organisation and its organisational unit, the chromonema, were confirmed independently of each other using different methods,
says Dr. Veit Schubert.
„To investigate the higher-level structure of mitotic chromosomes, the large chromosomes of the cultivated barley were used as a model. A single helical turn comprises 20 to 38 megabases of DNA and forms a ~400 nm thick fibre that we call a chromonema,
says Dr Amanda Camara, one of the first authors of the study.
The model suggests a general mechanism for the formation of condensed mitotic chromosomes that is applicable to all eukaryotes with a wide range of genome sizes. „We expect that after our study, chromonema-based organisation of chromosomes will be confirmed in a larger number of plant and animal species with large chromosomes. The identification of the principle of chromosome condensation in this work is a prerequisite for understanding chromatin dynamics during the cell cycle,
says Dr Amanda Camara.
Press release of the "idw - Informationsdienst Wissenschaft" from 02.03.2023
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