TheCell7e Ch10 Lecture

The Nucleus

• Main characteristic differentiating eukaryotic cells from prokaryotic cells.

• Houses the genome, serving as the repository of genetic information and the cell’s control center.

• The separation of the genome from mRNA translation sites is crucial for eukaryotic gene expression.

The Nuclear Envelope and Traffic

• Nuclear Envelope: Separates nuclear contents from cytoplasm and regulates the movement of proteins and RNAs via nuclear pore complexes.

• Components of the Nuclear Envelope:

  • Composed of two nuclear membranes.

  • Contains a nuclear lamina underneath, which provides structural support.

  • Has nuclear pore complexes crucial for selective transport.

• The nuclear envelope is continuous with the endoplasmic reticulum (ER) and directly connected to its lumen. The inner membrane contains integral proteins that bind to the nuclear lamina.

Nuclear Pore Complexes

• Nuclear pore complexes are large structures consisting of about 30 proteins (nucleoporins).

• Traffic Regulation:

  • Proteins/ RNAs must be exported from the nucleus for protein synthesis while nuclear proteins must be imported.

  • Transport Mechanisms:

    • Passive Diffusion: Small molecules can pass freely.

    • Selective Transport: Larger molecules, such as proteins and RNAs, require energy for transport.

Nuclear Lamina

• The nuclear lamina is a fibrous mesh made of lamins and other proteins, supporting the nuclear envelope's shape and anchoring chromatin.

• Lamins interact to form higher-order structures through dimer formation, leading to lamina assembly.

• Mutations in lamin genes can cause various inherited diseases, affecting the proper function of the nucleus.

Chromosome Organization

• Chromatin condenses during mitosis, forming visible metaphase chromosomes. In interphase, chromatin is decondensed and dynamic.

• Chromosome territories allow distinct organization within the nucleus, influencing gene transcription based on positioning.

• Techniques like in situ hybridization and chromosome conformation capture (3C) are used to study chromosome spatial organization within the nucleus.

• Chromosomes are grouped by gene-rich (central) and gene-poor (peripheral) locations during interphase.

Transcription and Replication Factories

• Locations for replication and transcription are compartmentalized within the nucleus.

• Transcription Factories: Sites where clustered genes are transcribed.

• Genes that are coregulated can spatially cluster in these factories, enhancing transcription efficiency.

Nuclear Bodies

• Definition: Structures within the nucleus that concentrate various proteins and RNAs specific to certain processes; are dynamic and not membrane-bound.

• Types of Nuclear Bodies:

  • Nucleolus: Site for rRNA transcription and ribosome assembly. Can produce millions of ribosomes during cell division.

  • Cajal Bodies: Involved in assembling snRNPs and other RNA-protein complexes.

  • Speckles: Storage and processing compartments for pre-mRNA splicing factors.

• mRNA Transport: mRNA is exported without karyopherins and is facilitated by distinct transporter complexes.

Regulation of Nuclear Transport

• Protein movement into and out of the nucleus often regulates cellular processes, notably gene expression.

• Transcription factors may remain in the cytoplasm if masked by associated proteins. Phosphorylation mediates their nuclear import.

• The functional dynamics of nuclear transport involve complex interactions with importins and exportins mediated by specific signal sequences.