Meiosis and Sexual Life Cycles

Transcription

1 Overview: Variatios o a Theme Chapter 13 Meiosis ad Sexual Life Cycles Livig orgaisms are distiguished by their ability to reproduce their ow kid Geetics is the scietific study of heredity ad variatio Heredity is the trasmissio of traits from oe geeratio to the ext Variatio is demostrated by the differeces i appearace that offsprig show from parets ad sibligs 13.1: Offsprig acquire gees from parets by iheritig I a literal sese, childre do ot iherit particular physical traits from their parets It is gees that are actually iherited 1

2 Iheritace of Gees Gees are the uits of heredity, ad are made up of segmets of DNA Gees are passed to the ext geeratio through reproductive cells called gametes (sperm ad eggs) Each gee has a specific locatio called a locus o a certai chromosome Most DNA is packaged ito Oe set of is iherited from each paret Compariso of Asexual ad Sexual Reproductio I asexual reproductio, oe paret produces geetically idetical offsprig by mitosis A cloe is a group of geetically idetical idividuals from the same paret I sexual reproductio, two parets give rise to offsprig that have uique combiatios of gees iherited from the two parets 0.5 mm Paret Bud (a) Hydra (b) Redwoods 2

3 13.2: Fertilizatio ad meiosis alterate i sexual life cycles A life cycle is the geeratio-to-geeratio sequece of stages i the reproductive history of a orgaism Two types of huma cells: Huma Cells Somatic cells (ay cell other tha a gamete) have 23 pairs of (2) Gametes (reproductive cells) have 23 () A karyotype is a ordered display of the pairs of from a cell The two i each pair are called homologous, or homologs Chromosomes i a homologous pair are the same legth ad carry gees cotrollig the same iherited characters APPLICATION APPLICATION TECHNIQUE 5 µm Pair of homologous replicated Cetromere Sister Metaphase chromosome 3

4 TECHNIQUE 5 µm Pair of homologous replicated Cetromere Sister Metaphase chromosome 4

5 The sex are called X ad Y Huma females have a homologous pair of X (XX) Huma males have oe X ad oe Y chromosome The 22 pairs of that do ot determie sex are called autosomes Each pair of homologous icludes oe chromosome from each paret The 46 i a huma somatic cell are two sets of 23: oe from the mother ad oe from the father 22 pr of autosomes, ad oe pr of sex A diploid cell (2) has two sets of For humas, the diploid umber is 46 (2 = 46) I a cell i which DNA sythesis has occurred, each chromosome is replicated Each replicated chromosome cosists of two idetical sister 2 = 6 Key Two sister of oe replicated chromosome Materal set of ( = 3) Pateral set of ( = 3) Cetromere Two osister i a homologous pair Pair of homologous (oe from each set) 5

6 A gamete (sperm or egg) cotais a sigle set of, ad is haploid () For humas, the haploid umber is 23 ( = 23) Each set of 23 cosists of 22 autosomes ad a sigle sex chromosome I a ufertilized egg (ovum), the sex chromosome is X I a sperm cell, the sex chromosome may be either X or Y Behavior of Chromosome Sets i the Huma Life Cycle Fertilizatio is the uio of gametes (the sperm ad the egg) The fertilized egg is called a zygote ad has oe set of from each paret The zygote produces somatic cells by mitosis ad develops ito a adult At sexual maturity, the ovaries ad testes produce haploid gametes Gametes are the oly types of huma cells produced by meiosis, rather tha mitosis Meiosis results i oe set of i each gamete Fertilizatio ad meiosis alterate i sexual life cycles to maitai chromosome umber The Variety of Sexual Life Cycles The alteratio of meiosis ad fertilizatio is commo to all orgaisms that reproduce sexually The three mai types of sexual life cycles differ i the timig of meiosis ad fertilizatio 6

