The Cell Cycle & Mitosis — Comprehensive Notes Summary & Study Notes

🧬 Cell Theory

Cell Theory states that all living things are composed of cells, that cells are the basic units of life, and that all cells come from pre-existing cells. This is the foundation for understanding growth, repair, and reproduction in multicellular organisms.

📏 Why Cells Are Small

Cells are small because as a cell's volume increases faster than its surface area, the membrane becomes insufficient for nutrient uptake and waste removal. When a cell cannot sustain exchange efficiently, it must divide.

🔁 Reasons Cells Divide

Cells divide for growth, repair, and reproduction. Division increases cell number, replaces damaged or dead cells, and produces genetically identical copies (clones) when needed.

⏳ Overview of the Cell Cycle

The Cell Cycle consists of three main parts: Interphase, Mitosis, and Cytokinesis. Cells spend most of their time in interphase, preparing for division by growing and replicating DNA.

🧪 Interphase (G1, S, G2)

Interphase is the longest stage and has three phases: G1 (First Gap) — cell growth and synthesis of proteins and organelles; S (Synthesis) — DNA replication occurs and chromatin duplicates; G2 (Second Gap) — final preparations for division and repair of DNA.

🚦 Cell Cycle Checkpoints

Checkpoints monitor conditions before division. Cells may pause the cycle if signals from other cells indicate not to divide, if nutrients are lacking, or if DNA is damaged. These checkpoints help maintain genomic integrity.

🧫 Mitosis — Purpose and Outcome

Mitosis is the process of nuclear division that ensures genetic continuity, producing two identical daughter nuclei. Mitosis is crucial for growth, tissue repair, and asexual reproduction in some organisms.

🔬 Prophase

In prophase, chromatin condenses into visible paired chromosomes (two sister chromatids joined at a centromere). Centrioles move to opposite poles, the nucleolus disappears, and the nuclear membrane breaks down.

➖ Metaphase

During metaphase, spindle fibers attach to the centromeres and chromosomes align at the cell's equatorial plane called the metaphase plate.

↔️ Anaphase

In anaphase, spindle fibers shorten and pull sister chromatids apart toward opposite poles, ensuring each new nucleus will receive an identical set of chromosomes.

🟢 Telophase

Telophase is the final stage of mitosis: chromosomes reach opposite poles, nuclear membranes begin to reform around each set, the nucleolus reappears, and cytokinesis often begins.

✂️ Cytokinesis — Division of the Cytoplasm

Cytokinesis divides the cytoplasm and organelles into two daughter cells. In animal cells, the membrane pinches in (a cleavage furrow). In plant cells, a cell plate forms that becomes a new cell wall.

🌱 Plant vs. Animal Cell Division

Both plant and animal cells undergo mitosis, but cytokinesis mechanisms differ: animal cells use membrane constriction while plant cells build a new wall due to the rigid cellulose structure.

☠️ Cell Death: Necrosis vs. Apoptosis

Necrosis is uncontrolled cell death from external factors like toxins or trauma. Apoptosis is programmed, controlled cell death used to remove unneeded or damaged cells without harming surrounding tissue.

✅ Summary and Key Terms

Mitosis and cytokinesis together produce two genetically identical daughter cells. Key terms to remember: chromatin, chromosome, sister chromatid, centromere, spindle fibers, centrioles, interphase, mitosis, cytokinesis, apoptosis, and necrosis.

📌 Final Note

Cell division is tightly regulated to balance growth and maintain organism health. Errors in regulation can lead to uncontrolled division or cell loss, emphasizing the importance of checkpoints and controlled cell death.