Lecture 21: Meiosis Lecture outlie 10/25/05 Review mitosis Fiish cell cycle Mutatios i checkpoit gees ca lead to cacer Sexual ad asexual life cycles Meiosis Stages of meiosis Idepedet assortmet ad Crossig over produce geetic variatio produces a exact copy of the paret cell. Used for growth ad asexual reproductio. The huma life cycle Key Haploid () Diploid (2) Haploid gametes ( = 23) Ovum () Gametes are the oly haploid cells Meiosis produces reduced (haploid) gametes, which are geetically uique. Necessary for sexual reproductio. Meiosis occurs durig gamete formatio Sperm Cell () Ovary Testis Diploid zygote (2 = 46) ad developmet Figure 13.5 Multicellular diploid adults (2 = 46) 1
Examples of Asexual Reproductio I asexual reproductio oe paret produces geetically idetical offsprig by mitosis. (basically just brachig) Aspes coected udergroud Paret www.robertturerphoto.com Bud 0.5 mm Figure 13.2 Hydra Sibligs share 1/2 of their gees, o average Each get oe chromosome from mother ad oe from father, but they might ot both get the same chromosome from a paret. Dadelios make asexual seeds Diploid cells have two copies of each chromosome Homologous cotai copies of the same gees Oe materal, oe pateral 2
The huma life cycle Key Haploid () Diploid (2) Haploid gametes ( = 23) Ovum () Gametes are the oly haploid cells Plats ad some algae Exhibit a alteratio of geeratios The life cycle icludes both diploid ad haploid multicellular stages Meiosis occurs durig gamete formatio Sperm Cell () Haploid multicellular orgaism (gametophyte) Spores Gametes Ovary Testis Diploid zygote (2 = 46) ad developmet Diploid multicellular orgaism (sporophyte) 2 2 Zygote Figure 13.5 Multicellular diploid adults (2 = 46) Figure 13.6 B (b) Plats ad some algae I most fugi, ad some protists Meiosis produces haploid cells that give rise to a haploid multicellular adult orgaism The haploid adult carries out mitosis, producig cells that will become gametes Figure 13.6 C Haploid multicellular orgaism 2 Gametes Zygote (c) Most fugi ad some protists Germ meas seed Soma meas body Germlie vs Soma Meiosis oly occurs i germ cells. Produces gametes Eggs ad sperm Huma oocytes start meiosis before birth ad arrest i prophase 1; the they mature oe at a time at ovulatio Huma spermatocytes produce sperm cotiuously 28 d for meiosis aother 35 d to become mature sperm 3
Meiosis 1 vs Meiosis diploid Not paired I: Separates homologous INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Sister remai attached Chiasmata Sister Spidle Tetrad Homologs pair i meiosis 1 Sister i meiosis 2 haploid Meiosis 2 PROPHASE II Cleavage furrow are still double Figure 13.8 METAPHASE II ANAPHASE II Sister Figure 13.8 Chromosome codesatio ad pairig II: Separates sister TELOPHASE I AND CYTOKINESIS Chromosomes duplicate Homologous (red ad blue) pair ad exchage segmets; 2 = 6 i this example Homologous Pairs of homologous Tertads lie up split up TELOPHASE II AND CYTOKINESIS Haploid daughter cells formig The Syaptoemal Complex holds the homologs together Segmets of this cartoo show the same Chromosome at differet times i meiosis 4
What makes Meiosis differet from? Syapsis ad crossig over Homologous physically coect ad exchage gees Paired homologous at metaphase 1 Separatio of homologues, producig haploid gametes At aaphase I of meiosis, homologous pairs I aaphase II of meiosis, the sister Meiosis produces geetic variatio Radom aligmet of i meiosis 1 Idepedet assortmet of materal ad pateral ito gametes Crossig over Futher shufflig of gees withi 2 23 = 8x10 6 possible combiatios of chromssomes i a gamete Radom fusio of gametes will produce about 2 23 x 2 23 =64 trillio diploid combiatios! Idepedet Assortmet Key Each pair of sorts its materal ad pateral homologues ito daughter cells idepedetly of the other pairs Materal set of Possibility 1 Pateral set of Two equally probable arragemets of at metaphase I Possibility 2 Metaphase II Daughter cells Figure 13.10 Combiatio 1 Combiatio 2 Combiatio 3 Combiatio 4 5
Crossig Over Crossovers are radom Prophase I of meiosis Produces recombiat that carry gees derived from two differet parets Metaphase I Tetrad Nosister Chiasma, site of crossig over This tetrad has 3 crossovers: 2&3 1&3 2&4 Metaphase II Daughter cells Figure 13.11 Recombiat Spidle fibers from oe pole attach to BOTH sister Crossovers are essetial for correct aligmet at metaphase 1 Spidle fibers from the other pole attach to the other homolog Chiasmata hold the pair together while Tug of war Aligs tetrads 6