Cell Cycle & M Phase Notes

📊 Chromosome & DNA Content in Onion Cells (16 chromosomes/cell)

  • G₁ phase: 16 chromosomes, DNA = 2C
  • After S phase: 16 chromosomes (but 32 chromatids!), DNA = 4C
  • After M phase: 16 chromosomes per daughter cell, DNA = 2C

😴 Quiescent Stage (G₀)

Some adult cells (like heart cells) don’t divide or divide rarely. They exit G₁ to enter G₀ – a sleepy but metabolically active stage! They only wake up when needed for repair.

🌿 Where Mitosis Happens

  • 🐝 Animals: Mostly in diploid somatic cells (except male honey bees – haploid mitosis!)
  • 🌻 Plants: Happens in BOTH haploid & diploid cells (e.g., haploid mitosis in gametophyte stage during alternation of generations)

💥 M Phase (Mitosis)

The most dramatic phase! Called equational division because parent & daughter cells have same chromosome number. It has 4 stages:

1️⃣ Prophase

  • Chromosomes condense & become visible 👀
  • Each chromosome has 2 sister chromatids glued at centromere
  • Centrosomes move to poles & form asters + spindle fibers
  • By end: Nucleus, ER, Golgi & nucleolus disappear! ✨

2️⃣ Metaphase

  • Nuclear envelope fully breaks down
  • Chromosomes align at equator (metaphase plate)
  • Kinetochores (on centromeres) attach to spindle fibers from BOTH poles
  • Best stage to study chromosome shape! 🔬

3️⃣ Anaphase

  • Centromeres split → sister chromatids separate → now called daughter chromosomes
  • Chromosomes dragged to poles (centromere leads, arms trail)

4️⃣ Telophase

  • Chromosomes reach poles & de-condense
  • Nuclear envelopes reform around both clusters 🎉
  • Nucleolus, Golgi & ER reappear!

✂️ Cytokinesis (Cytoplasm Splits!)

  • 🐾 Animal cells: Plasma membrane pinches in (forms furrow)
  • 🌱 Plant cells: Cell plate forms in center → becomes new cell wall
  • Mitochondria/plastids split between daughters
  • ⚠️ Fun fact: If cytokinesis skips → multinucleate cells form (e.g., coconut endosperm)! 🥥

🌟 NEET Must-Knows

  1. Chromosome numbers at G₁/S/G₂/M & DNA content (2C vs 4C)
  2. G₀ stage – why cells enter it & examples (heart cells)
  3. Mitosis in haploid cells – honey bees (animals) & plant gametophytes
  4. Key events in each mitotic stage (e.g., kinetochore in metaphase)
  5. Cytokinesis differences – furrow (animals) vs cell plate (plants)