Cell Division: Mitosis & Meiosis

Learning Objectives

• Distinguish mitosis vs meiosis in animal cells.
• Identify where each process occurs:
  • Mitosis ➔ somatic (body) cells (skin, immune, muscle, etc.).
  • Meiosis ➔ gametes (ova & sperm).
• Explain how genetic diversity (inheritance) arises through meiosis.
• Connect material to Section 3.7, Chapter 3 of the Tortora Anatomy & Physiology (3rd ed.).

General Overview of Cell Division

• Most cells divide to replace dead/injured cells or allow tissue growth (e.g., bulking up at the gym).
• Exception: neurons are usually post-mitotic.
• Two overarching mechanisms:
  1. Mitosis – one diploid parent ➔ two identical diploid daughters.
  2. Meiosis – one diploid parent ➔ four non-identical haploid gametes.
• Key terminology
  • Diploid (2n) = two copies of every chromosome.
  • Haploid (n) = one copy of every chromosome.

Chromosomes & Ploidy Refresher

• Chromosomes = DNA + histone proteins, packaged into chromatin → nucleosomes → condensed chromosomes.
• Human somatic cell inventory
  • 22 pairs of autosomes + 2 sex chromosomes = 46 total.
  • Displayed in a karyotype: ordered largest ➔ smallest with characteristic banding.
  • Typical female: XX; typical male: XY (Y physically smaller).
• Chromatin states
  • Interphase ➔ loose, string-like (“cat-tangled wool”).
  • Prophase onward ➔ tight X-shape for segregation.

The Cell Cycle (mitotic context)

• Non-dividing cells sit in G$_0$.
• Dividing cells run: Interphase + Mitotic (M) phase.
  1. G$_1$ – growth, organelle duplication.
  2. S phase – DNA replication (≈ 8 h).
  3. G$_2$ – further growth, protein synthesis, mitotic prep.
  4. M phase – Prophase → Metaphase → Anaphase → Telophase (+ cytokinesis).

Phases of Mitosis (PMAT mnemonic)

• Prophase (early)
  • Chromatin condenses to chromosomes.
  • Mitotic spindle forms; nuclear envelope starts disassembly.
• Prophase (late)
  • Nuclear envelope gone; spindle microtubules attach to kinetochores.
• Metaphase
  • Chromosomes align on metaphase plate (equator).
• Anaphase
  • Sister chromatids separate; migrate to opposite poles.
• Telophase / Cytokinesis
  • Chromosomes decondense.
  • Nuclear envelopes & nucleoli re-form.
  • Contractile ring cleaves cytoplasm ➔ two daughter cells.

Net Mitosis Outcome

• 1 diploid parent (2n=46) ➔ 2 diploid, genetically identical daughters.
• Roles: growth, routine replacement, tissue repair.

Introduction to Meiosis

• Etymology: Greek "meiosis" = reduction.
• Occurs in ovaries & testes/seminal vesicles.
• Purpose: produce gametes with half the chromosome count so that fertilization restores diploidy.
• Key fact: Two sequential divisions (Meiosis I & II).
  • End product: 4 haploid (n=23) cells.
  • Generates genetic diversity via crossing-over and independent assortment.

Detailed Steps of Meiosis

Pre-Game: Interphase

• DNA replicated → each chromosome = two sister chromatids.

Meiosis I (Reductional Division)

1. Prophase I
   • Homologous chromosomes pair (synapsis).
   • Crossing-over: exchange of DNA segments between nonsister chromatids ➔ recombinant chromosomes.
2. Metaphase I
   • Homologous pairs align along metaphase plate.
3. Anaphase I
   • Homologs separate to opposite poles; sister chromatids stay attached.
4. Telophase I + Cytokinesis
   • Two haploid daughter nuclei form (chromosomes still duplicated).

Meiosis II (Equational Division – mitosis-like)

1. Prophase II – spindles reform in each haploid cell.
2. Metaphase II – chromosomes align singly.
3. Anaphase II – sister chromatids finally split.
4. Telophase II + Cytokinesis – results = 4 genetically distinct haploids.

Sources of Genetic Diversity

• Crossing-over (Prophase I) ➔ novel allele combinations.
• Random orientation/assortment of homologs (Metaphase I) ➔ 2^{23} possible gamete chromosome combos per parent.
• Random fertilization multiplies diversity (sperm × egg).

Why Siblings Differ

• Each gamete carries a unique mosaic of parental maternal & paternal chromosome fragments.
• Combination with partner’s gamete further randomizes resulting zygote genome.

Comparative Summary – Mitosis vs Meiosis

(Table values wrapped in as required for numeric data.)

Life-Cycle Context Diagram (verbal)

• Adult diploid humans ( 2n ) continually undergo meiosis ➔ haploid gametes (n).
• Fertilization merges sperm & egg ➔ diploid zygote (2n).
• Zygote grows via mitosis, forming fetus → child → adult, repeating cycle.

Practical / Ethical / Evolutionary Implications

• Cancer arises when mitotic regulation fails (mutation accumulation).
• Genetic diversity produced by meiosis drives evolution & adaptation.
• Understanding ploidy crucial for:
  • Assisted reproductive technologies (IVF, genetic screening).
  • Diagnosing chromosomal disorders (e.g., trisomy 21).

Further Reading & Support

• Tortora, Derrickson & Burk, Principles of Anatomy & Physiology, 3rd ed., Ch 3, §3.7 (available via Massey e-library).
• Review lecture slides/recording to reinforce phase-by-phase imagery (wool analogy, karyotype photos).

Exam-Oriented Takeaways

• Remember PMAT (Pro-, Meta-, Ana-, Telo-) for mitosis and that meiosis simply repeats the sequence twice (with crossing-over & homolog separation first round).
• Be able to state concisely:
  • “Mitosis: 1 diploid → 2 identical diploids.”
  • “Meiosis: 1 diploid → 4 non-identical haploids; diversity via crossing-over & independent assortment.”
• Know chromosome counts: somatic 46 vs gamete 23$$.
• Understand WHY halving is essential (restoration to diploid after fertilization).