chapter 13 - Why Sex? ".most organisms have sex because the...

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1 Why Sex? “…most organisms have sex because the resulting genetic recombination allows Darwinian selection to work better [I]n water work better… [I]n water fleas, recombination does lead to fewer deleterious mutations…” Nielsen, Science 311: 2006 Water fleas Daphnia pulex may shift from being sexual to being asexual. Drawing by J.J. Paschoud, late 1700s Meiosis and Sexual Life Cycles Human genome Æ 30,000 genes, diverse alleles have evolved over time The human primate reproduces sexually asexual vs. sexual reproduction Asexual Æ one parent produces genetically identical offspring by mitosis or cell fission Sexual Æ genes from different parents combine and form genetically distinct and diverse offspring bacteria amoeba hydra macrogamete microgamete Facts Asexual Organisms Sexual Organisms Many unicellular Small er in size Short er generation time Some unicellular, most multicellular Larg er in size Low er genetic variability Short-term source of genetic variability Æ horizontal gene transfer , mutation rate Long er generation time High er genetic variability Short-term source of genetic variability Æ recombination , chromosome assortment pair of homologous chromosomes human chromosomes Æ mosaic of genes and alleles chromatids centromere Haploid ( n ) Diploid (2 n ) Haploid gametes ( n = 23) Ovum ( n ) Sperm cell ( n ) FERTILIZATION MEIOSIS A Life Cycle haploid n Testis Ovary Mitosis and development Multicellular diploid adults (2 n = 46) Diploid zygote (2 n = 46) diploid 2n
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2 Variety of Sexual Cycles Haploid Diploid Gametes n FERTILIZATION MEIOSIS n n Mitosis Diploid multicellular organism Zygote 2 n 2 n Animals Variety of Sexual Cycles Haploid Diploid Gametes n FERTILIZATION MEIOSIS n n n n n n n Gametes Mitosis Mitosis Spores Haploid multicellular organism (gametophyte) Diploid multicellular organism (sporophyte) Mitosis Diploid multicellular organism Zygote 2 n 2 n Animals Plants and some algae FERTILIZATION MEIOSIS Zygote Mitosis 2 n 2 n Variety of Sexual Cycles Haploid Diploid Gametes n FERTILIZATION MEIOSIS n n n n n n n n n n n n Gametes Gametes Mitosis Mitosis Mitosis Mitosis Spores Haploid multicellular organism (gametophyte)
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This note was uploaded on 04/16/2008 for the course BIO 131 taught by Professor Paz y mino during the Fall '08 term at UMass Dartmouth.

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chapter 13 - Why Sex? ".most organisms have sex because the...

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