MEIOSIS

Objectives Compare the end products of meiosis with those of mitosis. Summarize the events of meiosis I. Explain crossing-over and how it contributes to the production of unique individuals. Summarize the events of meiosis II. Compare spermatogenesis and oogenesis. Define sexual reproduction.

Formation of Haploid Cells Meiosis is a process of nuclear division that reduces the number of chromosomes in new cells to half the number in the original cell. Meiosis produces haploid gametes.

Meiosis leads to four haploid cells (gametes) rather than two diploid cells as in mitosis.

Meiosis I 1.prophase I 2.metaphase I 3.anaphase I 4.telophase I.

Prophase I DNA coils tightly into chromosomes. Spindle fibers appear. The nucleolus and nuclear membrane disassemble. Every chromosome lines up next to its homologue.(Synapsis) Each pair of homologus chromosome is called a Tetrad

Crossing-over, which is when portions of homologous chromosomes exchange genetics material, occurs during prophase I and results in genetic recombination.

Metaphase I Tetrads line up randomly along the midline of the dividing cell. Spindle fibers from one pole attach to the centromere of the homologus chromosome.

Anaphase I Each homologus chromosome moves towards one pole of the cell. The random separation of chromosomes is called independent assortment. Independent assortment results in genetic variation.

Telophase I Chromosomes reach end poles of the cells. Two nuclei are formed with haploid number of chromosomes. Cytokinesis I begins.

Meiosis II includes prophase II, metaphase II, anaphase II, and telophase II.

Spermatogenesis is the process by which sperm cells are produced. Oogenesis is the process that produces mature egg cells.