Cell Cycle & Cell Division – Comprehensive Study Notes

Origin & Importance of Cell Division

• Every multicellular organism begins as a single cell (zygote); through successive divisions, one cell becomes millions.
• Division must be orchestrated with DNA replication and cell growth to ensure intact genomes in progeny.
• Growth, tissue repair, reproduction, and the maintenance of chromosome number across generations all hinge on regulated cell cycles.

Definition & Scope of the Cell Cycle

• Cell cycle = ordered sequence that duplicates DNA, enlarges cytoplasm, and partitions everything into two daughter cells.
• Two overarching phases:
  • Interphase (prep phase between M phases)
  • M phase / Mitotic phase (actual division)
• Duration examples
  • Typical cultured human cell: \approx 24\text{ h} total; M phase only \approx 1\text{ h}; Interphase > 95\% of cycle.
  • Budding yeast: entire cycle \approx 90\text{ min}.

Interphase – Detailed Sub-phases

• G1 (Gap 1)
  • From end of mitosis to start of DNA replication.
  • Intense metabolic activity, continuous growth, normal function.
  • Organelle duplication largely occurs here.
  • DNA amount: constant (denote as 2C), chromosome number 2n.
• S (Synthesis)
  • Complete DNA replication → DNA content doubles: 2C \rightarrow 4C.
  • Chromosome number stays 2n; each chromosome gains a sister chromatid.
  • Centriole duplication in animal cells.
• G2 (Gap 2)
  • Protein synthesis for mitosis; further cytoplasmic growth.
  • DNA content remains 4C until mitosis starts.
• G0 (Quiescent stage)
  • Cells exit G1, remain metabolically active but non-dividing (e.g., heart muscle, many neurons).
  • Can re-enter the cycle on demand (wound healing, etc.).

Control, Variations & Special Cases

• Animals: mitosis normally occurs in diploid somatic cells; exception – haploid males of honey-bee (drones) divide mitotically.
• Plants: mitosis occurs in both diploid and haploid stages (e.g., gametophyte mitoses during alternation of generations).
• Continuous division tissues
  • Plants: apical & lateral meristems.
  • Animals: no direct meristem equivalent; however stem-cell niches (e.g., bone-marrow hematopoietic stem cells) perform analogous roles.

M Phase – Karyokinesis + Cytokinesis

• Also called equational division (chromosome number maintained).
• Four seamlessly progressive stages:

Prophase

• Follows S & G2; chromatin condensation begins → discrete chromosomes.
• Each chromosome: two sister chromatids joined at a centromere.
• Centrosomes (duplicated in S) migrate to opposite poles; each forms an aster; asters + spindle fibres = mitotic apparatus.
• Disappearance of nucleolus, ER, Golgi, and nuclear envelope marks late prophase.
• Example calculation (onion root tip, 16 chromosomes):
  • G1: 16 chromosomes, 2C DNA.
  • After S: 16 chromosomes, 4C DNA.
  • Post-M phase daughter cells: 16 chromosomes, 2C DNA each.

Metaphase

• Nuclear envelope fully disintegrated; chromosomes attain maximal condensation.
• Kinetochores (protein discs on centromeres) attach to spindle microtubules.
• Chromosomes align on the metaphase plate; one chromatid connected to each pole.

Anaphase

• Centromeres split simultaneously; sister chromatids (now daughter chromosomes) pulled toward opposite poles.
• Centromere leads, arms trail.

Telophase

• Chromosomes reach poles → decondense; individuality lost.
• Re-formation of nuclear envelope, nucleolus, ER, Golgi.

Cytokinesis

• Animal cells: cleavage furrow constricts from periphery inward.
• Plant cells: cell plate forms centrally → new middle lamella & walls.
• Failure of cytokinesis after karyokinesis → multinucleate syncytium (e.g., coconut liquid endosperm).

Significance of Mitosis

• Produces genetically identical diploid daughters → growth of multicellular body.
• Restores optimum nucleo-cytoplasmic ratio disturbed by cell enlargement.
• Enables continuous replacement & repair (epidermis, gut lining, blood, meristems).

