Nuclear shape is different in stem cells and differentiated cells and reflects important changes in the mechanics of the nuclear envelope (NE). The current framework emphasizes the key role of the nuclear lamina in nuclear mechanics and its alterations in disease 1, 2 . Whether active stress controls nuclear deformations and how this stress interplays with properties of the NE to control NE dynamics is unclear. We address this in the early Drosophila embryo, where profound changes in NE shape parallel the transcriptional activation of the zygotic genome. We show that microtubule (MT) polymerization events produce the elementary forces necessary for NE dynamics. Moreover, large-scale NE-deformations associated with groove formation require concentration of microtubule polymerization in bundles organized by Dynein. However, MT bundles cannot produce grooves when the farnesylated inner nuclear membrane protein Charleston/Kugelkern (Char/Kuk) is absent 3, 4 . Although it increases stiffness of the NE, Char/Kuk also stabilizes NE deformations emerging from the collective effect of MT polymerization forces concentrated in bundles. Finally we report that MT induced NE deformations control the dynamics of the chromatin and its organization at steady state. Thus, the NE is a dynamic organelle, whose fluctuations increase chromatin dynamics. We propose that such mechanical regulation of chromatin dynamics by MT may be important for gene regulation.