🧬 Cell Cycle & Division

• 1. The Cell Cycle Basics
  🖼️ Add Image: Circular Cell Cycle Diagram (G1, S, G2, M)
  [span_4](start_span)The biological sequence where a cell duplicates its genetic material, synthesizes necessary components, and ultimately divides into two daughter cells[span_4](end_span). [span_5](start_span)While human cells take roughly 24 hours to complete this cycle, yeast cells can finish it in just 90 minutes[span_5](end_span).
  • Interphase (The Prep Phase)
    [span_6](start_span)Often mislabeled as a "resting phase," this highly active period accounts for 95% of the cell cycle's duration[span_6](end_span).

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    • G1 Phase (Gap 1): The cell grows continuously and remains metabolically active, right before DNA replication begins[span_7](end_span).
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    • S Phase (Synthesis): The critical stage where DNA replicates, resulting in double the amount of genetic material per cell[span_8](end_span).
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    • G2 Phase (Gap 2): The cell synthesizes proteins to finalize preparations for actual division[span_9](end_span).
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    • G0 Phase (Quiescent Stage): An inactive exit state for cells (like adult animal cells) that no longer need to divide regularly[span_10](end_span).
• 2. Mitosis (Equational Division)
  🖼️ Add Image: Mitosis Flowchart (Prophase -> Metaphase -> Anaphase -> Telophase)
  [span_11](start_span)A type of division that maintains the exact same number of chromosomes in the parent and progeny cells[span_11](end_span). [span_12](start_span)In animals, it occurs in diploid somatic cells, whereas plants use it for both haploid and diploid cells[span_12](end_span).
  • Karyokinesis Stages
    🖼️ Add Image: Dividing Cell Diagrams (Metaphase spindle fibers, Early/Late Prophase)
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    • Prophase: Chromosomal material condenses into distinct structures, centrioles migrate to opposite poles, and organelles like the Golgi, ER, and nuclear membrane vanish[span_13](end_span).
    • Metaphase: The nuclear envelope is completely gone. [span_14](start_span)[span_15](start_span)Chromosomes align perfectly at the cell's center (metaphase plate), making this the best stage to study their morphology[span_14](end_span)[span_15](end_span).
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    • Anaphase: Centromeres split, and sister chromatids are pulled apart toward opposite poles of the cell[span_16](end_span).
    • Telophase: Chromosomes arrive at the poles and decondense (lose their discrete identity). [span_17](start_span)The nuclear membrane, nucleolus, and other organelles reassemble around the genetic material[span_17](end_span).
  • Cytokinesis
    [span_18](start_span)The final split of the cytoplasm[span_18](end_span). [span_19](start_span)Animal cells achieve this through a deepening plasma membrane furrow, while plant cells build a new cell wall from the center outward, starting with a cell plate[span_19](end_span).
• 3. Meiosis (Reductional Division)
  [span_20](start_span)[span_21](start_span)A specialized division that reduces the chromosome count by half, producing four haploid daughter cells[span_20](end_span)[span_21](end_span). [span_22](start_span)It features a single round of DNA replication but two division cycles[span_22](end_span).
  • Meiosis I (The Reduction Phase)
    🖼️ Add Image: Prophase I Hierarchy (Leptotene -> Zygotene -> Pachytene -> Diplotene -> Diakinesis)
    • Prophase I: A highly complex stage divided into five sub-phases: [span_23](start_span)
      - Leptotene: Chromosomes compact and become visible[span_23](end_span). [span_24](start_span)
      - Zygotene: Homologous chromosomes pair up (synapsis) to form bivalents/tetrads[span_24](end_span). [span_25](start_span)
      - Pachytene: The recombinase enzyme mediates crossing over, exchanging genetic material between non-sister chromatids[span_25](end_span). [span_26](start_span)
      - Diplotene: Synaptonemal complex dissolves, revealing X-shaped chiasmata at crossover sites[span_26](end_span). [span_27](start_span)
      - Diakinesis: Chiasmata undergo terminalisation, and the nuclear membrane breaks apart[span_27](end_span).
    🖼️ Add Image: Meiosis I Cell Sequence (Prophase I -> Metaphase I -> Anaphase I -> Telophase I)
    [span_28](start_span)Bivalents align at the equator (Metaphase I)[span_28](end_span). [span_29](start_span)Homologous chromosomes separate, but sister chromatids stay attached at their centromeres (Anaphase I)[span_29](end_span). [span_30](start_span)A pair of haploid cells (a dyad) is formed (Telophase I)[span_30](end_span).
  • Meiosis II (The Equational Phase)
    🖼️ Add Image: Meiosis II Cell Sequence (Prophase II -> Metaphase II -> Anaphase II -> Telophase II)
    [span_31](start_span)Functions very much like standard mitosis[span_31](end_span). [span_32](start_span)Chromosomes align at the equator (Metaphase II)[span_32](end_span). [span_33](start_span)Centromeres finally split, allowing sister chromatids to move to opposite poles (Anaphase II)[span_33](end_span). [span_34](start_span)This concludes with four distinct haploid daughter cells enclosed in nuclear membranes (Telophase II)[span_34](end_span).