Human Anatomy Lecture 2: The Cell Cycle

Human Anatomy Lecture 2: The Cell Cycle

• In humans, 100 billion cells die every 24 hours and need to be replaced.
• The cell cycle accomplishes this.

Stages of the Cell Cycle: Principal Events

• Interphase:
  • G1 Phase: Normal cellular activities, cell growth, organelle duplication, protein synthesis
  • S Phase: DNA replication
  • G2 Phase: Synthesis of proteins, microtubules
• Mitotic Phase:
  • Nuclear division
  • Cytokinesis: Cytoplasmic division

General Information

• Possible fates of "daughter" cells:
  • Differentiate (specialize), age, or die
  • Rest as "uncommitted" cells
  • Enter a new cell cycle immediately
• Capacity to grow and divide varies with cell type, as does the rate of these processes.

Three Major Categories of Cells Based on Division Ability

• Category 1: Cells that are extremely specialized and have lost their ability to divide:

  • Mature red blood cells
  • Neurons (nerve cells)
  • Skeletal muscle cells (may increase in size, not in number)

• Category 2: Cells that normally do not divide but can be induced to do so by an appropriate stimulus:

  • Liver cells
  • Lymphocytes (white blood cells)

• Category 3: Cells that continually undergo cell division because they are subject to frequent destruction:

  • Skin cells
  • Cells of the esophagus
  • Bone marrow cells

Terminology Related to Cell Division

• Chromosome: Dark-staining, threadlike body in the nucleus, composed of DNA and protein, contains genetic information, single strand
• Chromatid: Duplicated chromosome
• Gene: Unit of DNA found at a particular "locus" (location), provides instructions for the production of a specific protein

Centrioles

• Composed of microtubules
• Two cylindrical structures
• Serves as the organizing center for the mitotic spindle

Karyotype

• Number and type of chromosomes an individual has
• A normal human karyotype has 23 pairs of chromosomes (2n = 46)
• 22 pairs are autosomes
• 1 pair are sex chromosomes (XX in females, XY in males).

The Cell Cycle: Interphase vs. Mitotic Phase

• Interphase: The non-dividing stage of the cell cycle; focuses on DNA replication and growth
  • Noticeably longer than the mitotic phase
• Mitotic Phase: The actively dividing phase.

Interphase: Detailed Information

• S Phase: DNA replication, highly regulated, cell is very vulnerable during this phase, potential for damage to DNA is very high. Rapidly dividing cells are especially vulnerable (skin, GI tract, etc.)
• G2 Phase: Continued growth and protein synthesis

Stages of Mitosis

• Prophase:
  • Nuclear membrane disappears
  • Chromosomes condense
  • Centrioles migrate to opposite poles of the cell
• Metaphase:
  • Paired chromosomes line up along the equator of the cell (metaphase plate)
• Anaphase:
  • Chromosomes separate and migrate to opposite sides of the cell
• Telophase:
  • The nucleus divides into two identical cells with the same DNA composition
  • Cytokinesis occurs: Division into two daughter cells

Results of Mitosis

• Two identical daughter cells are produced
• Daughter cells may divide again
• Daughter cells may permanently remain in interphase

Mitosis vs. Meiosis

• Mitosis: Occurs in most body cells; results in two identical daughter cells.
  • Daughter cells have the same number of chromosomes as the parent cell
• Meiosis: Occurs in sex cells (ova and sperm); results in four daughter cells.
  • Daughter cells have half the number of chromosomes as the parent cell

Cellular Adaptations

• Cells change in response to stimuli or injury to adapt to unfavorable conditions.
  • Hypertrophy: Increase in size of cells
  • Atrophy: Decrease in size of cells
  • Hyperplasia: Increase in mitotic activity and number of cells
  • Metaplasia: Changes in cells in response to disease or chronic irritation, resulting in transformation into a different cell type
  • Neoplasm: "New cell mass", rapid multiplication (benign vs. malignant)

Up Next: Tissue 1 in the lecture hall