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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.
KEY TERMS: nuclear envelope, nuclear membrane, nuclear lamina, lamin
Nuclear pore complexes are large structures that provide the only routes through which molecules can travel between the nucleus and cytoplasm. Small molecules are able to diffuse freely through open channels in the nuclear pore complex. Macromolecules are selectively transported in an energy-dependent process.
KEY TERMS: nuclear pore complex
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. The small GTP-binding protein Ran is required for translocation through the nuclear pore complex and determines the directionality of transport.
KEY TERMS: nuclear localization signal, importin, Ran, nuclear export signal, exportin
The activity of some proteins, such as transcription factors, is controlled by regulation of their import to the nucleus.
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. Small nuclear RNAs are initially transported from the nucleus to the cytoplasm, where they associate with proteins to form snRNPs; then they return to the nucleus.
KEY TERMS: heterogeneous nuclear ribonucleoproteins (hnRNPs)
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.
KEY TERMS: heterochromatin, euchromatin, X chromosome inactivation, locus control region, insulator element
Some nuclear processes, such as DNA replication and pre-mRNA metabolism, may be localized to discrete subnuclear structures or domains.
The nucleolus is organized around the genes for ribosomal RNAs. It is the site of rRNA transcription and processing, and of ribosome assembly.
KEY TERMS: nucleolus, nucleolar organizing region
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 is mediated by small nucleolar RNAs (snoRNAs).
KEY TERMS: small nucleolar RNAs (snoRNAs)
Ribosomal subunits are assembled within the nucleolus from rRNAs and ribosomal proteins.
Entry into mitosis is signaled by activation of the Cdc2 protein kinase. In most cells, the nuclear envelope breaks down at the end of prophase. Depolymerization of the nuclear lamina results from phosphorylation of the lamins by Cdc2 and other protein kinases.
KEY TERMS: Cdc2
Phosphorylation of histones H1 and H3 is correlated with the condensation of mitotic chromosomes, and H3 phosphorylation is required for proper chromosome condensation. A complex of proteins called condensin is activated by Cdc2 phosphorylation and functions in chromosome condensation by wrapping the DNA into a compact structure.
Inactivation of Cdc2 at the end of mitosis leads to re-formation of the nuclear envelope and chromosome decondensation. Nuclear proteins are then selectively imported through nuclear pore complexes.