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Published byFlorence White Modified over 9 years ago
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Meiosis Sex cell (gamete) division= egg and sperm
Products= 4 cells all genetically different How is this different from mitosis? Mitosis=2 identical cells! (clones) Genes are located on the chromosomes.
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Chromosome number Homologous Chromosome – The chromosomes that make up a pair, one chromosome from each parent. Diploid= 2n, the # of chromosomes contained in “body” cells Gamete – sex cells that have half the number of chromosomes. Haploid – n, the # of chromosomes found in sex cells (gametes)
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Diploid Vs. 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)
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Quick Thinking… If a haploid cell in a frog is 13, what is the diploid cell? 26 If the muscle cell of a dog has 78 chromosomes, what does it’s egg cell have? 39
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Here’s Some More…. If Diploid= 8 Then Haploid= 4 If Haploid=70
Then Diploid= 140 If Diploid= 46 Then Haploid= 23 If Haploid = 56 Then Diploid= 112
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Phases of Meiosis Meiosis – cell division that reduces the # chromosomes, referred to as “reduction division”. Meiosis I: Interphase – chromosomes replicate (DNA) Prophase I Metapase I Anaphase I Telophase I
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Prophase I same as mitosis PLUS…
tetrads -2 homologous chromosomes pair up. Crossing over occurs =each tetrad “swaps” genes; provides for genetic variation
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Crossing Over!!!
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Metaphase I Same as mitosis, but the chromosomes line up as PAIRS (tetrads) at the plate
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Anaphase I Same as mitosis, except the pairs are separated, not the chromosome!!
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Telophase I Same as mitosis, except…
DNA does not unwind, it stays in chromosome form for the next set of division 2 cells not identical… why? Do the 2 daughter cells undergo interphase II?
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NO INTERPHASE II 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 do not replicate the DNA again…
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Can you guess the next stages?
Prophase II Metaphase II Anaphase II Telophase II Cytokinesis All look similar to mitosis, sisters get split in Anaphase II like mitosis!!!
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Prophase II Chromosomes condense. Spindles form in each new cell.
Spindle fibers attach to chromosomes
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Metaphase II Centromeres of chromosomes line up randomly at the equator of each cell
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Anaphase II Centromeres split Sister chromatids separate and move
to opposite poles
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Telophase II 4 nuclei form around chromosomes. Spindles break down.
Cell divides
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End Result…Finally! Sperm= 4 viable haploid (23) sperm
Egg= 1 viable egg (23), three polar bodies
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Meiosis = Genetic Variation
Independent Assortment -all pairs separate independently -gives approx. 8 million variations Crossing Over (prophase I) -adds recombination -limitless variations Random Fertilization - gametes from independent organisms squares the variations (64 million)
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