Meiosis

Meiosis Overview Meiosis - cell division in which gametes are made (eggs and sperm). The cells made will have half the number of chromosomes of the parent cell. Homologous – chromosomes with a corresponding partner. (one from mom & one from Dad)

Haploid – a cell that contains only one of each homologous chromosome pair; half the total.(sperm, egg) Diploid – a cell that contains both homologous chromosomes; full set.(body cells)

Meiosis I

Nucleus Interphase I Cell starts out as a DIPLOID cell. Either a SPERMATOCYTE or an OOCYTE Chromatin replicates. Cell Membrane

Prophase I Nucleus starts to disappear. Sister Chromatids pair up to form chromosomes, which now can be seen. Homologous Chromosomes pair up to form tetrads, (2 homologs together). 4. Centrioles move to the poles 5. Crossing-over can occur during the latter part of this stage.

Crossing Over Crossing over: the swapping of DNA which occurs between homologs. It gives us genetic variation or differences. This is one reason why you are your own unique person.

Metaphase I Spindle Fibers appear. Tetrads line up along the middle of the spindle. Fibers attach to centromeres. Spindle Fibers

Anaphase I Spindle fibers pull the tetrads apart, with sister chromatids remaining together. 2. Chromosomes are pulled to the centrioles.

Telophase I Spindle Fibers and centrioles disappear. Nucleus reappears around each of the new sets of chromosomes. Cell starts to form a break at the middle and starts to pinch in at the sides.

Telophase I (continued) Cytokinesis occurs

Continues with both cells, but this will only show one Meiosis II Continues with both cells, but this will only show one Movie

Prophase II 1. Chromosomes are already visible 2. Nucleus disappears 3. Centrioles move to the poles

Metaphase II 1. Spindle Fibers appear 2. Chromosomes are lined up along middle of cell

Anaphase II Spindle fibers pull sister chromatids to each pole.

Telophase II 1. Spindle fibers and centrioles disappear 2. A nucleus appears around each set of chromosomes 3. Cell starts to pinch in at the center

Telophase II (Continued) 1. Both cells divide (cytokinesis), leaving 4 new cells 2. Each with half the number of chromosomes the original cell had. 3. Each cell is now HAPLOID, called a gamete. 4. Gametes can be sperms or eggs.

Spermatogenesis- making sperm cells by meiosis. Makes 4 sperm

Oogenesis - making egg cells by meiosis. Makes 1 egg and 3 polar bodies Movie

Comparing Mitosis and Meiosis Happens all over body IPMAT Makes 2 cells Diploid cells—full set of chromosomes Make new body cells 1 set of divisions

Comparing Mitosis and Meiosis Reproductive organs IPMAT - PMAT Makes 4 cells Haploid—1/2 set of chromosomes Makes sex cells (Gametes) 2 sets of divisions Crossing over

Mitosis and Meiosis Mitosis allows for genetic continuity within an organism. Meiosis allows genetic variety during sexual reproduction.