Meiosis Meiosis Overview Meiosis cell division in which

Meiosis

Meiosis Overview • Meiosis - cell division in which gametes are made (eggs and sperm). • Occurs at the start of puberty • The cells made will have half the number of chromosomes (haploid n)

• Haploid –half the total of the number of the chromosomes(sperm and egg cells are haploid) • Diploid – a cell that contains full set of chromosomes(body cells)

Meiosis I

Interphase I Nucleus 1. Cell starts out as a DIPLOID cell. 2. DNA in chromosomes replicates. Cell Membrane

Prophase I 1. Nuclear membrane disappears and the nucleolus disappears 2. Homologous Chromosomes pair up to form tetrads, (2 chromosomes together). 4. Crossing-over can occur during 5. Centrioles move to the poles

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 1. Spindle Fibers appear. 2. Tetrads line up along the middle of the spindle. 3. Fibers attach to centromeres. Spindle Fibers

Anaphase I 1. Spindle fibers pull the tetrads apart, with sister chromatids remaining together. 2. Chromosomes are pulled to the centrioles.

Telophase I 1. Spindle Fibers and centrioles disappear. 2. Nucleus reappears around each of the new sets of chromosomes. 3. Cell starts to form a break at the middle and starts to pinch in at the sides.

Telophase I (continued) 4. Cytokinesis occurs

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 1. 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 Movie

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

Comparing Mitosis and Meiosis Mitosis 1. Happens all over body 2. PMAT 3. Makes 2 cells 4. Diploid cells—full set of chromosomes 5. Body cells (Somatic cells) 6. 1 set of divisions

Comparing Mitosis and Meiosis 1. Reproductive organs 2. PMAT 3. Makes 4 cells 4. Haploid— 1/2 set of chromosomes 5. Sex cells (Gametes) 6. 2 sets of divisions 7. Crossing over