Ordre et désordre dans les formes toroïdales de l'ADN condensé analysés par cryo-microscopie électronique

DNA toroids are liquid crystalline structures formed spontaneously by the condensation of DNA molecules in solution by a condensation agent such as spermine 4+. These toroids serve as models for understanding chromosome folding in certain double-stranded DNA viruses and for their potential in nano-engineering. Detailed characterisation of their three-dimensional organisation remains limited to a localised hexagonal order. This thesis aims to elucidate the fine structure and formation mechanism of DNA toroids, which are still poorly understood despite numerous theoretical studies and simulations.At the Solids Physics Laboratory (LPS), we have developed a protocol for controlling the curvature of toroids over a wide dimensional range, from a few tens to several hundreds of nanometres. This enables to study toroid morphogenesis by cryo Transmission Electron Microscopy (cryo-TEM), a now widely used technique for observing biological structures in their native state, after vitrification at low temperature.The images obtained by cryo-TEM revealed an hexagonal order within the DNA toroids, in agreement with previous results. We identified correlations between the DNA double helices, forming an electrostatic 'zipper'. Our study reveals that optimization of the helical correlations is associated with rearrangements within the toroid as it grows, with the establishment of correlation followed by polygonal shaping. In addition, a local decrease in the DNA helical repeat is measured in high curvature regions.We demonstrate an order-disorder segregation within toroids, with structural defects (DNA ends and “bridge” defects) concentrated in a specific sector of the toroid. This phenomenon plays a role in optimization of electrostatic interactions, including the electrostatic zipper.Lastly, we have initiated liquid phase electron microscopy, an emerging technique for studying the dynamics of biological processes at the nanoscale. We aim to follow toroid's formation, from their nucleation to their final state. We obtained preliminary images on bacteriophages, used here as a precursor of the toroid. This innovative approach would open up new perspectives for understanding the morphogenesis of DNA toroids and could potentially reveal fundamental mechanisms underlying their formation and stability.This study of DNA toroids combines experimental approaches to explore their structure, dynamics and formation mechanism. These results contribute to our fundamental understanding of the biophysics of condensed states of DNA.