Chapter 11: Chromosomes, the Cell Cycle, & Cell Division

Chapter 11: Chromosomes, the Cell Cycle, & Cell Division

1. Systems of Cell Reproduction

  • Necessity of Cell Division: Cell division is essential for reproduction, growth, and repair of organisms.

  • Three Steps of Cell Division:

    1. Replication of genetic material (DNA)
    2. Separation of DNA molecules
    3. Division of cytoplasm

  • Prokaryotic vs. Eukaryotic Reproduction:

    • Prokaryotes: Single circular DNA molecule; reproduce via binary fission.
    • Eukaryotes: DNA contained within a nucleus and divided via mitosis or meiosis.

2. Interphase and the Control of Cell Division

Phases of Cell Cycle
  • Mitotic Cell Cycle Phases:
    • Interphase (most of the cell cycle)
    • S Phase: DNA replication
    • G1 Phase: Cell growth and preparation for DNA synthesis
    • G2 Phase: Preparation for mitosis
Regulation of Cell Cycle
  • Cyclin-CDK Complexes:
    • Regulate cell cycle transitions (e.g., from G1 to S phase, G2 to M phase).
    • Cyclins activate CDKs, which phosphorylate proteins to facilitate transitions.
  • Checkpoint Controls:
    • Internal (e.g., p53, p21) and external signals (e.g., growth factors) control cell division.

3. Eukaryotic Chromosomes

  • Definition: Chromosomes are composed of DNA and proteins (Chromatin).
  • Structure:
    • Each chromosome consists of two sister chromatids, joined at the centromere.
  • Nucleosome Formation: DNA wraps around histone proteins, forming nucleosomes during interphase which condense further during mitosis.

4. Mitosis: Distributing Exact Copies of Genetic Information

Phases of Mitosis
  1. Prophase: Chromosomes condense, and the mitotic spindle begins to form.
  2. Prometaphase: Nuclear envelope disintegrates; spindle fibers attach to kinetochores.
  3. Metaphase: Chromosomes align at the metaphase plate.
  4. Anaphase: Chromatids are pulled apart to opposite poles of the cell.
  5. Telophase: Chromosomes de-condense, nuclear envelope reforms, and the cell prepares to split.

5. Cytokinesis: The Division of the Cytoplasm

  • Animal Cells: Cytokinesis occurs via a cleavage furrow formed by actin/myosin contractile rings.
  • Plant Cells: A cell plate forms during division, creating a new cell wall.

6. Reproduction: Sexual & Asexual

  • Asexual Reproduction: Produces genetically identical clones; variations arise through mutations.
  • Sexual Reproduction: Involves two gametes uniting to form a diploid zygote, leading to genetic diversity.
  • Gametogenesis: In meiosis, diploid cells undergo two rounds of division to produce haploid gametes.

7. Meiosis: A Pair of Nuclear Divisions

  • Purpose: Reduces chromosome number from diploid to haploid and increases genetic diversity.
  • Meiosis I and II: Two distinct divisions leading to four haploid cells.
  • Crossing Over: Genetic recombination during Prophase I enhances variability.

8. Meiotic Errors

  • Nondisjunction: Failure of homologous chromosomes to separate during meiosis, resulting in aneuploidy (abnormal number of chromosomes).

9. Cell Death

Types of Cell Death
  • Necrosis: Unprogrammed cell death due to stress, leading to cellular swelling and burst.
  • Apoptosis: Programmed cell death involving cellular detachment and DNA fragmentation.

Key Themes Reviewed

  1. Molecular Structure and Function: Understanding the relationship between molecule shape and biological function.
  2. Endergonic and Exergonic Processes: Coupled reactions for efficient cellular metabolism.
  3. Compartmentalization: Eukaryotic reactions occur within distinct cellular organelles.
  4. Gene Regulation Complexity: Multiple levels of control during gene expression.
  5. Mitosis and Meiosis: Differences in purpose and outcome regarding genomic consistency vs. variability.