Cell Reproduction, the Cell Cycle, and Mitotic Regulation

Genome Organization and Chromosome Compaction

• Prokaryotic Genome Structure: Composed of a single, circular double-stranded DNA molecule localized within the nucleoid region; prokaryotic DNA lacks histones and nucleosomes. Non-essential circular plasmids exist independently and can be horizontally transferred, often conferring adaptive traits such as antibiotic resistance.

• Eukaryotic Genome Structure: Consists of several linear double-stranded DNA molecules arranged as chromosomes. Somatic cells are diploid ($2n = 46$ chromosomes in humans), holding two homologous sets (one maternal, one paternal). Gametes (sperm and eggs) are haploid ($1n = 23$ chromosomes).

• Homologous Chromosomes and Alleles: Homologous chromosomes are identical in length and locus organization for specific genes coding for specific proteins; the sequence variation between homologous pairs is $< 1\%$. Alternative gene variants are alleles (e.g., blood type sequences $A$, $B$, and $O$). The $X$ and $Y$ sex chromosomes are the sole exception to homologous uniformity, sharing only minimal homology required for gamete production.

• Compaction Architecture: A human nucleus of $≈ 10\,\mu\text{m}$ houses $≈ 2\,\text{m}$ of linear DNA ($2\,\text{nm}$ diameter):

  • First Level (Nucleosomes): DNA winds around an octamer core of $8$ histone proteins ($146$ to $147\,\text{base pairs}$) to form nucleosomes ($10\,\text{nm}$ beads) connected by linker DNA, shortening the molecule sevenfold.

  • Second Level ($30\,\text{nm}$ Fiber): Nucleosomes and linker DNA condense with the linker histone $H1$ into a coiled $30\,\text{nm}$ chromatin fiber, achieving a fiftyfold compaction.

  • Third Level (Scaffold Packing): Fibrous proteins form radial loop domains ($300\,\text{nm}$) and heterochromatin ($700\,\text{nm}$), culminating in fully condensed metaphase chromosomes ($1400\,\text{nm}$) linked by cohesin proteins at the constricted centromere ($\approx 1\,\mu\text{m}$).

The Eukaryotic Cell Cycle and Mitosis

• Interphase Stages:

  • $G_1$ Phase (First Gap): Biochemical accumulation of energy reserves, nucleotide building blocks, and associated proteins necessary for DNA replication.

  • $S$ Phase (Synthesis): DNA replicates to produce identical pairs of sister chromatids bound at their centromeres; centrosomes duplicate (containing centriole pairs in animal cells; absent in plants and most fungi).

  • $G_2$ Phase (Second Gap): Final energy replenishment, synthesis of proteins required for chromosome manipulation, organelle duplication, and dismantling of the cytoskeleton.

• Karyokinesis (Mitosis Phases):

  • Prophase: Chromosomes condense via condensin proteins; nucleolus disperses; membranous organelles (Golgi apparatus, endoplasmic reticulum) fragment toward the cell periphery; mitotic spindle polymerizes as centrosomes migrate to opposing poles.

  • Prometaphase: Nuclear envelope disintegrates into vesicles; sister chromatids form protein kinetochores at their centromeric regions to bind kinetochore microtubules; non-kinetochore polar microtubules overlap to assist elongation; astral microtubules anchor spindle orientation.

  • Metaphase: Chromosomes reach maximum condensation and align precisely along the equatorial metaphase plate; cohesin maintains sister chromatid cohesion.

  • Anaphase: Cohesin proteins degrade, enabling separation of sister chromatids into individual daughter chromosomes pulled toward centrosome poles; overlapping polar microtubules slide apart, visibly elongating the cell.

  • Telophase: Chromosomes decondense into relaxed chromatin; mitotic spindle depolymerizes into tubulin monomers; nuclear envelopes reassemble around separated genomes; nucleoli reappear.

• Cytokinesis Mechanisms:

  • Animal Cells: Late anaphase triggers the assembly of an actin-myosin contractile ring along the metaphase plate, constricting the membrane inward to form a deepening cleavage furrow that cleaves the cell.

  • Plant Cells: Microtubules transport Golgi-derived vesicles containing glucose, enzymes, and structural proteins to form a phragmoplast; vesicles coalesce from the center outward to create a cell plate that fuses with existing cell walls.

