Cell Division

• Why cells divide — growth, repair, reproduction; cell cycle phases G₁, S, G₂, M.
• Mitosis — equational division producing genetically identical daughter cells (somatic).
• Meiosis — reduction division producing haploid gametes with crossing over and independent assortment.
• Cytokinesis differences in plant (cell plate) vs animal (furrow) cells.

Level 1 — Cell cycle and mitosis

Verbal: Interphase (G₁, S, G₂) is when cell grows and DNA replicates (S phase). M phase = mitosis + cytokinesis.

Symbolic: Mitosis: one division 2n→2n; meiosis: two divisions 2n→n; at metaphase 2n=12 → 12 chromosomes, 24 chromatids; crossing over in prophase I.

Mitosis stages (PMAT):

• Prophase: Chromatin condenses, spindle forms, nuclear envelope breaks down
• Metaphase: Chromosomes align at equatorial plate
• Anaphase: Sister chromatids separate to poles
• Telophase: Nuclear envelopes reform, chromosomes decondense

Result: 2n → 2n (two daughter cells same as parent). Essential for growth and asexual reproduction.

Level 2 — Meiosis and significance

Meiosis I (reductional): Homologous chromosomes pair (synapsis), crossing over at pachytene, separate in anaphase I → two haploid (n) cells each with replicated chromosomes.

Meiosis II (equational): Sister chromatids separate → four haploid gametes.

Significance of meiosis: Maintains chromosome number across generations; creates variation for evolution and breeding.

Regulation: Checkpoints (G₁/S, G₂/M, spindle) — cancer linked to uncontrolled division (preview).

NCERT spotlight — Mitosis in plants vs animals and meiosis significance

Plant cytokinesis builds cell plate from Golgi vesicles; animal cells form cleavage furrow from contractile ring of actin. Both follow mitotic chromosome separation.

Meiosis and variation: Crossing over in pachytene exchanges alleles between homologues. Independent assortment of homologous pairs at metaphase I produces 2^n gamete combinations for n bivalents.

Cancer link: Uncontrolled mitosis when cell cycle checkpoints fail — proto-oncogenes and tumour suppressors preview for medical entrance relevance.

A diploid cell has 2n = 12 chromosomes. How many chromosomes and chromatids per cell at (a) metaphase mitosis (b) metaphase meiosis I (c) metaphase meiosis II?

Step 1 — (a) Mitosis metaphase: 12 chromosomes, each with 2 sister chromatids → 24 chromatids total.
Step 2 — (b) Meiosis I metaphase: 6 bivalent pairs (12 chromosomes) at plate; each chromosome 2 chromatids → 24 chromatids.
Step 3 — After meiosis I: 2 cells with n = 6 chromosomes (each replicated).
Step 4 — (c) Meiosis II metaphase: 6 chromosomes aligned, 12 chromatids per cell.
Step 5 — Final four cells: each n = 6, unreplicated chromosomes after telophase II.
Step 6 — DNA content: 2C → 4C in S → back to 2C per mitotic daughter; meiosis ends at 1C per gamete (if started 2C).

Applications — growth, healing, and cancer therapy

Meristematic plant tissues divide by mitosis for growth — root and shoot apical meristems. Chemotherapy targets rapidly dividing cancer cells — side effects on hair follicles and gut lining also divide frequently. Meiosis shuffles alleles — genetic counselling uses understanding of nondisjunction (trisomy 21 Down syndrome) linked to meiosis I or II errors in egg/sperm formation.

Deep dive — cell cycle regulation and errors

Cell cycle phases G1 growth, S DNA replication, G2 prepare mitosis, M mitosis cytokinesis — G0 quiescent exit for differentiated cells like neurons mostly. Checkpoints G1/S DNA damage p53 pathway; G2/M; spindle assembly — failure → uncontrolled division cancer. Mitosis prophase centrioles move poles animal cells; plant lacks centrioles uses spindle from MTOC. Anaphase cohesin cleavage separates sister chromatids — molecular mechanism target cancer drugs paclitaxel stabilises microtubules blocking anaphase. Meiosis prophase I leptotene zygotene synaptonemal complex pachytene crossing over diplotene diakinesis — long phase majority meiosis duration. Nondisjunction failure separate homologues or chromatids → aneuploidy trisomy 21 Down syndrome extra chromosome 21 egg/sperm. Synaptonemal complex aligns homologues for recombination — unique to meiosis not mitosis. Comparison table mitosis one division 2n→2n growth repair; meiosis two divisions 2n→n gametes genetic variation sexual reproduction.

Review and practice drill

Review checklist: (1) Mitosis equational 2n to 2n. (2) Meiosis reduction crossing over. (3) PMAT mitosis phases. (4) Plant cell plate versus animal furrow cytokinesis. Practice: 2n=12 chromosomes at mitosis metaphase — 12 chromosomes 24 chromatids.

• List phases of mitosis in order.
• What is synapsis and when does it occur?
• Why is meiosis called reduction division?

Open the Practice tab for graded questions on Division.