Chapter 11: The Nucleus: Structure & Chromatin Organization

Significant attention is given to the clinical relevance of the nuclear lamina, discussing how mutations in lamin genes can lead to laminopathies, such as Emery-Dreifuss muscular dystrophy. The chapter elucidates the mechanisms of nucleocytoplasmic transport, distinguishing between the passive diffusion of small molecules and the selective, signal-mediated transport of macromolecules like proteins and RNAs. It explains the function of nuclear localization signals (NLS) and nuclear export signals (NES) which are recognized by specific receptors called karyopherins, including importins and exportins. A critical concept covered is the regulation of transport directionality by the Ran GTPase gradient, where a high concentration of Ran GTP in the nucleus drives the dissociation of import complexes and the formation of export complexes. The summary also distinguishes the specialized, Ran-independent mechanisms required for mRNA export, which involve distinct exporter complexes and helicases. Furthermore, the text explores the spatial organization of the genome, describing how chromosomes occupy discrete territories and are partitioned into topologically associating domains (TADs) and lamina-associated domains (LADs) that correlate with transcriptional activity. The functional compartmentalization of the nucleus is further illustrated through the concepts of replication and transcription factories, where DNA synthesis and RNA production occur in clustered foci. Finally, the chapter characterizes membrane-less nuclear bodies maintained by protein-protein and protein-RNA interactions, specifically the nucleolus, which functions as a ribosome production factory processing rRNAs and assembling subunits. Other nuclear organelles discussed include Cajal bodies, which facilitate snRNP maturation and telomerase assembly, nuclear speckles that store splicing factors, and Polycomb bodies involved in transcriptional repression via histone methylation.