The genetic information of all living organisms is contained in their DNA. Cells modify the degree of DNA compaction by epigenetics, which largely determines what genes are read out and which genes... Show moreThe genetic information of all living organisms is contained in their DNA. Cells modify the degree of DNA compaction by epigenetics, which largely determines what genes are read out and which genes are transcriptionally silent. Despite decades of research into this mechanism, there is no consensus on how cells realize the various degrees of DNA compaction in vivo. Eukaryotes, such as humans, compact their DNA into higher-order structures called compact chromatin fibers. We characterize these fibers through a combination of single-molecule force spectroscopy techniques like magnetic tweezers, and rigid base pair Monte Carlo simulations. We show that, for instance, the length and sequence of the linker DNA, the DNA between adjacent nucleosomes, control the mechanical properties of chromatin fibers. Our measurements suggest the formation of more than one higher-order fiber structure. A deeper understanding of the chromatin fiber and its compaction mechanism is important because the dysfunction of such regulation results in various medical conditions such as the epigenetic disorder type 1 diabetes, fragile X syndrome, or various cancers. Show less
In human cells, a meter-long DNA is condensed inside a micrometer-sized cell nucleus. Simultaneously, the genetic code must remain accessible for its replication and transcription to functional... Show moreIn human cells, a meter-long DNA is condensed inside a micrometer-sized cell nucleus. Simultaneously, the genetic code must remain accessible for its replication and transcription to functional proteins. Such plasticity of the genome is maintained by dynamic folding and unfolding of DNA-protein spools called nucleosomes. It is unclear, however, how this process is controlled when multiple nucleosomes stack on top of each other and form compact chromatin fibers. This is particularly important since nucleosomes are rarely present in isolation inside a densely packed cell nucleus. Therefore, the aim of this thesis was to increase the understanding of the chromatin fiber structure and its dynamics. Knowing these details would provide many new insights into the mechanisms of gene expression (epigenetic regulation) which, upon malfunction, may cause severe diseases. The presented work consists of an experimental approach involving the application of single-molecule force spectroscopy, and makes use of theoretical modelling based on statistical mechanics. By using magnetic tweezers, we stretched and twisted individual chromatin fibers reconstituted in vitro in order to unfold its nucleosomes. These studies show that folding of nucleosomes into chromatin fibers opens up a plethora of regulatory pathways for controlling the level of DNA organization in cells. Show less
Since the discovery of the right-handed helical structure of DNA, 61 years have passed. The DNA molecule, which encodes genetic information, is also found twisted into coils. This extra twist of... Show moreSince the discovery of the right-handed helical structure of DNA, 61 years have passed. The DNA molecule, which encodes genetic information, is also found twisted into coils. This extra twist of the helical structure, called supercoiling, plays important roles in both DNA compaction and gene regulation. The DNA in eukaryotic cells is packaged into chromatin. Using single-molecule force spectroscopy, I resolved force/torque induced structural changes of DNA and chromatin fibers. I showed that the structural changes of chromatin fibers can be described by four conformations. I showed for the first time the folding and unfolding of a chromatin fiber under torsion. Th e anisotropic response of chromatin fibers to supercoiling reflects its leftŸ-handed chirality. These findings give a detailed structural insight of a supercoiled chromatin fiber, yielding a better understanding of the response of chromatin during transcription Show less
In eukaryotic cells, genomic DNA is organized in chromatin fibers composed of nucleosomes as structural units. A nucleosome contains 1.7 turns of DNA wrapped around a histone octamer and is... Show moreIn eukaryotic cells, genomic DNA is organized in chromatin fibers composed of nucleosomes as structural units. A nucleosome contains 1.7 turns of DNA wrapped around a histone octamer and is connected to the adjacent nucleosomes with linker DNA. The folding of chromatin fibers effectively increases the compaction of genomic DNA, but it remains accessible for enzymatic reactions. This apparent paradox motivates a detailed study of the dynamics of chromatin. A structural model at the molecular level will shed light on how cells regulate the compaction and dynamics of genomic DNA. This thesis presents the results of an experimental study on the dynamics of chromatin higher-order folding. Using magnetic tweezers, we observed force-dependent structural changes within chromatin fibers at the single nucleosome level. Show less
