Cell Biology: The Cell Nucleus (Le Noyau)

Comprehensive vocabulary flashcards covering the structural characteristics, nuclear envelope, nuclear pore complex, nucleo-cytoplasmic transport, nucleolus, and chromatin organization based on the lecture by Pr Guy Stéphane Padzys.

Cellular Nucleus

A prominent, membrane-bound organelle characteristic of eukaryotic cells that occupies approximately $6\%$ of total cell volume and stores genetic information in the form of DNA.

Nucleo-Cytoplasmic Ratio ($V_n/V_c$)

The volumetric ratio of the nucleus to the entire cell, which approaches $1$ in undifferentiated cells and drops to below $0.15$ in differentiated adult cells.

Polylobed Nucleus

A segmented nuclear morphology characteristic of polymorphonuclear leukocytes (granulocytes), featuring multiple connected lobes.

Multinucleated Cells (Osteoclasts)

Cells containing multiple nuclei, exemplified by bone-resorbing osteoclasts which typically house between $30$ and $50$ nuclei.

Nuclear Basophilia

The property of the nucleus to readily bind basic dyes such as hematein, which stains chromatin blue due to the electronegative phosphate groups of the DNA backbone.

Brachet Test

A histochemical differential staining method utilizing methyl green to stain nuclear DNA/chromatin green and pyronin to stain cytoplasmic RNA red, verified through DNase and RNase digestion.

Outer Nuclear Membrane

The $6\text{ to }8\,\text{nm}$ thick trilaminar lipid bilayer facing the cytoplasm, studded with ribosomes on its outer surface and directly continuous with the rough endoplasmic reticulum.

Inner Nuclear Membrane

The $6\text{ to }8\,\text{nm}$ thick trilaminar lipid bilayer in direct contact with the nucleoplasm and chromatin, lined internally by the meshwork of the nuclear lamina.

Perinuclear Space

The $20\text{ to }50\,\text{nm}$ wide intermembrane lumen separating the inner and outer nuclear membranes, which remains continuous with the lumen of the endoplasmic reticulum.

Nuclear Pore Complex (NPC)

An octagonal macromolecular structure with an outer diameter of $120\,\text{nm}$ and a height of $200\,\text{nm}$, composed of cytoplasmic and nuclear rings each possessing 8 globular subunits made of roughly $50$ distinct nucleoporins.

Nuclear Basket

A basket-like structure projecting into the nucleoplasm formed by 8 filaments, each $100\,\text{nm}$ long, originating from the nuclear ring of the NPC and converging onto a smaller distal ring.

Nuclear Lamina

A fibrous protein meshwork underlying the inner nuclear membrane, assembled from intermediate filament proteins called lamins that maintain nuclear shape, anchor chromatin, and stabilize pore complexes.

Type A Lamins

Intermediate filament proteins (specifically lamin A and lamin C) encoded by the LMNA gene that are expressed in all differentiated cell types.

Type B Lamins

Intermediate filament proteins (specifically lamin B1 and lamin B2) that are uniquely present in undifferentiated cells.

Laminopathies

A diverse class of genetic disorders arising from mutations in the LMNA gene encoding type A lamins, leading to pathologies such as Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy, Dunnigan-type lipodystrophy, and Charcot-Marie-Tooth disease.

Hutchinson-Gilford Progeria Syndrome

A severe premature aging laminopathy caused by a mutation in the LMNA gene on chromosome $1$, producing a truncated lamin A variant known as progerin that accumulates in the nuclear envelope and causes severe nuclear deformities.

Passive Nuclear Diffusion

The bidirectional, non-energy-dependent transport through nuclear pores of small molecules, ions, nucleotides, and small proteins with a molecular weight below $5\,\text{kDa}$.

Nuclear Localization Signal (NLS)

A targeting peptide sequence on nuclear proteins rich in basic, positively charged amino acids (lysine and arginine) that directs their active import through the nuclear pore complex.

Importin

A soluble cytosolic nuclear import receptor belonging to the karyopherin family that binds the NLS of cargo proteins and transiently interacts with nucleoporins to cross the NPC.

FG Repeats

Short hydrophobic peptide repeats consisting of phenylalanine (F) and glycine (G) residues located on nucleoporins that line the central pore channel and provide binding sites for transport receptors.

Ran GTPase

A monomeric molecular switch protein that binds GTP or GDP, regulating the directionality and assembly/disassembly of nuclear import and export complexes across the nuclear envelope.

Ran-GAP (GTPase-Activating Protein)

A cytosolic regulatory protein that triggers Ran to hydrolyze its bound GTP into GDP, transforming active Ran-GTP into Ran-GDP in the cytoplasm.

Ran-GEF (Guanine Nucleotide Exchange Factor)

A chromatin-bound nuclear regulatory protein that catalyzes the exchange of GDP for GTP on Ran, maintaining high levels of Ran-GTP inside the nucleoplasm.

Karyopherins

A conserved family of nuclear transport receptors encoded by a common gene family that includes both nuclear import receptors (importins) and nuclear export receptors (exportins).

Nucleolus

A non-membrane-bound, spherical nuclear subcompartment enriched in RNA and proteins that is visible during interphase, disappears during mitosis, and serves as the site of ribosome biogenesis.

Nucleolus Organizer Region (NOR)

Tandemly repeated clusters of ribosomal DNA (rDNA) located on the short arms of human acrocentric chromosomes 13, 14, 15, 21, and 22, encoding 18S, 5.8S, and 28S rRNAs.

45S Pre-rRNA

The primary large ribosomal RNA precursor transcribed in the nucleolus, which is subsequently cleaved to generate mature 18S, 5.8S, and 28S rRNAs.

5S rRNA

A distinct ribosomal RNA species transcribed outside the nucleolus in the nucleoplasm from chromosome $1$, which is subsequently imported into the nucleolus to be assembled into the large ribosomal subunit.

Nucleoplasm

The viscous colloidal ground substance of the nucleus that embeds the nucleolus, chromatin, and a fibrous nucleoskeleton containing actin, serving as the site of DNA replication and transcription.

Euchromatin

The decondensed, transcriptionally active form of chromatin distributed throughout the nucleoplasm during interphase, representing approximately $10\%$ of total cellular chromatin.

Constitutive Heterochromatin

The permanently condensed, transcriptionally silent fraction representing approximately $90\%$ of total chromatin, localized mainly around centromeres, telomeres, and the nuclear periphery against the lamina.

Barr Body (Sex Chromatin)

A dense mass of heterochromatin located against the inner nuclear envelope representing the randomly inactivated X chromosome in somatic cells of female mammals, visible in $20\text{ to }40\%$ of interphase nuclei.

Barr Body Calculation Rule

The quantitative formula establishing that the number of visible Barr bodies in a diploid somatic cell equals the total number of X chromosomes minus one ($\text{Number of Barr bodies} = \text{Number of X chromosomes} - 1$).