Mitosis and Meiosis

Lily pollen meiosis, anaphase I

Lily pollen meiosis, anaphase I (Photo credit: Carolina Biological Supply Company)

Biology 42 – Human Biology (Spring 2007)

Mitosis and Meiosis

[ Students and professors, please read. ]

What are the functions of mitosis and meiosis in the life cycle of humans?

Mitosis:  Describe the phases of mitosis using the terms centrioles, nucleolus, spindle, and furrowing.

[Unfortunately, the images in the original document wouldn’t copy.]

Prophase Chromatin in the nucleus begins to condense and becomes visible in the light microscope as chromosomes. The nucleolus disappears. Centrioles begin moving to opposite ends of the cell and fibers extend from the centromeres. Some fibers cross the cell to form the mitotic spindle.
Prometaphase The nuclear membrane dissolves, marking the beginning of prometaphase. Proteins attach to the centromeres creating the kinetochores. Microtubules attach at the kinetochores and the chromosomes begin moving.
Metaphase Spindle fibers align the chromosomes along the middle of the cell nucleus. This line is referred to as the metaphase plate. This organization helps to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome.
Anaphase The paired chromosomes separate at the kinetochores and move to opposite sides of the cell. Motion results from a combination of kinetochore movement along the spindle microtubules and through the physical interaction of polar microtubules.
Telophase Chromatids arrive at opposite poles of cell, and new membranes form around the daughter nuclei. The chromosomes disperse and are no longer visible under the light microscope. The spindle fibers disperse, and cytokinesis or the partitioning of the cell may also begin during this stage.
Cytokinesis In animal cells, cytokinesis results when a fiber ring composed of a protein called actin around the center of the cell contracts pinching the cell into two daughter cells, each with one nucleus. In plant cells, the rigid wall requires that a cell plate be synthesized between the two daughter cells.

Meiosis:  1. how do the terms diploid (2n) and haploid (n) pertain to meiosis? 2. know how sex is determined.

Meiosis I & II

What is meiosis I?

In meiosis I, chromosomes in a diploid cell resegregate, producing four haploid daughter cells. It is this step in meiosis that generates genetic diversity.

The phases of meiosis I & II

Prophase I DNA replication precedes the start of meiosis I. During prophase I, homologous chromosomes pair and form synapses, a step unique to meiosis. The paired chromosomes are called bivalents, and the formation of chiasmata caused by genetic recombination becomes apparent. Chromosomal condensation allows these to be viewed in the microscope. Note that the bivalent has two chromosomes and four chromatids, with one chromosome coming from each parent.
Prometaphase I The nuclear membrane disappears. One kinetochore forms per chromosome rather than one per chromatid, and the chromosomes attached to spindle fibers begin to move.
Metaphase I Bivalents, each composed of two chromosomes (four chromatids) align at the metaphase plate. The orientation is random, with either parental homologue on a side. This means that there is a 50-50 chance for the daughter cells to get either the mother’s or father’s homologue for each chromosome.
Anaphase I Chiasmata separate. Chromosomes, each with two chromatids, move to separate poles. Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids.
Telophase I Nuclear envelopes may reform, or the cell may quickly start meiosis II.
Cytokinesis Analogous to mitosis where two complete daughter cells form.

Meiosis II is similar to mitosis. However, there is no “S” phase. The chromatids of each chromosome are no longer identical because of recombination. Meiosis II separates the chromatids producing two daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid.

Comparing Meiosis and Mitosis

  • Chromosome behavior
    1. Mitosis: Homologous chromosomes independent
    2. Meiosis: Homologous chromosomes pair forming bivalents until anaphase I
  • Chromosome number- reduction in meiosis
    1. Mitosis- identical daughter cells
    2. Meiosis- daughter cells haploid
  • Genetic identity of progeny:
    1. Mitosis: identical daughter cells
    2. Meiosis: daughter cells have new assortment of parental chromosomes
    3. Meiosis: chromatids not identical, crossing over

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