Chapter 9: Chromosomes, Mitosis, and Meiosis
Lecture 2: Meiosis

Asexual vs. Sexual Reproduction
1. Asexual Reproduction
        - Reproduction without sex
        - Mitosis or binary fission
        - Involves only one parent
        - Offspring are genetically identical to each other and to the parent
        - Clones
2. Sexual Reproduction
        - Involves the processes of fertilization or syngamy and meiosis.

Fertilization or Syngamy
1. Fusion of haploid gametes (egg and sperm)
2. Produces a diploid zygote
3. The 2n zygote grows into the adult.

1. Reduction division
2. 2n --> 1n
3. Daughter cells (= gametes) have half the number of chromosomes as the mother cell.
4. The human sexual life cycle was examined.

How does meiosis differ from mitosis? Lets examine four differences.
Meiosis vs. Mitosis - #1
1. Meiosis occurs in germinal (germ line) tissues.
        - Animals: ovaries and testes
        - Plants: pistils and anthers

Meiosis vs. Mitosis - #2
1. Meiosis involves two successive nuclear divisions in which the chromosomes are duplicated only once. This is a reduction division.
2. The two divisions of meiosis are termed Meiosis I and Meiosis II.

Meiosis I:
        - Homologous chromosomes first pair, then separate from each other.
        - Homologous chromosome separation results in haploidization (i.e., reduction division)

Homologous Chromosomes
1. Also called homologues .
2. Chromosomes that carry the same genes and originated from different sources.
3. The sources are the two parents.

Meiosis II:
1. Is only a simple mitosis increasing the number of haploid cells from 2 to 4.
2. Like mitosis, sister chromatids separate during anaphase II.

Meiosis vs Mitosis - #3
1. At the completion of meiosis, there are 4 daughter cells each with ½ the number of chromosomes as the mother cell.

Meiosis vs Mitosis - #4
1. The 4 daughter cells are genetically different, due to:
        - Crossing over
        - Independent assortment

Meiosis: The Process

Prophase I -- Events
1. Chromatin begins to condense to form chromosomes.
2. Chromosomes are duplicated (i.e., two sister chromatids joined at the centromere).
3. Duplex chromosomes.
4. Homologous chromosomes pair, termed synapsis.
5. Crossing over occurs.
6. Nucleolus disappears.
7. Nuclear membrane disappears.
8. Spindle apparatus begins to form.

Prophase I -- Synapsis
1. The paired homologous chromosomes are called:
        - bivalents (2 chromosomes)
        - tetrads (4 chromatids)
2. Synaptonemal complex holds (binds) homologues together.
3. A TEM micrograph, a diagram and a three-dimensional model of the synaptonemal was shown.
4. While the homologous are paired, crossing over occurs.

Crossing Over
1. Is the reciprocal (or equal) exchange of genetic material (genes) between nonsister chromatids of a synapsed pair of homologous chromosomes
2. It occurs only during prophase I of meiosis.
3. Does not occur in mitosis.
4. Chiasma: point where crossing over occurs.
5. Plural is chiasmata
6. Several models of crossing over were presented.
7. The results of crossing over is that the two sister chromatids of a single chromosome are no longer genetically identical.

Metaphase I
1. Synapsed homologues are moved to the equatorial plate by the kinetochore spindle fibers.
2. Independent assortment occurs.
3. A model of spindle fiber attachment to mitotic vs. meiotic metaphase chromosomes was shown.

Independent Assortment
1. Known as Mendel's Second Law.
2. Briefly stated: How one pair of homologues lines up on the equatorial plane is independent of how any other pair of homologues lines up.
3. Several models of independent assortment were presented.

Anaphase I
1. Homologous chromosomes segregate and move to opposite poles.
2. NOTE: sister chromatids do not separate in anaphase I of meiosis.
3. When homologous chromosomes segregate, the chromosome number is reduced in half.
        - 2n -->1n
4. The result is reduction division.
5. Independent Assortment is completed.
6. Several examples of anaphase I were shown.
7. The separation on homologues is termed disjunction.

Telophase I
1. Individual chromosomes gather together at the two poles.

1. Resting stage between meiosis I and II.
2. May or may not occur.
3. Chromosomes do not replicate.
4. It is not an interphase.

Meiosis II
1. Simply a mitotic division which increases the number of haploid cells from 2 to 4.
2. Prophase II, Metaphase II, Anaphase II, Telophase II.

Prophase II
1. Chromatin condenses to form chromosomes.
2. Nuclear envelop and nucleolus disappear.
3. Spindle apparatus begins to form.

Metaphase II
1. Duplex chromosomes align on the metaphase plate.

Anaphase II
1. Former sister chromatids segregate and move to opposite poles of the spindle.
2. Sister chromosomes are now called daughter chromosomes.

Telophase II
1. Chromosomes unwinds (form chromatin).
2. Nucleolus and nuclear envelop reform.
3. Spindle apparatus disappears.
4. Each new cell has a haploid number of chromosomes.
5. All chromosomes are in the unduplicated state (simplex chromosomes).
6. The 4 cells are genetically different

Products of Meiosis
1. Animals: 4 haploid cells function as gametes.
2. Plants: 4 haploid cells divide by mitosis to produce haploid plants which eventually produce gametes.

Animal Life Cycle
1. See Solomon Figure 9-12a, page 210.

Plant Life Cycle
1. See Solomon Figure 9-12c, page 210.

Meiosis: A Pictorial Review
1. Light micrographs of the stages of meiosis were shown. See the PowerPoint presentation.

The Evolutionary Consequences of Sex
1. How sex evolved is sketchy.
2. But sexual reproduction has an enormous impact on how species evolve today.
3. This is because of the ability of sexual reproduction to rapidly generate new genetic combinations.

Three Mechanisms of Genetic Diversity
1. Independent assortment
2. Crossing over
3. Random Fertilization

What is the importance of generating genetic diversity?
1. Genetic diversity is the raw material of evolution..

Comparison of mitosis and meiosis: Diagrammatical.
Using diagrams, the difference between mitosis and meiosis were shown. See the PowerPoint presentation. In the example, 2n = 4.

Mitosis vs. Meiosis: Four comparisons.
1. DNA Replication:
Mitosis: Occurs during interphase before nuclear division begins.
Meiosis: Occurs during interphase before nuclear division begins.

2. Number of Divisions:
Mitosis: One division, consisting of prophase, metaphase, anaphase and telophase.
Meiosis: Two divisions, each consisting of prophase, metaphase, anaphase and telophase; DNA replication does not occur between the two nuclear divisions (meiosis I and meiosis II); an event unique to meiosis is that during meiosis I, the homologous chromosomes synapse (join along their length), forming tetrads (groups of four chromatids); crossing over occurs when the homologous are synapsed.

3. Number of Daughter Cells and Genetic Composition
Mitosis: Two daughter cells which are diploid (2n) and genetically identical.
Meiosis: Four daughter cells which are haploid (1n) and genetically different to each other or the original mother cell.

4. Importance in the Animal Body
Mitosis: Development of multicellular adult from zygote; growth and tissue repair.
Meiosis: Production of gametes; reduces chromosome number in half; genetic variability.

Mitosis vs. Meiosis
1. Handout (Get from Dr. Ruch).

Meiosis in Humans
1. Gametogenesis
        - Spermatogenesis (sperm formation)
        - Oogenesis (egg [ovum] formation)
2. Diagrams of spermatogenesis and oogenesis were examined. See the PowerPoint presentation and Solomon Figures 48-6, page 1049 and 48-13, page 1055.

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