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Chromosome Content, Cell Cycle, Mitosis, and Meiosis: Study Notes for Anatomy & Physiology

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A) Cell Types Based on Chromosome Content

1. Gametes

Gametes are specialized reproductive cells (egg and sperm) that carry half the genetic material of an organism.

  • Haploid (n): Gametes contain 23 different chromosomes, representing one set of genetic information.

  • Includes ova (eggs) and sperm.

2. Somatic Cells

Somatic cells make up all body tissues except those involved in reproduction. They result from mitosis and have a full set of chromosomes.

  • Diploid (2n): Somatic cells contain 46 chromosomes (23 pairs).

  • Examples: muscle cells, nerve cells, liver cells, etc.

  • Homologous chromosomes: Each pair consists of one chromosome from each parent, similar in length, centromere position, and gene content (e.g., eye color).

  • Alleles: Variants of a gene found at the same locus on homologous chromosomes, responsible for different traits (e.g., brown vs. green eyes).

  • Each somatic cell contains:

    • 23 pairs of chromosomes (46 total)

    • 22 pairs are autosomal chromosomes (numbered 1-22)

    • 1 pair are sex chromosomes (XX for female, XY for male)

B) Cell Cycle – for Growth and Repair of Tissues

The cell cycle describes the sequence of events that somatic cells undergo for growth, DNA replication, and division.

  • Diploid somatic cell (2n) divides to produce two genetically identical diploid cells (2n).

  • Phases:

    1. Interphase: Cell growth and DNA replication.

    2. Mitotic (M) Phase: Cell division.

1. Interphase

  • G1 Phase:

    • Growth and metabolism.

    • Centrosome replication begins.

    • Cells that do not divide again remain in G0 (e.g., nerve and some muscle cells).

  • S Phase:

    • DNA replication occurs; chromosomes are duplicated but remain as chromatin.

    • Chromosomes become individually visible only during mitosis/meiosis.

    • Each chromosome consists of two sister chromatids attached at the centromere.

    • Kinetochore: Protein structure at the centromere, site of spindle microtubule attachment.

  • G2 Phase:

    • Further growth and metabolism.

    • Production of enzymes and proteins needed for cell division.

2. Mitotic Phase

  • Mitosis: Division of nuclear material (chromosomes) in four phases:

    1. Prophase

    2. Metaphase

    3. Anaphase

    4. Telophase

  • Cytokinesis: Division of the cytoplasm.

  • Result: Two identical diploid cells enter interphase (G1).

C) Somatic Cell Formation: Mitosis

1. Prophase

  • Chromosomes condense and become visible.

  • Nuclear envelope and nucleolus disappear.

  • Centrosomes move to opposite poles of the cell.

  • Spindle microtubules grow out of centrosomes and attach to kinetochores.

2. Metaphase

  • Chromosomes align at the cell equator.

3. Anaphase

  • Spindle microtubules shorten and pull kinetochores apart.

  • Sister chromatids separate, resulting in 92 individual chromosomes (in humans).

  • 46 chromosomes migrate to each pole.

  • Cytokinesis begins.

4. Telophase

  • Chromosomes uncoil and become thread-like.

  • Nuclear envelope and nucleolus reappear.

  • Spindle disassembles.

  • Cytokinesis completes cell division.

D) Gamete Formation: Meiosis = Reproductive Nuclear Division

Meiosis produces gametes (sperm and ova) with half the chromosome number of somatic cells, ensuring genetic diversity.

  • 1 cell (diploid, 2n) produces 4 genetically unique gametes (haploid, n).

  • Meiosis involves two consecutive divisions: Meiosis I and Meiosis II.

1. Meiosis I

  • Homologous chromosomes separate, reducing chromosome number from 2n to n.

  • Phases:

    1. Prophase I: Homologous chromosomes pair up (tetrads).

    2. Metaphase I: Tetrads align at the equator.

    3. Anaphase I: Homologous chromosomes separate and migrate to opposite poles.

    4. Telophase I: Two haploid cells form, each with 23 replicated chromosomes.

2. Meiosis II

  • Sister chromatids separate, similar to mitosis.

  • Phases:

    1. Prophase II: Chromosomes condense in each haploid cell.

    2. Metaphase II: Chromosomes align at the equator.

    3. Anaphase II: Sister chromatids separate.

    4. Telophase II: Four unique haploid gametes are produced.

After Meiosis

  • Each gamete has 23 chromosomes (haploid, n): one copy of each autosome and one sex chromosome.

Comparison Table: Mitosis vs. Meiosis I vs. Meiosis II

Phase

Mitosis

Meiosis I

Meiosis II

Prophase

Chromosomes condense

Tetrads form

Chromosomes condense

Metaphase

46 chromosomes line up at equator

23 tetrads line up at equator

23 chromosomes line up at equator

Anaphase

Kinetochores split & 46 chromosomes migrate to each pole

Tetrads split & 23 replicated chromosomes migrate to each pole

Kinetochores split, sister chromatids separate & 23 chromosomes migrate

After Cytokinesis

2 identical diploid cells, 23 pairs of chromosomes in each

2 haploid cells, copies of each of 23 chromosomes with sister chromatids

4 gametes, each with 23 chromosomes

E) Why Meiosis?

Meiosis is essential for sexual reproduction, ensuring that gametes are haploid so that fertilization restores the diploid chromosome number.

  • Turns diploid germ cells into haploid gametes (ova/sperm).

  • 23 chromosomes from haploid sperm combine with 23 chromosomes from haploid ovum to form a zygote with 46 chromosomes (diploid, 2n).

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