7 I aimals, meiosis produces gametes, which udergo o further cell divisio before fertilizatio Gametes are the oly haploid cells i aimals Gametes fuse to form a diploid zygote that divides by mitosis to develop ito a multicellular orgaism Key Haploid () Diploid (2) Gametes MEIOSIS FERTILIZATION 2 Zygote 2 Diploid multicellular Mitosis orgaism Haploid multicellular orgaism (gametophyte) Mitosis Mitosis Mitosis Spores Gametes MEIOSIS FERTILIZATION MEIOSIS 2 2 Diploid Zygote multicellular Mitosis orgaism (sporophyte) 2 Zygote Haploid uicellular or multicellular orgaism Gametes Mitosis FERTILIZATION (a) Aimals (b) Plats ad some algae (c) Most fugi ad some protists Key Haploid () Diploid (2) MEIOSIS 2 Diploid multicellular orgaism Gametes Zygote 2 FERTILIZATION Mitosis 13.3: Meiosis reduces the umber of chromosome sets from diploid to haploid Like mitosis, meiosis is preceded by the replicatio of the DNA (S-Phase) Meiosis takes place i two sets of cell divisios, called meiosis I ad meiosis II The two cell divisios result i four daughter cells, rather tha the two daughter cells i mitosis Each daughter cell has oly half as may as the paret cell (a) Aimals 7

8 The Stages of Meiosis Iterphase Homologous pair of i diploid paret cell I the first cell divisio (meiosis I), homologous separate Meiosis I results i two haploid daughter cells with replicated ; it is called the reductioal divisio I the secod cell divisio (meiosis II), sister separate Meiosis II results i four haploid daughter cells with ureplicated ; it is called the equatioal divisio Chromosomes replicate Homologous pair of replicated Sister Diploid cell with replicated Iterphase Homologous pair of i diploid paret cell Iterphase Homologous pair of i diploid paret cell Chromosomes replicate Homologous pair of replicated Chromosomes replicate Homologous pair of replicated Sister Diploid cell with replicated Sister Diploid cell with replicated Meiosis I Meiosis I 1 Homologous separate 1 Homologous separate Haploid cells with replicated Haploid cells with replicated Meiosis II 2 Sister separate Haploid cells with ureplicated 8

9 Meiosis I is preceded by iterphase, i which are replicated to form sister The sister are geetically idetical ad joied at the cetromere The sigle cetrosome replicates, formig two cetrosomes Sister Homologous Prophase I Metaphase I Aaphase I Cetrosome (with cetriole pair) Chiasmata Spidle Fragmets of uclear evelope Cetromere (with kietochore) Metaphase plate Microtubule attached to kietochore Sister remai attached Homologous separate Cleavage furrow Telophase I ad Cytokiesis Prophase II Metaphase II Aaphase II Sister separate Telophase II ad Cytokiesis Haploid daughter cells formig Divisio i meiosis I occurs i four phases: Prophase I Metaphase I Aaphase I Telophase I ad cytokiesis Sister Prophase I Cetrosome (with cetriole pair) Chiasmata Spidle Metaphase I Cetromere (with kietochore) Metaphase plate Aaphase I Sister remai attached Telophase I ad Cytokiesis Homologous Fragmets of uclear evelope Microtubule attached to kietochore Homologous separate Cleavage furrow 9

10 Prophase I Prophase I typically occupies more tha 90% of the time required for meiosis Chromosomes begi to codese I syapsis, homologous loosely pair up, aliged gee by gee I crossig over, osister exchage DNA segmets Each pair of forms a tetrad, a group of four Each tetrad usually has oe or more chiasmata, X-shaped regios where crossig over occurred Prophase I Metaphase I Metaphase I I metaphase I, tetrads lie up at the metaphase plate, with oe chromosome facig each pole Sister Cetrosome (with cetriole pair) Chiasmata Spidle Cetromere (with kietochore) Metaphase plate Microtubules from oe pole are attached to the kietochore of oe chromosome of each tetrad Microtubules from the other pole are attached to the kietochore of the other chromosome Homologous Fragmets of uclear evelope Microtubule attached to kietochore 10

11 Aaphase I I aaphase I, pairs of homologous separate Oe chromosome moves toward each pole, guided by the spidle apparatus Sister remai attached at the cetromere ad move as oe uit toward the pole Telophase I ad Cytokiesis I the begiig of telophase I, each half of the cell has a haploid set of ; each chromosome still cosists of two sister Cytokiesis usually occurs simultaeously, formig two haploid daughter cells I aimal cells, a cleavage furrow forms; i plat cells, a cell plate forms No chromosome replicatio occurs betwee the ed of meiosis I ad the begiig of meiosis II because the are already replicated Aaphase I Sister remai attached Telophase I ad Cytokiesis Homologous separate Cleavage furrow 11