• $G_0$ Phase (Quiescence): Inactive state outside the mitotic division cycle entered temporarily due to resource/growth factor scarcity, or permanently by non-dividing mature cells (e.g., cortical neurons, cardiac muscle cells).

Cell Cycle Timing and External Regulation

• Duration Variations: Cell-cycle length spans from $≈ 8\,\text{minutes}$ in fruit fly embryos (successive rounds of nuclear mitosis without cytokinesis), to $24\,\text{hours}$ in standard mammalian culture, to an entire organismal lifespan in quiescent tissues.

• Temporal Breakdown ($24\,\text{hour}$ Mammalian Cycle):

  • $G_1$ phase: $≈ 9\,\text{hours}$

  • $S$ phase: $≈ 10\,\text{hours}$

  • $G_2$ phase: $≈ 4.5\,\text{hours}$

  • Mitotic ($M$) phase: $≈ 0.5\,\text{hours}$

• External Stimulatory and Inhibitory Signals:

  • Cell Crowding: Physical contact and density-dependent signals arrest division.

  • Hormonal Control: Growth factors such as human growth hormone ($\text{HGH}$ / $\text{hGH}$) stimulate cycle entry; deficiency results in dwarfism, whereas hypersecretion induces gigantism.

  • Surface-to-Volume Ratio: Cell enlargement leads to a decreasing surface-area-to-volume ratio, causing physiological inefficiency that acts as an intrinsic trigger for division.

Internal Checkpoints and Molecular Regulators

• Surveillance Checkpoints:

  • $G_1$ Checkpoint (Restriction Point): Assesses DNA integrity, cell size, and nutritional reserves; irreversible commitment step determining whether the cell proceeds into $S$ phase, repairs damage, or halts into $G_0$.

  • $G_2$ Checkpoint: Blocks mitotic entry until complete chromosome replication is confirmed and damaged DNA is fully repaired.

  • $M$ Checkpoint (Spindle Checkpoint): Occurs at late metaphase; prevents irreversible anaphase onset until all kinetochores are securely anchored to spindle fibers from opposite cell poles.

• Positive Regulation (Cyclins and Cdks):

  • Concentrations of four cyclins ($\text{Cyclin D}$, $\text{E}$, $\text{A}$, $\text{B}$) fluctuate predictably during specific phases, whereas cyclin-dependent kinase ($\text{Cdk}$) protein levels remain stable.

  • Full $Cdk$ activation requires binding to a specific cyclin and subsequent phosphorylation by an activating kinase; active $Cdk$/cyclin complexes phosphorylate downstream regulatory proteins to drive phase transitions.

  • Cyclins are degraded by cytoplasmic enzymes immediately after checkpoint clearance.

• Negative Regulation (Tumor Suppressors):

  • $p53$ ($53\,\text{kDa}$): Master sensor activated by DNA damage or hypoxia in $G_1$; halts the cell cycle, recruits DNA repair enzymes, and triggers programmed cell death (apoptosis) if damage is unrepairable.

  • $p21$ ($21\,\text{kDa}$): Induced transcriptionally by $p53$; directly binds and inhibits $Cdk$/cyclin complexes to enforce cell-cycle arrest.

  • Retinoblastoma Protein ($\text{Rb}$): Monitors cell size; in its dephosphorylated, active state, $\text{Rb}$ binds and sequesters the transcription factor $\text{E2F}$, blocking transcription of genes required for the $G_1/S$ transition. Inactivating phosphorylation of $\text{Rb}$ releases $\text{E2F}$ to advance the cycle.

Cancer and Cell-Cycle Control Failure

• Pathogenesis of Cancer: Uncontrolled proliferation resulting from accumulated mutations in cell-cycle regulatory genes, leading to accelerated division rates that outpace normal homeostatic apoptosis.

• Proto-oncogenes to Oncogenes: Proto-oncogenes encode normal positive regulators (e.g., $\text{Cdks}$, growth signals). Gain-of-function mutations produce oncogenes, causing hyperactive or cyclin-independent $\text{Cdk}$ activity that overrides checkpoints.

