chapter 10

In sexually reproducing organisms, germ-line cells in organs called produce haploid .
- • The fusion of haploid gametes is called fertilization and creates the single-celled diploid zygote.
  The zygote contains two versions of each chromosome:
  - a maternal homologue from the haploid gamete (egg) of the mother
  - a paternal homologue from the haploid gamete (sperm) of the father
The gametes are produced by . whereas somatic cells are produced by in a sexual reproductive cycle.
- • Sexual life cycle.
  Germ-line cells (2 n = diploid) are produced in organs called gonads (ovaries and testes).
  These cells undergo meiosis to form gametes (eggs and sperm, n = haploid).
  The rest of the diploid body cells are called somatic cells and reproduce by mitosis.
  Fertilization from the fusion of gametes creates the diploid zygote, which undergoes many rounds of mitosis to produce the organism.

Meiosis involves two cell divisions, each containing a , , , and .

• Meiosis consists of two rounds of cell division and produces four haploid cells.
Meiosis I is shown in the outer circle: this is the reduction division where the daughter cells become haploid.
Meiosis II is shown in the inner circle: this is where the sister chromatids of each chromosome separate.
Review:

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Prophase I. Homologous chromosomes pair up (synapsis). `Crossing over` occurs.	Metaphase I. Homologous pairs align along metaphase plate (plane).	Anaphase I. Homologous chromosomes separate, moving to opposite poles.	Telophase I/Cytokinesis I. One set from the paired chromosomes arrives at each pole. The cell divides into two.

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Prophase II. Chromosomes re-condense. Spindle fibers form.	Metaphase II. Spindles attach to chromosomes, which align along equator.	Anaphase II. Sister chromatids separate and move to opposite poles.	Telophase II. Chromatids arrive at each pole. Cell division begins.	Cytokinesis II. Cell division is complete. Each cell is haploid.

Unique features of meiosis include , homologous , and division.
- •
  Meiosis unique features:
  1. Synapsis and homologous recombination
  2. Reduction division
  Review:
- • Synapsis and crossing over
  In prophase I, homologous chromosomes pair up in a process called synapsis,
  Non-sister chromatids of the homologous chromosomes exchange portions in a process called crossing over.
  The chromosomes in daughter cells of meiosis I have undergone homologous recombination: they are genetically different from those in the parent cell.
  Review:
- • Meiosis I is a reduction division, producing 2 different haploid cells with 2 chromatids on each chromosome.
  Meiosis II produces 4 different haploid gametes, with 1 chromatid on each chromosome.
  Note that chromosome duplication occurs before meiosis I, but NOT before meiosis II.
- • Synapsis and crossing over
  In prophase I, homologous chromosomes pair up in a process called synapsis,
  Non-sister chromatids of the homologous chromosomes exchange portions in a process called crossing over.
  The chromosomes in daughter cells of meiosis I have undergone homologous recombination: they are genetically different from those in the parent cell.
  Review:
- • Meiosis I is a reduction division, producing 2 different haploid cells with 2 chromatids on each chromosome.
  Meiosis II produces 4 different haploid gametes, with 1 chromatid on each chromosome.
  Note that chromosome duplication occurs before meiosis I, but NOT before meiosis II.
Mitosis and meiosis comparison.
- • Meiosis/meiosis comparison.
  Meiosis involves two cell divisions after DNA replication. It thus produces 4 haploid gametes.
  Crossing over occurs in prophase I of meiosis.
  Mitosis involves a single cell division after DNA replication. It thus produces 2 diploid somatic cells.
  Summary:
Another consequence of meiosis and sexual reproduction is increased genetic .
- Crossing .
  - • Crossing over.
    In prophase I, homologous chromosomes pair up (synapsis).
    Non-sister chromatids of the 2 homologues may exchange DNA segments (crossing over).
    The exchange results in homologous recombination: the sister chromatids of a chromosome may be genetically different.
- Independent .
  - • Independent assortment.
    The orientation of homologous chromosomes on the metaphase plane during metaphase I is random.
    A cell with 3 chromosome pairs has 8 (2³) different possible ways of aligning the paternal and maternal chromosomes during metaphase I.
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    Review:
  - • Independent assortment.
    A cell with 3 chromosome pairs can produce gametes with 2³, or 8 different combinations of gametes.
    Humans with 23 chromosome pairs can produce 2²³, or 8,388,608 different gametes.
- Random .

• Random fertilization.
A zygote is formed by the random union of independently-produced gametes.
For humans, the number of different gametes is 2²³ * 2²³, or 8,388,608², giving 70,368,744,177,664 (70 trillion) possible combinations.