Cell Division

• Cell division is essential for:
• Growth – increasing the number of cells in an organism.
• Repair – replacing damaged or dead cells.
• Asexual reproduction – producing genetically identical offspring (mitosis).
• Sexual reproduction – producing gametes (meiosis).
• The cell cycle describes the life of a cell from one division to the next. It has two major parts:
• Interphase: growth phase 1 to synthetic phase to growth phase

Most of the cell’s life is spent here.

G1 (Gap 1): Cell grows; organelles increase.

S (Synthesis): DNA replicates → each chromosome forms two identical chromatids.

G2 (Gap 2): Further growth; proteins for mitosis produced; DNA checked for errors.

• Mitosis: Division of the nucleus.
• Cytokinesis: Division of the cytoplasm → two identical daughter cells.

• Purpose: Growth, repair, asexual reproduction. Outcome: Two genetically identical diploid cells (same number of chromosomes as parent).
• Stages of mitosis:
• Prophase: Chromosomes condense; nuclear membrane breaks down.
• Metaphase: Chromosomes line up at the equator.
• Anaphase: Sister chromatids pulled to opposite poles.
• Telophase: New nuclear membranes form.

Cytokinesis: Cell splits into two identical daughter cells.

• Purpose: Production of gametes for sexual reproduction. Outcome: Four genetically different haploid cells (half the chromosome number).
• Key Features
• Meiosis I: Homologous chromosomes separate → variation from crossing over & independent assortment.
• Meiosis II: Sister chromatids separate → similar to mitosis.

Cell Division in Prokaryotes.

Prokaryotes divide by binary fission, not mitosis.

• DNA replicates
• Cell elongates
• Cell splits into two identical cells
• Cell Division, Cancer & Regulation:
• Mutations in regulators → uncontrolled cell division → cancer.
• There are regulatory proteins that control cell cycle.
• Their job is to make sure cells divide only when needed and stop when division is not safe.
• Key regulatory proteins include:
• Cyclins — proteins that rise and fall to control each stage of the cell cycle.
• CDKs (Cyclin-dependent kinases) — enzymes activated by cyclins to push the cell through checkpoints.
• Cancer often begins when the regulatory proteins that normally control cell division stop working properly, allowing cells to divide too fast, ignore signals, and become abnormal.
• When regulatory proteins fail:
• Cells divide without stopping
• Cells ignore signals to die (apoptosis)
• Abnormal cells accumulate
• A tumour form
• Some cells may break away and spread (metastasis)