Meiosis Sex cell (gamete) division= egg and sperm Sex cell (gamete) division= egg and sperm Products= 4 cells all haploid, genetically different Products= 4 cells all haploid, genetically different How is this different from mitosis? How is this different from mitosis? Mitosis=2 identical cells! (clones) Mitosis=2 identical cells! (clones)

Diploid Vs. Haploid Diploid= 2n, the # of chromosomes contained in “body” cells Diploid= 2n, the # of chromosomes contained in “body” cells Haploid= n, the # of chromosomes found in sex cells (gametes) Haploid= n, the # of chromosomes found in sex cells (gametes) Why do sex cells have to be haploid? Why do sex cells have to be haploid? They must have half the # of chromosomes to produce a viable gamete (23 mom+ 23 dad= zygote) They must have half the # of chromosomes to produce a viable gamete (23 mom+ 23 dad= zygote)

Quick Thinking… If a haploid cell in a frog is 13, what is the diploid cell? If a haploid cell in a frog is 13, what is the diploid cell? If the muscle cell of a dog has 78 chromosomes, what does it’s egg cell have? If the muscle cell of a dog has 78 chromosomes, what does it’s egg cell have? 39 39

Here’s Some More…. I If Diploid= 8 Then Haploid= 4 If Haploid=70 Then Diploid= 140 If Diploid= 46 Then Haploid= 23 If Haploid = 56 Then Diploid= 112

o In order to create haploid cells, 2 divisions must take place o Meiosis I and Meiosis II

Phases of Meiosis Prophase I: Prophase I: Chromosomes coil Chromosomes coil Spindle appears and moves Spindle appears and moves Nuclear membrane disappears Nuclear membrane disappears Homologous chromosomes pair up into tetrads Homologous chromosomes pair up into tetrads Crossing over occurs =each tetrad “swaps” genes; provides for genetic variation Crossing over occurs =each tetrad “swaps” genes; provides for genetic variation Just like mitosis Meiosis Only! TETRAD

Crossing Over!!! 2 Homologous Chromosomes swap DNA 2 Homologous Chromosomes swap DNA Increase genetic diversity Increase genetic diversity

Metaphase I Chromosomes move to equator Chromosomes move to equator Spindle attaches to centromeres Spindle attaches to centromeres Chromosomes line up as PAIRS (tetrads) at the plate Chromosomes line up as PAIRS (tetrads) at the plate

Anaphase I Spindle fibers shorten and begin to pull on centromeres Pairs are separated, not the chromosome!!

Telophase I Chromosomes reach the poles Chromosomes reach the poles Nuclear membrane reforms Nuclear membrane reforms Spindle Dissolves Spindle Dissolves DNA does not unwind, it stays in chromosome form for the next set of division DNA does not unwind, it stays in chromosome form for the next set of division 2 cells not identical… why? 2 cells not identical… why? Do the 2 daughter cells undergo interphase II?

NO INTERPHASE II The DNA stays wound for round 2 of meiosis for one reason. The DNA stays wound for round 2 of meiosis for one reason. We now have 2 diploid chromosomes, we must divide again to reach the haploid state. We now have 2 diploid chromosomes, we must divide again to reach the haploid state. We do not replicate the DNA again… We do not replicate the DNA again…

Can you guess the next stages? Prophase II Prophase II Metaphase II Metaphase II Anaphase II Anaphase II Telophase II Telophase II Cytokinesis Cytokinesis All look similar to mitosis, sisters get split in Anaphase II like mitosis!!! All look similar to mitosis, sisters get split in Anaphase II like mitosis!!!

Spermatogenesis vs. Oogenesis Sperm= 4 viable haploid (23) sperm Sperm= 4 viable haploid (23) sperm Egg= 1 viable egg (23), three polar bodies Egg= 1 viable egg (23), three polar bodies

Meiosis = Genetic Variation 1) Independent Assortment (p. 144) -all pairs separate independently -gives approx. 8 million variations 2) Crossing Over (prophase I) -adds recombination -limitless variations 3) Random Fertilization - gametes from independent organisms squares the variations (64 million)

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