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#cell nucleus

6 public questions tagged with this topic.

Cell Organelles

The primary function of the nucleolus is:

Electron microscopy reveals nucleolus as darkly staining subnuclear domain due to high RNA protein density, tripartite with fibrillar centers containing inactive rDNA, dense fibrillar component where transcription and early processing occurs, granular component assembling pre-ribosomes. RNA polymerase I with transcription factor UBF drives high-rate synthesis of forty seven S precursor containing eighteen S, five point eight S, twenty eight S RNAs flanked by five prime external transcribed spacer and internal transcribed spacers ITS1 and ITS2. Early cleavages by U3 snoRNP at five prime ETS occur in dense fibrillar component, subsequent modifications include methylation by fibrillarin and pseudouridylation by dyskerin guided by snoRNAs. Pre-forty S and pre-sixty S assembled with imported ribosomal proteins one hundred copies transit to granular component before export. DNA replication initiates at many origins in nucleoplasm during S phase, ATP production mitochondrial oxidative phosphorylation, RNA modification widespread but concentrated for rRNA here. Actinomycin intercalates rDNA halting Pol I causing nucleolar disruption, confirming function as ribosomal RNA processing and assembly center coordinating growth with protein synthesis demand.

Ref: Scheer Annu Rev Cell Biol; nucleolus FC DFC GC rRNA transcription processing assembly.

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Cell Organelles

The nuclear lamina is composed of:

Meshwork underlying inner nuclear membrane known as nuclear lamina provides mechanical strength preventing envelope rupture under cytoskeletal tension transmitted via LINC complex and organizes chromatin positioning. It consists of intermediate filament proteins lamins subclassified into B-type encoded by LMNB1 and LMNB2 constitutively farnesylated for membrane anchorage, and A-type encoded by LMNA via alternative splicing producing lamin A including farnesylated precursor processed to mature form and lamin C lacking farnesylation, expressed after differentiation. Lamins assemble via parallel coiled-coil dimers forming head-to-tail polymers assembling into ten nanometer filaments forming orthogonal lattice. They interact with integral membrane proteins emerin, LAP2beta, LBR and chromatin adaptor BAF. Phosphorylation by CDK1 at mitosis drives depolymerization allowing envelope breakdown. Tubulin forms twenty five nanometer microtubules, actin seven nanometer microfilaments, spectrin and ankyrin support erythrocyte membrane, histone H1 compacts linkers between nucleosomes, therefore not components of lamina lattice. Mutations cause progeria and muscular dystrophies. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.

Ref: Burke Annu Rev Cell Dev Biol; lamins A/B/C type V IF, farnesylation, lattice under inner membrane.

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Cell Organelles

Which nuclear domain is associated with transcriptionally inactive chromatin?

Mammalian nucleus partitions chromatin into active and inactive states with distinct cytology and epigenetic marks. Euchromatin appears lightly stained, decondensed, nuclease hypersensitive and enriched in histone acetylation, H3K4 trimethylation, H3K36 methylation associated with ongoing transcription and replication early S phase, located interiorly. Heterochromatin appears electron dense, concentrates at periphery adjacent to nuclear lamina, around nucleolus and at centromeres, enriched in H3K9 trimethylation, H4K20 trimethylation, HP1 binding and DNA methylation repressing transcription by limiting accessibility of RNA polymerase II and transcription factors. Facultative heterochromatin such as inactive X chromosome marked by Xist and H3K27 trimethylation can switch to euchromatin under developmental cues, while constitutive heterochromatin containing repetitive satellites remains silent. Nucleoplasm describes soluble nuclear compartment, lamins are intermediate filament proteins forming structural mesh not chromatin type. ATAC-seq and DNase-seq distinguish open euchromatin from closed heterochromatin genome-wide. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.

Ref: Grewal Annu Rev Genet; heterochromatin H3K9me3 HP1 transcriptionally silent peripheral, euchromatin active.

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Cell Organelles

The nuclear matrix contains proteins such as:

Nuclear interior contains skeletal framework operationally defined as material insoluble after detergent extraction, DNase digestion and high-salt removal of chromatin. This nuclear matrix comprises lamins A and B at periphery plus internal matrin family proteins Matrin-3, Matrin F, hnRNPs, SAF-A and actin-related proteins that form ten nanometer filaments and fibrogranular networks visible by resinless embedding EM. Matrin-3 is RNA- and DNA-binding protein with zinc-finger domains retaining hyperedited RNAs and anchoring matrix attachment regions to organize loops, mutated in familial amyotrophic lateral sclerosis. Lamin B remains associated throughout extraction, lamin A/C contributes to internal foci. Kinesin and dynein are microtubule motors functioning in cytoplasm, actin and myosin generate contractile forces in cytoplasm not nucleus except for nuclear myosin one, SNARE and COPI mediate vesicle fusion and coating in secretory pathway not nucleus. Matrix organizes replication factories, transcription hubs and splicing speckles, providing structural context for genome function. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.

Ref: Berezney Annu Rev Cell Biol; nuclear matrix lamin matrin-3 scaffold MAR organizes transcription factories.

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Cytoskeleton

What is the function of the nuclear lamina?

Interphase nucleus experiences mechanical stresses from cytoskeletal pulling forces migration through constricted spaces and chromatin dynamics requiring reinforcement beyond lipid bilayer envelope alone. Nuclear lamina provides mechanical support as dense meshwork 10 to 20 nm thick apposed to inner nuclear membrane beneath lamina associated chromatin domains. Composed of A type lamins lamin A and C splice isoforms and B type lamins B1 B2 type V intermediate filament proteins forming coiled coil dimers assembling head to tail polymers into orthogonal filaments crosslinked into sheet. Mesh confers elastic stiffness about 2 to 5 fold increasing nuclear rigidity buffers deformation prevents rupture under high strain anchors nuclear pore complexes basket tethers lamina associated domains LADs enriched heterochromatin regulating transcription silencing. Additional functions include mechanotransduction signaling via emerin to regulate gene expression and DNA repair scaffolding through 53BP1. Disassembly mediated by CDK1 phosphorylation disassembles filaments during mitosis enabling nuclear envelope breakdown chromosome capture. Additional regulatory inputs including phosphorylation, small GTPases, and cargo adaptors fine tune filament assembly stability and motor activity matching cellular demands during division, migration, and mechanical stress responses efficiently.

Ref: Dechat et al., Genes Dev 2008 – Nuclear lamina provides mechanical support to nucleus function.

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