Nuclear Pore

Nuclear pores are large protein-lined channels in the nuclear envelope that regulate exchange of materials between the nucleus and cytoplasm. Each nuclear pore is part of a nuclear pore complex, which acts as a gateway for molecules (such as mRNAs, ribosomal subunits, and proteins) to pass into or out of the nucleus.

• The nuclear envelope (the double membrane around the nucleus) is perforated with many nuclear pores. Small molecules and ions can diffuse through, but larger molecules (like RNA and proteins) require active transport via the pore complexes.
• Nuclear pore complexes ensure directionality: for example, they enable mRNA and ribosome components to exit the nucleus, and allow nuclear proteins (like transcription factors or polymerases) to enter. Transport signals (NLS or NES sequences on proteins) are used for recognition by the NPC.
• Only eukaryotic cells have nuclear pores. (Prokaryotes have no nucleus and thus no nuclear envelope or pores.) So any mention of nuclear pores implies a eukaryotic cell context.

• If a question asks "How does mRNA get from the nucleus to the cytoplasm-" or "through what structure do ribosomal subunits leave the nucleus-", the answer is via nuclear pores.
• Be prepared for a question that simply asks for the function of nuclear pore complexes: correct answer being control of traffic between nucleus and cytoplasm (e.g., letting out RNA, letting in nucleus-targeted proteins).
• A tricky point: an exam might ask which structures are absent in bacteria - one answer could be "nuclear pores," since bacteria lack a nucleus. Remember: nuclear pores are a feature of the eukaryotic nuclear envelope.