Organelle connectivity — Which organelle is directly continuous with the outer membrane of the nucleus in eukaryotic cells?

Introduction:
Organelle topology matters for trafficking and gene expression. This question examines your knowledge of which organelle is physically continuous with the nuclear envelope, enabling coordinated transcription–translation coupling across compartments.

Given Data / Assumptions:

• Eukaryotic cell with a double-membrane nucleus.
• Focus on continuity with the outer nuclear membrane.

Concept / Approach:
The outer membrane of the nuclear envelope is continuous with the rough endoplasmic reticulum (RER). Ribosomes stud the RER surface, linking mRNA export from the nucleus to co-translational translocation into the ER lumen or membrane.

Step-by-Step Solution:

Verification / Alternative check:
Electron microscopy shows ribosome-studded outer nuclear membrane blending seamlessly into RER cisternae. Biochemically, shared membrane proteins and lumenal continuity further support this connection.

Why Other Options Are Wrong:

• Mitochondrion: close association but not continuous with nuclear envelope.
• Lysosome: distinct endomembrane organelle; no direct continuity.
• Golgi apparatus: receives cargo from ER via vesicles; not continuous.
• Peroxisome: independent, formed via ER contributions and division; not continuous.

Common Pitfalls:
Assuming physical proximity equals membrane continuity; only the ER is topologically continuous with the outer nuclear membrane.

Final Answer:
Endoplasmic reticulum.