Question

Which part of the nuclear membranes permit the free passage of the products synthesized in the nucleus into the surrounding cytoplasm?

• A. Pores
• B. G proteins
• C. MHC molecules
• D. Coated pits

Hint:

The Nuclear Pore Complex (NPC) is one of the largest macromolecular assemblies in the cell.
It regulates transport in a highly selective manner, relying on nuclear localization signals (NLS) for import and nuclear export signals (NES) for export.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The eukaryotic nucleus is enclosed by the nuclear envelope, a double-membrane barrier consisting of an inner and outer nuclear membrane.
To allow communication between the nucleoplasm and the cytoplasm, specialized channels must span across both membranes.

Step 2: Detailed Explanation:
The inner and outer nuclear membranes fuse at specific points to form circular openings known as nuclear pores.
Each pore is lined by a massive, multi-subunit protein structure called the Nuclear Pore Complex (NPC).
The NPC acts as a selective gatekeeper:
1. Passive diffusion: Small molecules (water, ions, and small proteins up to $\approx 40\,\text{kDa}$) can pass freely through the pores via passive diffusion.
2. Active transport: Large macromolecules, such as fully assembled ribosomal subunits, mRNAs, and tRNAs synthesized in the nucleus, are actively transported out into the cytoplasm using specific transport receptors (exportins).
Similarly, nuclear proteins synthesized in the cytoplasm (like histones and polymerases) are imported into the nucleus using importins.
The other options do not facilitate this transport:
- G proteins (B) are involved in signal transduction.
- MHC molecules (C) are cell-surface antigen-presenting proteins.
- Coated pits (D) are invaginations of the plasma membrane involved in receptor-mediated endocytosis.

Step 3: Final Answer:
Therefore, the nuclear pores are the structures that permit the passage of macromolecules between the nucleus and the cytoplasm, corresponding to option (A).