Cell Nucleus and Cycle
Interphase Nucleus
The interphase nucleus, ranging from 5 to 10 nm, houses the genetic information. This corresponds to the state of the nucleus when the cell is not dividing, a phase occurring in all eukaryotic cells.
Cell Size and Position
Cell size is related to the cytoplasm, which directly influences cell position. This position, often central, can vary depending on cell type and form, constantly changing and adapting to the cell’s configuration. In isodiametric cells, the nucleus is typically spherical.
Nuclear Structure
- Nuclear Envelope: Encloses the DNA and defines nuclear behavior. It’s a double membrane with an inner space called the perinuclear cistern (25-40 nm).
- Inner Nuclear Membrane: Contains a dense material composed of three polypeptides: lamin A, B, and C, arranged in a square grid. Its function is to organize the envelope and participate in its dissolution and reformation during mitosis.
- Pore Complex: Perforates the nuclear envelope (50-100 nm diameter). It has four main units: columnar subunit, ring subunit, luminal subunit, and ring molecular subunit.
- Molecular Transit: Regulates transport between the cytosol and the nucleus. Transport from the cytosol to the nucleus includes histones, DNA and RNA polymerases, gene regulatory proteins, and RNA processing proteins. Transport from the nucleus to the cytosol includes tRNAs and mRNAs, a targeted and complex process. Small molecules pass through passive diffusion, while macromolecules require energy-dependent selective mechanisms.
- Cellular Localization Signals: Large proteins require a label for the central channel.
- Nucleolus: A spherical structure (1-3 μm) visible under light microscopy. Its size depends on the cell’s age and functional state. Its structure includes reconstituted chromatin fibers from different chromosomes (acrocentric with secondary constrictions called nucleolar organizers (NORs) containing rRNA genes), a granular part (PG), and a fibrous part (PF). The nucleolus occupies 25% of the total cell volume.
Cell Cycle
A cell reproduces through an orderly sequence of events, duplicating its contents and dividing into two daughter cells. This process is essential for both unicellular (generating a new individual) and multicellular (long and complex division sequences) organisms.
Interphase
The longest stage (95% of the cycle). Chromosomes are not visible, existing as chromatin. Intense metabolic activity occurs as the cell enlarges and prepares for division.
- G1 Phase: Some cells remain in the G0 phase (quiescent state) indefinitely. G1 duration is highly variable, depending on external conditions and extracellular signals. It’s a period of general growth and cytoplasmic organelle formation. The R point (restriction point) occurs in G1, where the cell decides whether to proceed with the cell cycle.
- S Phase: Synthesis of new genetic material and histone proteins (approximately 9 hours).
- G2 Phase: The premitotic period, from replication’s end to division’s beginning. The cell prepares for division, and centrioles are duplicated (approximately 4 hours).
M Phase
The shortest phase. The nuclear envelope disintegrates, chromatin condenses, and two stages occur: mitosis and cytokinesis.
Cell Cycle Regulation
A biochemical mechanism of interacting regulatory proteins (cyclins and cyclin-dependent kinases (CDKs)) directs and coordinates the cycle’s basic processes. Checkpoints exist at the end of G1, the end of G2, and metaphase. CDK activity fluctuates as the cycle progresses. Four cyclin classes exist: G1 cyclin, G1/S cyclin (activate CDKs at the end of G1, enabling DNA replication), S cyclin (bind CDKs during S phase for DNA replication), and M cyclin (promote mitosis).
MPF (mitosis-promoting factor) activity peaks at mitosis’s end through cyclin B degradation. MPF promotes chromatin condensation, nuclear envelope disintegration, Golgi apparatus and ER fragmentation, and microtubule formation/stabilization.
CDK Activation
- Inactive: The active site is blocked by the T-loop region.
- Cyclin Binding: Dislodges the T-loop, partially activating CDK.
- CAK Phosphorylation: Cdk-activating kinase fully activates CDK by enhancing substrate binding.
CDK Activity Determinants
Cyclin levels are the primary determinant, but others exist: Wee1 kinase (inhibits via phosphorylation) and Cdc25 (phosphatase, increases CDK activity).
Mitosis
The process of nuclear division, creating two daughter nuclei with identical chromosome number and type as the parent nucleus. It involves two key elements: the chromatic device (chromosomes) and the achromatic device (centrioles, asters, and spindle).
- Prophase: Chromosomes become visible, the nucleus disintegrates, and the mitotic spindle forms. Astral microtubules position the poles, kinetochore microtubules bind to chromosome kinetochores, and polar microtubules separate the poles.
- Prometaphase: The mitotic spindle is fully formed, connecting chromosomes to kinetochore microtubules. Active chromosome movement begins, and small vesicles form.
- Metaphase: Chromosomes align at the equatorial plate, attached to spindle fibers by kinetochores.
- Anaphase: Chromosomes separate and move to the poles, synchronized with kinetochore microtubule shortening and polar microtubule elongation.
- Telophase: Chromosome migration completes, chromosomes decondense into chromatin, the nucleolus reforms, and a new nuclear envelope develops. Spindle microtubules disappear.
Cytokinesis
Cytoplasm division, beginning at anaphase’s end.
- Animal Cytokinesis: Occurs via strangulation by a contractile ring (actin and myosin filaments). The ring shrinks, forming a cleavage furrow that divides the cell.
- Plant Cytokinesis: Occurs via septum formation (cell plate or phragmoplast).