Meiosis – Overview

• Specialized division that halves chromosome number to form haploid gametes/spores; fertilization later restores diploid state.
• Key hallmarks:
  1. Two sequential divisions (meiosis I & II) but one S phase.
  2. Homologous pairing (synapsis) and crossing-over between non-sister chromatids.
  3. Result: four genetically distinct haploid cells.

Meiosis I – Reductional Division

Prophase I (longest, 5 sub-stages)

1. Leptotene – chromosomes start becoming visible; gradual condensation.
2. Zygotene – homologues pair (synapsis); formation of synaptonemal complex → unit called a bivalent/tetrad.
3. Pachytene – clearly visible tetrads; appearance of recombination nodules; crossing-over catalyzed by recombinase enzyme → genetic exchange.
4. Diplotene – synaptonemal complex dissolves; homologues begin to separate but remain connected at chiasmata (X-shaped crossover sites). In many vertebrate oocytes diplotene arrest can last months/years.
5. Diakinesis – terminalization of chiasmata; maximum condensation; spindle assembly; nuclear envelope & nucleolus disappear → metaphase I transition.

Metaphase I

• Bivalents orient on equatorial plate; kinetochore microtubules attach to homologues from opposite poles.

Anaphase I

• Homologous chromosomes separate (disjunction); sister chromatids remain joined.

Telophase I & Cytokinesis

• Reappearance of nuclear envelope & nucleolus; produces dyad (two haploid cells with duplicated chromosomes).
• Brief interkinesis (no DNA replication) precedes meiosis II.

Meiosis II – Equational Division (mitosis-like)

Prophase II

• Nuclear envelope breaks again; chromosomes (still of two chromatids) re-condense.

Metaphase II

• Chromosomes align at equator; kinetochores of sister chromatids attach to opposite poles.

Anaphase II

• Centromeres split; sister chromatids segregate.

Telophase II & Cytokinesis

• Chromosomes decondense; nuclei reform; cytokinesis yields a tetrad of haploid cells.

Significance of Meiosis

• Constancy of chromosome number across generations despite sexual reproduction.
• Genetic variability via independent assortment & crossing-over → raw material for evolution.

Comparative Snapshot – Mitosis vs. Meiosis (Key Contrasts)

• Number of divisions: 1 vs 2.
• DNA replication: once, prior to division(s), in both.
• Pairing of homologues: absent in mitosis, present in prophase I.
• Crossing-over: only in meiosis.
• Chromosome number in products: conserved (diploid) vs halved (haploid).
• Genetic identity: identical daughters vs unique recombinants.

Practical & Conceptual Questions Raised in Text

• Continuous growth in plants: Which tissues? (Answer: meristems).
• Mitosis in haploid organisms (male honey-bee, plant gametophytes).
• Onion root tip exercise regarding n and C values.
• Examples of equal vs unequal meiotic products (e.g., microspore tetrad vs oogenesis polar bodies).
• Can DNA replicate without cell division? (Yes – endoreduplication, polytene chromosomes).
• Can mitosis occur without prior DNA replication? (No, would compromise genomic integrity).

Numerical & Symbolic References (LaTeX Notation)

• DNA doubling: 2C \rightarrow 4C during S.
• Human cell-cycle span: \approx 24\text{ h}; M phase \approx 1\text{ h}.
• Yeast cycle: \approx 90\text{ min}.
• Onion root example: 16 chromosomes per somatic nucleus.

Real-World & Evolutionary Connections

• Cancer originates from loss of cell-cycle control.
• Plant breeding exploits meiotic recombination for new trait assortments.
• Endosperm syncytium (coconut water) arises via nuclear divisions without cytokinesis.

Ethical & Philosophical Notes

• Understanding cell division informs stem-cell therapy, cloning, fertility treatments.
• Balancing medical intervention (e.g., anticancer mitotic inhibitors) with potential side-effects on healthy dividing cells is an ongoing ethical challenge.