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Chromosome Dynamics

DNA must allow for various processing events; it is transcribed into RNA to make the stored information available to the cell; it is replicated and identical copies of itself are equally distributed to its daughter cells; it is frequently repaired, when damaged by endogenous or exogenous sources. All these processes, whether concerning condensed or uncondensed, mitotic or meiotic chromosomes, make them highly flexible and dynamic structures, which change their nucleotide composition as well as their morphology and position.

Regions on chromosomes that undergo replication or repair are transported to the respective centers of activity, replication factories and repair centers. Likewise, interphase chromosomes or parts thereof shuttle between internal transcriptionally active nuclear domains and the nuclear periphery, depending on their transcriptional activity in certain developmental stages or tissues. The most dramatic chromosome movements occur during mitosis and meiosis when daughter chromosomes (chromatids) or homologous chromosomes, respectively, are distributed to daughter nuclei at the onset of anaphase. Both divisions are preceded by the condensation of chromosomes from the open chromatin conformation in interphase to highly compacted chromosomes in mitosis and meiosis. During meiosis additional, distinct forms of intranuclear chromosomal movements are observed. Largely depending on the organism and its strategies to pair homologous chromosomes, these movements may be undirected ("stirring") in order to promote chance contacts between homologous chromosomes, or they may direct chromosomes towards certain compartments in the nucleus where homology recognition takes place.

The long-distance movements of condensed chromosomes during mitotic and meiotic cell divisions are mediated by spindle microtubules inserting at the kinetochore. An intricate collaboration of numerous factors ensures the coordination of chromosome movements with other events in the cell cycle, and the interaction of microtubules with kinetochores is subject to checkpoint controls to ensure the timely and correct segregation of chromosomes. In mitosis this is the separation of sister chromatids, while in meiosis I pairs of sister centromeres are separated from their homologs. A crucial factor in chromosome and cell division is the centrosome, (or spindle pole in yeast) which is essential for correct spindle assembly and positions the spindle relative to the cell cortex.

The telomere at the end of linear chromosomes is important for chromosome integrity and maintenance. It protects them from being recognized and processed as DNA double strand breaks. Furthermore, it harbors a machinery that replenishes chromosomal parts that are lost during every replication step. This process is essential for sustained maintenance of genome integrity over multiple cell generations and has recently been shown to be a key factor that can mediate ageing in mammals or protect against it.

Projects within the graduate program cover a wide range of aspects of chromosome dynamics, from actual chromosome movements, to DNA recombination or chromatin reorganization. Projects concentrating on movement study either microtubule-mediated chromosome segregation and the roles of kinetochores, spindle and centrosomes, or actin-mediated movements observed in meiotic prophase, which contribute to the meiotic homology search and support synapsis and recombination. Recombination projects study requirements for the initiation of recombination by DSBs, as well as the repair of these breaks. Finally, there are projects dealing with the reorganization of chromatin during activation of transcription, and, conversely, the modifications leading to and maintaining a transcriptionally silent chromatin state.