Exploring the Cell Nucleus: Structure, Functions, and Components
The Cell Nucleus
Core Features
- Present in all eukaryotic cells except red blood cells.
- Components: nuclear envelope, chromatin (DNA), and the nucleolus.
- Form: Highly variable (spherical, oval) depending on cell type and cell cycle stage.
- Size: Can vary, but remains constant within a single cell type.
- Position: Characteristic of each cell type. Central in embryonic cells, lateralized in fat cells, basal in secreting cells.
- Number: Usually one nucleus per cell, but exceptions exist (e.g., enucleated cells like paramecia with a macronucleus and micronucleus). Plurinucleated cells can arise from:
- Successive nuclear divisions without cell division, forming a plasmodium.
- Merger of uninucleate cells, forming a syncytium.
The Nuclear Envelope
A complex structure at the boundary between the nucleus and cytoplasm in eukaryotic cells.
- Outer Nuclear Membrane: Contains ribosomes on its cytoplasmic surface and is often connected to the endoplasmic reticulum (smooth or rough).
- Perinuclear or Intermembrane Space: The space between the inner and outer nuclear membranes.
- Inner Nuclear Membrane: Contains a fibrillar layer called the nuclear lamina. The nuclear lamina anchors chromatin and regulates nuclear envelope growth.
Nuclear Pores
Circular openings where the inner and outer nuclear membranes fuse. These pores have a consistent diameter for each cell type and act as aqueous channels regulating molecular exchange between the nucleus and cytosol.
- Allow free passage of water-soluble molecules.
- Regulate active transport of macromolecules like RNA and proteins.
- The number of pores varies and is generally higher in cells with greater transcriptional activity.
Pore Complex
A cylindrical structure composed of eight protein particles surrounding a central channel. A dense material called the diaphragm is associated with these particles. A central granule within the pore corresponds to ribosomes. Protein fibrils extend from both sides of the pore.
Chromatin
The complex of DNA and proteins that forms a compact structure within the eukaryotic cell nucleus.
Features
- Composed of DNA and proteins.
- Histone Proteins: Basic proteins rich in amino acids. Five types: H1, H2A, H2B, H3, and H4, all with low molecular weight.
- Non-histone Proteins: A diverse group, including enzymes involved in DNA replication, transcription, and regulation.
Chromatin Types
- Euchromatin: The most abundant and least compacted form of chromatin.
- Heterochromatin: More condensed chromatin. Two types:
- Constitutive: Always condensed and never transcribed.
- Facultative: Condensed in some cells but not others, leading to cell-specific gene expression.
Ultrastructure
Electron microscopy reveals a fibrillar structure composed of chromatin fibers coiled around each other. Upon decondensation, each fiber resembles a “string of beads” called nucleosomes.
- Each nucleosome consists of a core particle (histone octamer) and a DNA strand wrapped around it.
- Histone H1 is not part of the nucleosome core.
- The chromatin fiber folds into a solenoid structure with varying degrees of coiling.
- First-degree coiling results in a 30 nm fiber.
- Second-degree coiling leads to a 300 nm fiber and further compaction into a “supercoil.”
Nucleoplasm and Nucleolus
Nucleoplasm
Also known as karyoplasm or nuclear matrix. The semi-fluid matrix within the nucleus, containing various biomolecules like nucleotides, proteins (involved in DNA and RNA metabolism), enzymes for transcription and replication, gene regulatory proteins, and histones.
Nucleolus
A prominent structure within the nucleus, usually one per nucleus but sometimes two or more. It is a rounded organelle located near the nuclear envelope. Its size correlates with cellular activity, particularly protein synthesis.
- Some nucleoli have an internal filamentous component called the nucleolonema.
- Visible under light microscopy during interphase, disappears during mitosis when chromosomes condense, and reappears during telophase.
Functions of the Nucleolus
- Synthesis and processing of rRNA.
- Packaging of ribosomal subunits for export to the cytosol.
Ultrastructure of the Nucleolus
- Granular Zone: Contains ribosomal subunits.
- Dense Fibrillar Zone: Site of rRNA transcription.
- Fibrillar Zone: Contains rRNA genes.