1 meiosis 2 Making haploid cells 3 Most

1 meiosis

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Making haploid cells 3 • Most body cells are diploid (2 n) and undergo mitosis. • Gamete – reproductive cell is a haploid (n). • In humans these are eggs and sperms. • The gametes are found in organisms which reproduce sexually. • Then fertilization occurs when two gametes fuse to form the first cell of an organism.

4 23 23 46 The gametes form from germ cells through a process known as meiosis

meiosis Two stage form of nuclear division in which the number of chromosomes is halved Meiosis I – first division – reduces the number of chromosomes by half 5

Prophase I is the st 1 stage 6 • Chromosomes condense • The homologous chromosomes pair up. • Matching pairs of chromosomes – one from mom and one from dad. • The nuclear membrane disappears. • Spindle fibers form.

Crossing over occurs • Exchange of DNA segments on homologous chromosomes. • Creates unique chromosomes with each containing genetic material from each parent. • Contributes to evolution and adaptation 7

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Metaphase I Homologous pairs of chromosomes move to the middle of the cell 9

Anaphase I Homologous chromosomes are separated (pulled apart) by the spindle fibers. 10

Telophase I • 2 diploid cells form • Spindle fibers disappear 11

Meiosis II 12 2 nd division that separates copies of the chromosome CHROMOSOMES DO NOT DUPLICATE!

Prophase II • Spindle fibers form • Nuclear membrane disappears • NO CROSSING OVER 13

Metaphase II Chromosomes line up in the middle of the cell 14

Anaphase II Sister chromatids are separated 15

Telophase II • Nucleus reforms • Chromosomes uncoil forming chromatin • Spindle fibers disappear 16

Cytokinesis Cell division Cytoplasm pinches in half Haploid: 23 Chromosomes 17

Results of meiosis. 18 • 4 unique haploid cells • In males – 4 sperms • In females – 1 egg and 3 polar bodies (non-reproductive cell)

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Pg 344 21

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Compare Mitosis & Meiosis: pg 396 - 397 23 Mitosis: Meiosis: Same: Pg 345 - 349 Pg 394 - 395