Chapter 14 – How Eukaryotic Cells Sort and Transmit Chromosomes: Mitosis and Meiosis

  • Outline of Topics:

    • The eukaryotic cell cycle

    • Mitotic cell division

    • Meiosis

    • Sexual reproduction

    • Variations in chromosome structure and number

14.1 The Eukaryotic Cell Cycle

  • Key Concepts:

    • Cell Division:

    • Highly regulated process vital to all organisms.

    • Origin of life traced back nearly 4 billion years.

    • Cell Cycle:

    • Series of events leading to cell division.

    • Involves chromosome processing: replication, alignment, separation.

Chromosome Structure and Inheritance

  • Chromosomes compact enough to visualize via light microscope during division.

  • Karyotype:

    • Photographic representation of chromosomes in dividing cells.

    • Reveals number, size, and shape of chromosomes.

  • Types of Chromosomes:

    • Eukaryotic chromosomes occur in homologous pairs:

    • Example: Humans have 23 pairs (46 total); 1 pair of sex chromosomes (XX or XY).

    • Definitions:

    • Diploid (2n): 2 sets of chromosomes.

    • Haploid (1n): 1 set of chromosomes.

Chromosomal Pairing and Homologs

  • Homologous Chromosomes:

    • Nearly identical in size and genetic content.

    • Different versions (alleles) can confer different traits (e.g., eye color).

  • DNA Sequence Similarity:

    • Generally >99%; small differences can affect traits.

  • Sister Chromatids:

    • Formed after DNA replication, joined at the centromere.

Phases of the Eukaryotic Cell Cycle (cell cycle occurs prior to mitosis/meiosis cell division)

  • Cell Cycle Phases:

    1. G1 Phase: Cell growth; responds to environmental signals.

    2. S Phase: DNA replication.

    3. G2 Phase: Preparation for division; synthesizes proteins necessary for sorting chromosomes.

    4. M Phase: Includes mitosis and cytokinesis.

  • Checkpoints:

    • Regulate progression:

    • G1 Checkpoint (Restriction Point)

    • G2 Checkpoint

    • Metaphase Checkpoint

    • Checkpoint proteins detect issues (e.g., DNA damage) and halt cycle if necessary, preventing mutations or cancer.

14.2 Mitotic Cell Division

  • Points to Remember:

    • Mitosis: Produces two genetically identical daughter cells.

    • Mother cell divides into daughter cells.

    • Sister Chromatids:

    • Formed when chromosomes are replicated and compacted.

Spindle Apparatus and Chromosome Sorting

  • Organizer of chromosomes, made up of microtubules; begins at centrosomes.

  • Three Microtubule Types:

    1. Astral Microtubules - position the spindle apparatus.

    2. Polar Microtubules - push the poles of the spindle apart.

    3. Kinetochore Microtubules - attach to kinetochores of sister chromatids for chromosome sorting.

Key Events of Mitosis

  • Stages of Mitosis:

    1. Prophase: Chromosomes condense and become visible; the nuclear envelope breaks down.

    2. Prometaphase: Kinetochores form; spindle attaches.

    3. Metaphase: Chromosomes align at the cell's equatorial plane.

    4. Anaphase: Sister chromatids are pulled apart to opposite poles of the cell.

    5. Telophase: Chromosomes decondense; nuclear envelopes reform around each set of chromosomes

  • Cytokinesis:

    • Division of cytoplasm; differs in plants (cell plate formation) and animals (cleavage furrow formation).

14.3 Meiosis and Sexual Reproduction

  • Meiosis: Produces four haploid daughter cells from one diploid cell via two rounds of division (Meiosis I and II).

    • four genetically diverse haploid cells

  • Features of Meiosis:

    • Homologous chromosomes pair in Meiosis I separate; sister chromatids separate in Meiosis II.

Key Events of Meiosis I

  • Stages Include:

    1. Prophase I: Chromosomes condense, crossing over occurs; bivalents (pair) form.

    2. Prometaphase I: Complete nuclear envelope dissociation, spindle formation.

    3. Metaphase I: Bivalents align at the metaphase plate.

    4. Anaphase I: Homologs separate.

    5. Telophase I and Cytokinesis: Two haploid cells are formed.

  • No S Phase Between Meiosis I and II.

14.4 Sexual Reproduction

  • Benefits of Sexual Reproduction:

    • Increases genetic diversity via random alignment and crossing over.

    • For diploid species, random chromosomal alignments lead to a large number of combinations (e.g., 2^{23}).

  • Life Cycles:

    • Diploid-dominant (most animals), haploid-dominant (most fungi), alternation of generations (some plants).

14.5 Variation in Chromosome Structure and Number

  • Key Concepts:

    • Variations in chromosome structure (deletions, duplications, inversions, translocations) can create diseases and species evolution.

    • Chromosome Structure Classifications:

    • Metacentric, submetacentric, acrocentric, telocentric based on centromere location.

  • Aneuploidy and its Effects:

    • Monosomy (2n - 1), Trisomy (2n + 1); significant genetic impact.

    • Nondisjunction leads to abnormal chromosome distribution in cell division.