The Nucleus, Chromatin, and Interphase Structure

Structure and Overview of the Nucleus

Size and Dimensions: The nucleus averages $5\,\mu\text{m}$ in diameter, making it larger than any of the cytoplasmic organelles.
Shape: Most commonly spherical or oval, but its shape generally conforms to the overall shape of the cell.
Core Regions: The nucleus contains three distinct, recognizable structures:
- Nuclear envelope (membrane)
- Nucleoli
- Chromatin

The nucleus showing the nuclear envelope, nuclear pores, nucleolus, chromatin, and cisterns of rough ER

The Nuclear Envelope and Nucleoplasm

Double Membrane Barrier:
- The nucleus is bounded by the nuclear envelope, a double membrane barrier separated by a fluid-filled space (similar in structure to the mitochondrial membrane).
- Outer Nuclear Membrane: Continuous with the rough endoplasmic reticulum (ER) of the cytoplasm and studded with ribosomes on its external face.
- Inner Nuclear Membrane: Lined by the nuclear lamina.
Nuclear Lamina:
- A netlike network composed of lamins, which are rod-shaped proteins that assemble to form intermediate filaments.
- Functions: Maintains the shape of the nucleus and serves as a structural scaffold to organize DNA within the nucleus.
Nuclear Pores and Pore Complexes:
- The nuclear envelope is punctured at various points by nuclear pores.
- Each pore is lined by an intricate complex of protein particles termed the nuclear pore complex.
- Function: Forms an aqueous transport channel that regulates the entry and exit of molecules (such as mRNAs) and large particles into and out of the nucleus.
Selective Permeability and Transport:
- Like other cellular membranes, the nuclear envelope exhibits selective permeability, but substances pass much more freely here than elsewhere in the cell.
- Small Molecules: Pass unhindered through the relatively large nuclear pore complexes.
- Large Molecules: Protein molecules imported from the cytoplasm and RNA molecules exported from the nucleus move through the central channel of the pores via an energy-dependent process guided by soluble transport proteins.
Nucleoplasm ( $\text{nu'kle-o-plazm}$ ):
- A jellylike fluid enclosed by the nuclear envelope in which all other nuclear elements are suspended.
- Contains dissolved salts, nutrients, and other essential solutes, similar in composition to the cytosol.

The Nucleolus and Ribosomal Assembly

Definition: Nucleoli ( $\text{nu-kle'o-li}$ ; meaning "little nuclei") are dark-staining, spherical bodies located inside the nucleus where ribosomal subunits are assembled.
Membrane Status: Nucleoli are non-membrane-bounded structures.
Abundance and Size:
- Typically, there are one or two nucleoli per nucleus, though more may be present.
- They are largest in growing cells actively synthesizing large quantities of tissue proteins.
Composition and Assembly Process:
- Nucleoli are aggregations of all components necessary to synthesize and assemble ribosomal subunits.
- They center around the regions of DNA that code for ribosomal RNA (rRNA).
- As rRNA molecules are transcribed, they combine with proteins (which are manufactured on cytoplasmic ribosomes and imported into the nucleus) to form two distinct types of ribosomal subunits.
- Completed ribosomal subunits exit the nucleus through nuclear pores into the cytoplasm, where they unite to form functional ribosomes.

Chromatin Structure and Gene Regulation

Microscopic Appearance:
- Under a standard light microscope, chromatin ( $\text{kro'mah-tin}$ ) appears as a fine, unevenly stained network.
- Advanced structural techniques reveal chromatin as a system of bumpy threads weaving through the nucleoplasm.
Chemical Composition:
- $\approx 30\%$ DNA (the cell's genetic material).
- $\approx 60\%$ globular histone proteins ( $\text{his'tŃn}$ ), which function in DNA packaging and regulation.
- $\approx 10\%$ RNA chains (newly synthesized or actively forming).
Fundamental Unit — Nucleosomes ( $\text{nu'kle-o-sŃmz}$ , meaning "nuclear bodies"):
- Consist of flattened, disc-shaped cores or clusters of eight histone proteins.
- A DNA double helix (with a $2\,\text{nm}$ diameter) winds twice (resembling a ribbon of Velcro®) around each nucleosome core (with a $10\,\text{nm}$ diameter).
- Nucleosomes are connected sequentially like "beads on a string" by segments of linker DNA.

Chromatin and chromosome structure illustrating DNA double helix, histones, nucleosomes, and chromosome condensation

Histone Functions and Epigenetic Regulation:
- Physical Packaging: Histones provide a structural mechanism to pack approximately $2\text{ meters}$ worth of DNA per cell into a highly compact, organized arrangement.
- Gene Regulation:
  - Methylation: The attachment of methyl groups to histone proteins in a nondividing cell shuts down adjacent DNA regions.
  - Acetylation: The addition of acetyl groups to histones exposes specific DNA segments (genes), allowing them to direct the synthesis of proteins or various RNA species.
Functional States of Chromatin:
- Extended Chromatin: Active chromatin segments that are uncoiled to allow gene transcription; generally invisible under light microscopy. Active body cells possess large quantities of extended chromatin.
- Condensed Chromatin: Inactive chromatin segments that remain coiled and packaged; dark-staining and easily detectable under microscopy.

Chromosomes and Cell Division

Formation of Chromosomes:
- When a cell prepares to divide, chromatin threads coil and condense enormously to form short, barlike bodies called chromosomes (meaning "colored bodies").
Functional Significance of Condensation:
- Extreme compactness prevents long, delicate chromatin strands from becoming tangled or broken during the physical movements that occur during cell division.

Interphase and the Cell Cycle

Definition and Overview:
- Interphase is the period extending from cell formation to cell division.
- Historically termed the "resting phase" by early cytologists due to the lack of visible structural movements, but this is a misnomer; the cell is actively carrying out metabolic processes and only resting from division.
- More accurate designations include the metabolic phase or growth phase.
General Cell Growth:
- Throughout interphase, the cell carries out life-sustaining routines and grows by synthesizing proteins and building organelles.
- Chromatin reproduction (DNA replication) is restricted exclusively to the $\text{S}$ subphase.
Subphases of Interphase:
- $\text{G}_1$ Subphase (Gap 1):
  - The cell is metabolically active, rapidly synthesizing proteins and growing vigorously.
  - Most variable in duration: can last from several minutes to hours, days, or years.
  - $\text{G}_0$ Phase: A non-dividing state entered by cells that permanently cease division.
  - Direct division-related activities are absent during most of $\text{G}_1$ , but centrioles begin to replicate near the end of this phase.
- $\text{S}$ Phase (Synthetic):
  - DNA is replicated to ensure that both future daughter cells receive identical copies of genetic material.
  - New histone proteins are synthesized and assembled with DNA into chromatin.
  - Proper execution of the $\text{S}$ phase is essential for a correct subsequent mitotic phase.
- $\text{G}_2$ Subphase (Gap 2):
  - A brief final preparation phase before division.
  - Enzymes and other essential proteins required for cell division are synthesized and transported to their appropriate locations.
  - Centriole replication (initiated in $\text{G}_1$ ) is fully completed.
  - $\text{G}_2/\text{M}$ Checkpoint: Located at the end of $\text{G}_2$ ; the cell checks that all DNA has been completely replicated and any damaged DNA is repaired before proceeding to cell division.