1. 8-3 Meiosis
A process that reduces the number of chromosomes in new cells to half the number in the original cell.
These haploid reproductive cells are called gametes.
Human gametes are sperm and egg cells, and they contain 23 chromosomes.
Meiosis - Bill Nye.mp4

2. Stages of Meiosis
Cells preparing to divide by meiosis undergo the three stages of interphase, G1, S, and G2.
Reminder: during interphase, cells grow to a mature size and copy their DNA.
Thus, cells begin meiosis with a duplicate set of chromosomes just as cells beginning mitosis do!
***Cells undergoing meiosis divide twice, resulting in four haploid cells rather than two diploid cells.
The stages of cell division are called meiosis I and meiosis II.
meiosis 1

3. Meiosis I: 4 phases

4. Prophase 1: As in mitosis, DNA coils tightly into chromosomes and spindle fibers appear. Nucleus and nucleolus disassemble.
Synapsis: The pairing of homologous chromosomes which does not occur in mitosis.
Each pair of homologous chromosomes is called a tetrad. Genetic material is exchanged in a process called crossing over, which results in genetic recombination.

5. Metaphase I: the tetrads line up randomly; spindle fibers appear and attach to the centromere.
Anaphase I: homologous chromosomes move to opposite poles. Since it is random, this separation is called independent assortment and thus results in genetic recombination.
Telophase I: The chromosomes reach the opposite side of the cell and cytokinesis begins.

6. Meiosis II DNA is not copied, and the nuclear membrane may or may not form before this stage begins.
Prophase II: spindle fibers form and begin to move chromosomes toward the midline of the cell.
Metaphase II: chromosomes move to the midline, facing opposite poles. (Very orderly.)
Anaphase II: chromatids separate; proceed to opposite poles.
Telophase II: nuclear membrane forms around chromosomes in 4 new cells. Cytokinesis occurs.

8. Formation of Gametes
In animals, meiosis produces haploid reproductive cells called gametes.
Meiosis occurs only within reproductive organs, testes and ovaries.
In the testes, meiosis forms four haploid cells called spermatids.
Oogenesis is the production of mature egg cells, or ova.
As the original cell divides, the cytoplasm is not divided equally, so only one egg cell is produced by meiosis. The other three products are called polar bodies, and degenerate.

9. meiosis

10. Section 8-3 Review List two ways that meiosis differs from mitosis.
During which stage of meiosis is the diploid number of chromosomes reduced to the haploid number?
How many chromosomes do human gametes normally contain?
Explain the role of crossing-over in ensuring genetic variation.
Describe the primary differences between spermatogenesis an oogenesis.