Write a short note on nuclear envelope.

Nuclear Envelope (Nuclear Membrane):

Structure:

(i) Double Membrane System:

• Outer membrane: Continuous with rough ER; may bear ribosomes
• Inner membrane: Smooth; associated with nuclear lamina (intermediate filament network)
• Perinuclear space: 10–50 nm gap between membranes; continuous with ER lumen

(ii) Nuclear Pores:

• Octagonal structures (~3000–4000 per nucleus)
• Diameter: ~100 nm
• Formed by fusion of outer and inner membranes
• Nuclear Pore Complex (NPC): ~30 different proteins (nucleoporins)
  • Central channel: ~9 nm functional diameter
  • Regulates bidirectional transport

Functions:

1. Selective Barrier:
   • Separates nucleoplasm from cytoplasm
   • Maintains distinct nuclear environment
2. Regulated Transport:
   • Import: Histones, DNA/RNA polymerases, transcription factors (via importins)
   • Export: mRNA, tRNA, ribosomal subunits (via exportins)
   • Passive diffusion: Small molecules (<40 kDa)
   • Active transport: Large molecules (require signal sequences)
3. Structural Organization:
   • Nuclear lamina provides mechanical support
   • Anchors chromatin to envelope
4. Cell Division:
   • Disassembles during prophase (in most organisms)
   • Reassembles during telophase around daughter nuclei

Clinical relevance: Mutations in nuclear envelope proteins cause laminopathies (muscular dystrophy, progeria)