IB Biology Topic B2.2.6: Functional Benefits of the Double Membrane of the Nucleus Notes

The Nuclear Envelope: A Selective Double Membrane

1. The nuclear envelope consists of two phospholipid bilayers, an inner and an outer membrane, separated by a narrow perinuclear space.
   1. The outer membrane is continuous with the rough endoplasmic reticulum (rER) and often has ribosomes attached to its cytoplasmic face.
2. The double membrane separates the contents of the nucleus from the cytoplasm.
   1. This barrier protects the DNA from cytoplasmic enzymes such as nucleases and proteases that could damage it.
   2. It also creates a distinct chemical environment where DNA replication, transcription, and RNA processing can proceed independently of the cytoplasm.

Nuclear Pores: Controlling Molecular Traffic

1. The two membranes are joined at intervals by nuclear pore complexes.
   1. These are large protein structures that span both membranes and regulate the two-way transport of molecules between the nucleus and cytoplasm.
2. Small molecules and ions pass through nuclear pores freely.
   1. Larger molecules need active transport with a signal: proteins bound for the nucleus carry a nuclear localisation signal (NLS) recognised by transport proteins called importins.
   2. In the other direction, mRNA, tRNA, and ribosomal subunits are exported to the cytoplasm after processing.