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Meiosis
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Vocab
Heredity – the passing of traits/genes from one generation to the next
Genetics – the study of heredity
Genes – segments of DNA that code for a specific trait
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Cell Division / Asexual Reproduction
Asexual reproduction – 1 parent ….identical offspring (clones)*single celled eukaryotes…..yeast, paramecium, amoeba*simple multicellular eukaryotes...hydra, jellyfish, *binary fission……bacteria *mitosis ……identical daughter cells (clones), same # of chromosomes, same DNA
buddingbudding
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Sexual reproduction – 2 parents = diff offspring
+ =
What are the disadvantages of asexual reproduction?What are the advantages to asexual reproduction?
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Karyotype – picture of 23 pairs of chromosomes (46)
Homologous chromosomes – identical chromosomes…same length, centromere, same staining pattern…exception…males sex chromosome
1-22 autosomes 23 sex chromsomes
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Human female karyotype
46 chromosomes23 pairs
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Human male karyotype
46 chromosomes23 pairs
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Homologous chromosomes• Paired chromosomes– both chromosomes of a pair carry “matching” genes
• control same inherited characters• homologous = same information
diploid2n2n = 4
single stranded homologouschromosomes
double strandedhomologous chromosomes
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How do we make sperm & eggs?• Must reduce 46 chromosomes 23
23
2346
egg
sperm
46
meiosis 46
fertilization
23
23
gametes
zygote
Gametes – single set of 22 chromosomes 1 sex chromosome…(n)=23
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Meiosis: production of gametes• Happens in gonads – Conserves chromosome #
• diploid haploid• 2n n
– humans: 46 23• meiosis reduces chromosome
number• makes gametes
– fertilization restores chromosome number• haploid diploid• n 2n
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Meiosis I – Division of homologous pairs
Interphase I : -G1, S, G2 *replicates only once!
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Prophase I : -chromosomes condense & form homologous chromosomes called tetrads or bivalents-crossing over occurs forming chiasmata-synapsis allows for maternal chromsomes to pair up w/ its paternal homolog-centrosomes move to poles, meiotic spindle forms-nuclear membrane disappears -90% of time
Meiosis I – Division of homologous pairs
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Meiosis I – Division of homologous pairs
Metaphase I : -homologous pairs align at metaphase plate-kinetochore microtubules attach to one chromosome of each pair
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Meiosis I – Division of homologous pairs
Anaphase I : -homologous chromosomes move toward opposite poles*separate the pair
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Meiosis I – Division of homologous pairs
Telophase I & Cytokinesis : -two new nuclei form-homologous chromosomes form into chromatin -nuclear membrane reappears-divide cytoplasm
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Meiosis I Animation
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Meiosis II – Division of sister chromatids
Interphase II : -G1, G2 * No replication!!
Prophase II : -spindle apparatus forms-sister chromatids become visible -move toward metaphase plate -nuclear membrane disappears
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Meiosis II – Division of sister chromatids
Metaphase II : -chromosomes on metaphase plate -kinetochore microtubules attach to sister chromatids of each chromosome
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Meiosis II – Division of sister chromatids
Anaphase II : -sister chromatids move toward opp poles
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Meiosis II – Division of sister chromatids
Telophase II : -nuclear membrane reforms -chromatin forms -cytokinesis produces 4 haploid (n) cells
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Meiosis II Animation
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Meiosis I and II
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Meiosis 1
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Meiosis 2
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One way meiosis generates genetic variability is through the different ways in which maternal and paternal chromosomes are combined in the daughter cells.
The number of possible chromosome combinations in the haploid nuclei is potentially very large. In general, the number of possible chromosome combinations is 2n, where n is the number of chromosome pairs.
For example, in fruit flies, which have 4 chromosome pairs, the number of possible combinations is 2n, or 16. For humans, with 23 chromosome pairs, there are over 8 million metaphase arrangements. 2 parents = zygote : 223 x 223 = over 70 trillion
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3 Sources of Genetic Variability
1. Independent Assortment of homologous chromosome in meiosis I 2. Crossing over during prophase I 3. Random fertilization of egg and sperm
http://www.pbs.org/wgbh/nova/body/how-cells-divide.html
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Differences across kingdoms• Not all organisms use haploid & diploid stages in
same way– which one is dominant (2n or n) differs– but still alternate between haploid & diploid
• must for sexual reproduction
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http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter12/animations.html#
http://www.youtube.com/watch?v=0gF8bCE4wqA&safe=activeHela Cell