The Nucleus: Structure & Chromosome Organization | Chapter 10 – The Cell: A Molecular Approach (7th)

This chapter explores the nucleus, the defining feature of eukaryotic cells that serves as the central repository for genetic information and the primary control center for cellular activity. By sequestering the genome within the nuclear envelope, eukaryotes can utilize sophisticated gene regulation mechanisms, such as posttranscriptional RNA processing and regulated transcription factor transport, that are unavailable to prokaryotes. The nuclear envelope itself is a complex double-membrane system where the outer membrane is continuous with the endoplasmic reticulum. Beneath the inner membrane lies the nuclear lamina, a fibrous meshwork of lamin proteins that provides structural support and anchors chromatin. Defects in these lamin proteins or their associated membrane receptors, like emerin, lead to a variety of tissue-specific diseases known as laminopathies, including Emery–Dreifuss muscular dystrophy and Hutchinson-Gilford progeria syndrome. Traffic between the nucleus and cytoplasm is exclusively mediated by nuclear pore complexes, which are massive protein assemblies that allow for the passive diffusion of small molecules while facilitating the selective, signal-directed transport of macromolecules. This selective transport relies on nuclear localization signals (NLS) for import and nuclear export signals (NES) for export, which are recognized by specialized receptors called importins and exportins. The directionality of this movement is fueled by a concentration gradient of the GTP-binding protein Ran, which exists primarily in its GTP-bound state within the nucleus and its GDP-bound state in the cytoplasm. The internal landscape of the nucleus is equally organized, with individual chromosomes occupying distinct regions known as chromosome territories. Transcriptionally active euchromatin is typically found in the nuclear interior, often clustering in "transcription factories" where multiple genes are processed by concentrated RNA polymerases. Conversely, inactive heterochromatin often localizes to the nuclear periphery or the edges of the nucleolus in specialized regions called lamina-associated domains (LADs) or nucleolus-associated domains (NADs). The chapter also details various membrane-less "nuclear bodies" that compartmentalize nuclear functions. The most prominent is the nucleolus, a dedicated factory for transcribing ribosomal RNA (rRNA) and assembling ribosomal subunits. Other specialized bodies include Polycomb bodies for gene silencing, Cajal bodies for the maturation of splicing machinery and telomerase, and nuclear speckles, which serve as storage sites for splicing factors. Together, these structural and functional elements transform the nucleus into a highly dynamic organelle capable of precisely coordinating the complex life of a eukaryotic cell. 📘 Read full blog summaries for every chapter: https://lastminutelecture.com 📘 Have a book recommendation? Submit your suggestion here: https://forms.gle/y7vQQ6WHoNgKeJmh8 Thank you for being a part of our little Last Minute Lecture family! ⚠️ Disclaimer: These summaries are created for educational and entertainment purposes only. They provide transformative commentary and paraphrased overviews to help students understand key ideas from the referenced textbooks. Last Minute Lecture is not affiliated with, sponsored by, or endorsed by any textbook publisher or author. All textbook titles, names, and cover images—when shown—are used under nominative fair use solely for identification of the work being discussed. Some portions of the writing and narration are generated with AI-assisted tools to enhance accessibility and consistency. While every effort has been made to ensure accuracy, these materials are intended to supplement—not replace—official course readings, lectures, or professional study resources. Always refer to the original textbook and instructor guidance for complete and authoritative information.