Overview of Cell Cycle and Reproduction

Sep 6, 2024

Lecture on the Cell Cycle and Cellular Reproduction

Introduction to the Cell Cycle

  • The cell cycle refers to the entire life of a cell until it reproduces, not just cell division.
  • Focus is on DNA conservation and chromosome behavior during cell division.

DNA and Chromosomes

  • Chromosomes: Condensed DNA and organizational proteins, visible during mitosis and meiosis.
  • Chromatin: Less condensed form of DNA, active in the nucleus.
  • Chromosomes are visible and inactive when condensed.

Phases of the Cell Cycle

Interphase

  • Takes up most of the cell cycle, divided into:
    • G1 Phase (First Gap Phase): Cell grows and metabolizes nutrients.
    • S Phase (Synthesis Phase): DNA replication occurs.
    • G2 Phase (Second Gap Phase): Preparation for division.
  • G1 and G2 were termed 'gap phases' because they seem inert under a microscope.

M Phase (Mitotic Phase)

  • Composed of mitosis and cytokinesis, brief and rapid process.

Mitosis: Nuclear Division

Phases of Mitosis

  1. Prophase

    • Chromosomes condense and become visible.
    • Mitotic spindle begins to form.
    • Nuclear membrane intact, no nucleolus visible.

  2. Prometaphase

    • Nuclear membrane dissolves.
    • Chromosomes begin to align, spindle fibers attach.

  3. Metaphase

    • Chromosomes align at the cell's equator, forming the metaphase plate.

  4. Anaphase

    • Sister chromatids separate and move to opposite poles.

  5. Telophase

    • Chromatids arrive at poles, nuclear membrane reforms, cell starts to divide.

Cytokinesis

  • Final step where the cytoplasm divides, resulting in two daughter cells.

Regulation of the Cell Cycle

  • Density-Dependent Inhibition: Cells stop dividing when crowded.
  • Anchorage Dependence: Cells must be attached to something to divide.
  • Cancer cells ignore these regulations, leading to tumors.

Meiosis: Formation of Gametes

Differences from Mitosis

  • Occurs in two rounds: Meiosis I and Meiosis II.
  • Results in four genetically unique haploid cells (gametes).

Meiosis I

  1. Prophase I

    • Homologous chromosomes pair up and exchange segments (crossing over).

  2. Metaphase I

    • Paired homologous chromosomes align on the metaphase plate.

  3. Anaphase I

    • Homologous chromosomes separate to opposite poles.

  4. Telophase I and Cytokinesis

    • Cells divide, forming two haploid cells.

Meiosis II

  • Similar to mitosis but without preceding DNA replication.
  • Sister chromatids separate, resulting in four haploid gametes.

Genetic Variation in Meiosis

  • Independent Assortment: Random distribution of homologous chromosomes.
  • Crossing Over: Exchange of genetic material between homologous chromosomes.
  • Random Fertilization: Vast number of possible genetic combinations.

Errors in Meiosis

  • Non-Disjunction: Failure of chromosomes to separate properly.
    • Can lead to conditions like Down Syndrome.
  • Mutations: Can introduce genetic diversity but also cause errors.

These notes summarize the key points discussed in the lecture about cell cycles, mitosis, and meiosis, emphasizing the processes, their regulation, and the potential errors that can occur.