Zellkern

The Zellkern, or cell nucleus, is a membrane-bound organelle found in most eukaryotic cells. It houses the cell’s genetic material, organized as chromosomes. In humans, the nucleus typically measures about 5 micrometers in diameter, and it is absent in prokaryotic cells.

Structure and organization: The nucleus is enclosed by a double membrane called the nuclear envelope, consisting of an outer and an inner membrane. The outer membrane is continuous with the endoplasmic reticulum, and the two membranes enclose the perinuclear space. The inner membrane is supported by a network of intermediate filaments known as the nuclear lamina, which helps maintain nuclear shape. Inside the nucleus, DNA is packaged into chromatin, which can exist as euchromatin (transcriptionally active) or heterochromatin (more compact and less active). The nucleolus, a substructure within the nucleus, is the site of ribosomal RNA synthesis and ribosome assembly. Nuclear pores across the envelope regulate the selective exchange of proteins, RNA, and ribonucleoprotein particles between the nucleus and cytoplasm, mediated by transport receptors and a Ran GTPase gradient.

Function: The nucleus protects and organizes genetic material and provides regulated access to it. It coordinates DNA replication, transcription, and RNA processing, and serves as a hub for chromatin organization that influences gene expression. The nucleolus focuses on ribosome production, while the nuclear envelope and pores control the traffic of macromolecules between the nucleus and cytoplasm.

Dynamics and health relevance: Nuclear architecture can change during the cell cycle; in many organisms the envelope breaks down during mitosis and re-forms afterward. Defects in nuclear envelope components, such as lamins or nuclear pore proteins, can cause diseases known as nuclear envelopathies or laminopathies, including certain muscular dystrophies and lipodystrophies, and are linked to aging and cancer in some contexts.