Lecture Notes: Cell Division and Chromosomes

Introduction

• Focus on how cells divide and the role of chromosomes.
• Importance in multicellular eukaryotic organisms (humans, plants, walruses) for growth, development, and repair.
  • Critical for young organisms to grow.
  • Necessary for older organisms to replace lost cells (e.g., skin cells).

The Cell Cycle

• Defined as the life cycle of a cell from formation to division into two new cells.
• Composed of three main stages:
  1. Growth
  2. DNA Replication
  3. Mitosis and Division (Cytokinesis)

Stage 1: Growth

• Cell grows in size.
• Increases number of sub-cellular structures (e.g., mitochondria, ribosomes).

Stage 2: DNA Replication

• DNA is duplicated to ensure each new cell has a full set of DNA.
• DNA Structure:
  • Normally spread out in long strings when the cell is not dividing.
  • Condenses into chromosomes forming packets of DNA during preparation for division.
  • Each chromosome contains genes controlling characteristics (e.g., eye color).
  • Eukaryotic cells have pairs of chromosomes: one from mother, one from father.
  • Humans have 23 pairs, totaling 46 chromosomes.

Chromosome Duplication

• Each chromosome duplicates, remaining attached to original, forming an X shape.
• Two halves of each chromosome (arms) are identical:
  • Right arm is a duplicate of the left arm.

Stage 3: Mitosis and Cytokinesis

• Mitosis:
  • Chromosomes line up along the cell's center.
  • Spindle fibers attach to each chromosome's halves.
  • Fibers pull chromosomes apart to opposite cell sides (poles).
  • Chromosomes split into two arms, dividing genetic material evenly.
• Cytokinesis:
  • Cell membrane and cytoplasm split, forming two daughter cells.
  • Each daughter cell receives an identical set of chromosomes.

Conclusion

• Daughter cells can contribute to the organism's growth, development, or repair.
• The cycle can repeat as needed.

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