Eukaryotic Cells and Chromosome Transmission

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 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, nuclear envelope dissipates.

  2. Prometaphase: Kinetochores form; spindle attaches.

  3. Metaphase: Chromatids align at the metaphase plate.

  4. Anaphase: Sister chromatids separate and move to poles.

  5. Telophase: Chromosomes decondense; nuclear envelopes reform.

• 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).

• Features of Meiosis:

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

Key Events of Meiosis I

• Stages Include:

  1. Prophase I: Chromosomes condense, crossing over occurs; bivalents 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.