Structure of the Nuclear Envelope: The nuclear envelope separates the contents of the nucleus from the cytoplasm, maintaining the nucleus as a distinct biochemical compartment that houses the genetic material and serves as the site of transcription and RNA processing in eukaryotic cells. The nuclear envelope consists of the inner and outer nuclear membranes (which are joined at nuclear pore complexes) and an underlying nuclear lamina.
The Nuclear Pore Complex: Nuclear pore complexes are large structures that provide the only routes through which molecules can travel between the nucleus and the cytoplasm. Small molecules can diffuse freely through open channels in the nuclear pore complex. Macromolecules are selectively transported in an energy-dependent process.
Selective Transport of Proteins to and from the Nucleus: Proteins destined for import to the nucleus contain nuclear localization signals that are recognized by receptors that direct transport through the nuclear pore complex. Proteins that shuttle back and forth between the nucleus and the cytoplasm contain nuclear export signals that target them for transport from the nucleus to the cytoplasm. In most cases, the small GTP-binding protein Ran is required for translocation through the nuclear pore complex and determines the directionality of transport.
Regulation of Nuclear Protein Import: The activity of some proteins, such as transcription factors, is controlled by regulation of both their import to, and export from, the nucleus.
Transport of RNAs: RNAs are transported through the nuclear pore complex as ribonucleoprotein complexes. Messenger RNAs, ribosomal RNAs, and transfer RNAs are exported from the nucleus to function in protein synthesis. Several classes of small nuclear RNAs are initially transported from the nucleus to the cytoplasm where they associate with proteins to form RNPs; they then return to the nucleus.
Chromosomes and Higher-Order Chromatin Structure: The interphase nucleus contains transcriptionally inactive, highly condensed heterochromatin as well as decondensed euchromatin. Interphase chromosomes are organized within the nucleus and divided into large looped domains that function as independent units.
Sub-Compartments within the Nucleus: Some nuclear processes, such as DNA replication and mRNA metabolism, may be localized to discrete subnuclear structures.
Ribosomal RNA Genes and the Organization of the Nucleolus: The nucleolus is associated with the genes for ribosomal RNAs. It is the site of rRNA transcription, rRNA processing, ribosome assembly, and the modification of several small RNAs.
Transcription and Processing of rRNA: The primary transcript of the rRNA genes is 45S pre-rRNA, which is processed to yield 18S, 5.8S, and 28S rRNAs. Processing of pre-rRNA and other small RNAs is mediated by small nucleolar RNAs (snoRNAs).
Ribosome Assembly: Ribosomal subunits are assembled within the nucleolus from rRNAs and ribosomal proteins.
