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What is Meiosis? 1. Meiosis produces daughter cells that have one half the number of chromosomes as the parent cell (haploid) 2N (diploid) N (haploid) 2. Meiosis enables organisms to reproduce sexually. Gametes (sperm and eggs) are haploid. 3. Meiosis involves 2 divisions producing a total of 4 daughter cells.
Phases of Meiosis • A cell undergoing meiosis will divide 2 times; • The first division is Meiosis I and the 2nd is Meiosis II • The phases have the same names as those of mitosis. • A number indicates the division number (1st or 2nd): Meiosis I prophase 1, metaphase 1, anaphase 1, and telophase 1 Meiosis II prophase 2, metaphase 2, anaphase 2, and telophase 2
MEIOSIS 1 vs. MEIOSIS 2 • MEIOSIS 1 - the number of cells is doubled but the number of chromosomes is not • Results in 1/2 as many chromosomes per cell • MEIOSIS 2 - is like MITOSIS • Number of chromosomes does not get reduced
Order of Events • Diploid Cell (2N): from mitotic division, the Oogonium (Spermatogonium) enters meiosis with DIPLOID (2N) chromosomes but TETRAPLOID (4N) DNA • Chromosomes then duplicate to produce SISTER CHROMATIDS (or HOMOLOGOUS DYADS).
Meiosis I • Prophase I: Dyad pairs align to create "TETRADS", non-sister chromatids connect and trade sections at a "CHIASMA", a process called "CROSSING OVER".
Meiosis I • Metaphase I: SPINDLE FIBERS attach to each dyad at the KINETOCHORE. • Tension from spindle fibers aligns the tetrads at the cell equator.
Meiosis I • Anaphase I: Chiasmata break apart and sister chromatids begin migrating toward opposite poles.
Meiosis I • Telophase I: CLEAVAGE FURROW forms beginning the process of CYTOKINESIS (cell division)daughter cells are HAPLOID (1N)
Meiosis II • Prophase II: Spindle formation begins and centrosomes begin moving toward poles.
Meiosis II • Metaphase II: Tension from spindle fibers aligns chromosomes at the metaphase plate.
Meiosis II • Anaphase II: CHROMATIDS separate and begin moving to the poles.
Meiosis II • Telophase II: CLEAVAGE FURROW forms beginning CYTOKINESIS.
Results • Gamete (1N): NUCLEAR ENVELOPES form and chromosomes disperse as CHROMATIN. • Meiosis has produced 4 DAUGHTER CELLS, each with 1N chromosomes and 1N DNA. • During fertilization, male and female 1N gametes will fuse to form a 2N ZYGOTE.