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Ways to show the number of chromosomes in a cell.

DIPLOID. HAPLOID. n 1 copy of each chromosome Sex cells. 2n 2 copies of each chromosome Body cells. Ways to show the number of chromosomes in a cell. n. Definition: Cell division where cells divide twice. Done by:

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Ways to show the number of chromosomes in a cell.

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  1. DIPLOID HAPLOID • n • 1 copy of each • chromosome • Sex cells • 2n • 2 copies of each • chromosome • Body cells Ways to show the number of chromosomes in a cell.

  2. n Definition: Cell division where cells divide twice. Done by: Sex cells in multi-cellular organisms (diploid) Meiosis Variation: Crossing over – genes switch between homologs. Independent assortment – homologous chromosomes line up differently every time. Mutations Sexual reproduction Results in: 4 cells with ½ DNA (haploid cells) 2n Zygote (baby) n + =

  3. What Happens During Meiosis?

  4. MEIOSIS I • A diploid (2N) cell divides to produce 2 daughter cells. • Creates genetic variation.

  5. Prophase I • Homologous chromosomes pair up to form tetrads (groups of four chromatids). • Crossing over occurs when homologous chromosomes overlap.

  6. Metaphase I • Tetrads line up at the center of the cell.

  7. Anaphase I • One homologous chromosome is pulled to each side of the cell.

  8. Telophase I • Two daughter nuclei are formed. • Followed by cytokinesis and then immediately meiosis II. • NO replication!

  9. MEIOSIS II • Two cells each divide to form four haploid (N) gametes. • In females, one viable egg + three “polar bodies” • In males, four viable sperm

  10. Prophase II • Chromosomes condense in each cell.

  11. Metaphase II • In each cell, chromosomes line up in the middle.

  12. Anaphase II • In each cell, sister chromatids are pulled apart.

  13. Telophase II • 4 new nuclei form, 2 in each cell. • Followed by cytokinesis, resulting in 4 haploid (N) gametes.

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