Nucleus: Structural Organization and Function

Introduction

• The nucleus is the largest membrane-bound organelle in eukaryotic cells.
• Often called the “control center” of the cell because it stores genetic material (DNA) and regulates cellular activities.
• Typically spherical/ovoid, occupying 10–20% of cell volume.
• Discovered by Robert Brown (1831)

Structural Organization of Nucleus

Nuclear Envelope

• Double membrane system surrounding the nucleus
  • Outer membrane: continuous with rough endoplasmic reticulum, often studded with ribosomes.
  • Inner membrane: lined with nuclear lamina (protein meshwork of lamin A, B, C) providing structural support.
    
• Nuclear pores:
  • Large protein complexes (nuclear pore complexes, NPCs).
  • Regulate bidirectional transport of proteins, RNAs, and ribonucleoproteins.

Nucleoplasm (Nuclear Sap)

• Gel-like matrix filling the nuclear space.
• Contains ions, nucleotides, enzymes, and soluble proteins.
• Provides medium for diffusion of small molecules and macromolecules.

Chromatin

• Complex of DNA + histone + non-histone proteins.
• Two forms (visible under light/electron microscope):
  • Euchromatin: loosely packed, transcriptionally active DNA.
  • Heterochromatin: highly condensed, transcriptionally inactive DNA.
• During cell division → condenses into chromosomes.

Nucleolus

• Dense, non-membranous structure within nucleus.
• Site of ribosomal RNA (rRNA) synthesis and ribosome subunit assembly.
• Contains:
  • Fibrillar centers (rDNA transcription sites).
  • Dense fibrillar component (processing of pre-rRNA).
  • Granular component (assembly of ribosomal subunits).

Nuclear Matrix

• Insoluble fibrillar network inside nucleus.
• Provides structural framework, organizes chromatin, regulates replication and transcription.

Other Intranuclear Structures

• Cajal bodies: involved in RNA splicing and modification of snRNPs.
• Speckles: storage of splicing factors.
• PML (promyelocytic leukemia) bodies: roles in apoptosis, DNA repair, and viral defense.

Functions of Nucleus

1. Genetic Information Storage
   • Stores DNA in chromatin form.
   • Maintains genome integrity.
     
2. Gene Expression Regulation
   • Controls transcription (DNA → RNA).
   • Chromatin remodeling determines active/inactive regions.
     
3. RNA Synthesis & Processing
   • Transcribes mRNA, tRNA, rRNA, snRNA.
   • RNA splicing, capping, polyadenylation occur in nucleus.
     
4. Ribosome Biogenesis
   • Nucleolus assembles rRNA + ribosomal proteins → ribosome subunits.
     
5. Cell Division
   • Chromatin condensation → chromosome segregation during mitosis and meiosis.
     
6. Nuclear Transport
   • Selective movement of molecules via nuclear pore complexes.
     
7. Signal Integration
   • Responds to cytoplasmic/nuclear signals for growth, differentiation, and stress responses.