• Tumor Suppressor Genes: Segments of DNA encoding negative regulators ($\text{Rb}$, $p53$, $p21$) that arrest the cycle in the presence of genomic damage. Inactivating loss-of-function mutations disable these critical brakes on proliferation.

• $p53$ Pathology: Mutated $p53$ occurs in $> 50\%$ of all human tumors. A dysfunctional $p53$ cannot arrest damaged cells at $G_1$, recruit repair machinery, activate $p21$, or trigger apoptosis, resulting in rapid accumulation of oncogenic mutations across subsequent daughter generations.

Prokaryotic Cell Division and Mitotic Evolution

• Binary Fission Mechanism:

  • Prokaryotes divide without karyokinesis due to the absence of a membrane-bound nucleus and multiple chromosomes.

  • Bidirectional DNA replication begins at the origin of replication near the cell membrane attachment site and proceeds along both strands simultaneously.

  • Origins move toward opposite cell poles as the cell membrane and wall elongate.

  • Cytokinesis is orchestrated by $\text{FtsZ}$ (filamenting temperature-sensitive mutant Z), which polymerizes into a ring at the cell midpoint, recruiting repair and synthesis proteins to form an inward-growing septum until two identical daughter cells are pinched apart.

• Evolutionary Continuity of Cytoskeletal Division Elements:

  • Bacterial $\text{FtsZ}$ and eukaryotic tubulin are homologous proteins sharing common ancestry; both utilize GTP hydrolysis for dynamic assembly and disassembly.

  • Evolutionary Spectrum of Division Apparatus:

  • Prokaryotes: Single circular chromosome in nucleoid; binary fission driven by an $\text{FtsZ}$ ring.

  • Certain Protists: Linear chromosomes in an intact nuclear envelope; mitotic spindle traverses the intact envelope to elongate the cell; microfilaments form an animal-like cleavage furrow; centrioles absent.

  • Other Protists: Histone-bound linear chromosomes; mitotic spindle organized by centrioles penetrates an intact nuclear envelope; microfilament-mediated cleavage furrow.

  • Animal Cells: Histone-bound linear chromosomes; nuclear envelope completely dissolves; centrosome-anchored mitotic spindle orchestrates karyokinesis; microfilaments form a cleavage furrow.

Questions & Discussion

• Question: Which of the following is the correct order of events in mitosis?

  • a. Sister chromatids line up at the metaphase plate. The kinetochore becomes attached to the mitotic spindle. The nucleus reforms and the cell divides. Cohesin proteins break down and the sister chromatids separate.

  • b. The kinetochore becomes attached to the mitotic spindle. Cohesin proteins break down and the sister chromatids separate. Sister chromatids line up at the metaphase plate. The nucleus reforms and the cell divides.

  • c. The kinetochore becomes attached to the cohesin proteins. Sister chromatids line up at the metaphase plate. The kinetochore breaks down and the sister chromatids separate. The nucleus reforms and the cell divides.

  • d. The kinetochore becomes attached to the mitotic spindle. Sister chromatids line up at the metaphase plate. Cohesin proteins break down and the sister chromatids separate. The nucleus reforms and the cell divides.

  • Response: The correct sequence is d: The kinetochore becomes attached to the mitotic spindle, sister chromatids line up at the metaphase plate, cohesin proteins break down and the sister chromatids separate, and finally the nucleus reforms and the cell divides.

• Question: Rb and other proteins that negatively regulate the cell cycle are sometimes called tumor suppressors. Why is the name tumor suppressor appropriate for these proteins?

  • Response: In their active state, these proteins prevent cells from proceeding through division when conditions are unfavorable, cell size is inadequate, or DNA damage is present. By halting the cell cycle and preventing uncontrolled cellular proliferation, they prevent the formation of tumors.

• Question: Human papillomavirus can cause cervical cancer. The virus encodes E6, a protein that binds p53. Based on this fact and what is known about p53, what effect does E6 binding have on p53 activity?

  • a. E6 activates p53

  • b. E6 inactivates p53

  • c. E6 mutates p53

  • d. E6 binding marks p53 for degradation

  • Response: The viral protein $E6$ acts by binding to and inactivating $p53$ (or targeting it for ubiquitin-mediated degradation), thereby disabling the $G_1$ checkpoint, blocking apoptosis of damaged cells, and facilitating tumorigenesis.