12 Divisio i meiosis II also occurs i four phases: Prophase II Metaphase II Aaphase II Telophase II ad cytokiesis Meiosis II is very similar to mitosis Prophase II Metaphase II Aaphase II Sister separate Telophase II ad Cytokiesis Haploid daughter cells formig Prophase II I prophase II, a spidle apparatus forms I late prophase II, (each still composed of two ) move toward the metaphase plate Metaphase II I metaphase II, the sister are arraged at the metaphase plate Because of crossig over i meiosis I, the two sister of each chromosome are o loger geetically idetical The kietochores of sister attach to microtubules extedig from opposite poles 12

13 Prophase II Metaphase II Aaphase II I aaphase II, the sister separate The sister of each chromosome ow move as two ewly idividual toward opposite poles Telophase II ad Cytokiesis I telophase II, the arrive at opposite poles Nuclei form, ad the begi to ucodese. Cytokiesis separates the cytoplasm At the ed of meiosis, there are four daughter cells, each with a haploid set of ureplicated Each daughter cell is geetically distict from the others ad from the paret cell 13

14 Aaphase II Sister separate Telephase II ad Cytokiesis Haploid daughter cells formig A Compariso of Mitosis ad Meiosis Mitosis coserves the umber of chromosome sets, producig cells that are geetically idetical to the paret cell Meiosis reduces the umber of sets from two (diploid) to oe (haploid), producig cells that differ geetically from each other ad from the paret cell The mechaism for separatig sister is virtually idetical i meiosis II ad mitosis MITOSIS MEIOSIS Paret cell Chiasma MEIOSIS I Prophase Replicated chromosome Chromosome replicatio 2 = 6 Chromosome replicatio Prophase I Homologous chromosome pair Property Mitosis SUMMARY Meiosis DNA replicatio Occurs durig iterphase before mitosis begis Occurs durig iterphase before meiosis I begis Metaphase Metaphase I Number of divisios Oe, icludig prophase, metaphase, aaphase, ad telophase Two, each icludig prophase, metaphase, aaphase, ad telophase Syapsis of homologous Does ot occur Occurs durig prophase I alog with crossig over betwee osister ; resultig chiasmata hold pairs together due to sister chromatid cohesio Aaphase Telophase 2 2 Daughter cells of meiosis I Aaphase I Telophase I Haploid = 3 MEIOSIS II Number of daughter cells ad geetic compositio Role i the aimal body Two, each diploid (2) ad geetically idetical to the paret cell Eables multicellular adult to arise from zygote; produces cells for growth, repair, ad, i some species, asexual reproductio Four, each haploid (), cotaiig half as may as the paret cell; geetically differet from the paret cell ad from each other Produces gametes; reduces umber of by half ad itroduces geetic variability amog the gametes Daughter cells of mitosis Daughter cells of meiosis II 14

15 Three evets are uique to meiosis, ad all three occur i meiosis l: Syapsis ad crossig over i prophase I: Homologous physically coect ad exchage geetic iformatio At the metaphase plate, there are paired homologous (tetrads), istead of idividual replicated At aaphase I, it is homologous, istead of sister, that separate Sister chromatid cohesio allows sister of a sigle chromosome to stay together through meiosis I Protei complexes called cohesis are resposible for this cohesio I mitosis, cohesis are cleaved at the ed of metaphase I meiosis, cohesis are cleaved alog the chromosome arms i aaphase I (separatio of homologs) ad at the cetromeres i aaphase II (separatio of sister ) 13.4: Geetic variatio produced i sexual life cycles cotributes to evolutio Mutatios (chages i a orgaism s DNA) are the origial source of geetic diversity Mutatios create differet versios of gees called alleles Reshufflig of alleles durig sexual reproductio produces geetic variatio Origis of Geetic Variatio Amog Offsprig The behavior of durig meiosis ad fertilizatio is resposible for most of the variatio that arises i each geeratio Three mechaisms cotribute to geetic variatio: Idepedet assortmet of Crossig over Radom fertilizatio 15

16 Idepedet Assortmet of Chromosomes Homologous pairs of oriet radomly at metaphase I of meiosis I idepedet assortmet, each pair of sorts materal ad pateral homologues ito daughter cells idepedetly of the other pairs The umber of combiatios possible whe assort idepedetly ito gametes is 2, where is the haploid umber For humas ( = 23), there are more tha 8 millio (2 23 ) possible combiatios of Possibility 1 Possibility 2 Two equally probable arragemets of at metaphase I Possibility 1 Possibility 2 Two equally probable arragemets of at metaphase I Metaphase II 16

17 Possibility 1 Possibility 2 Two equally probable arragemets of at metaphase I Metaphase II Crossig Over Crossig over produces recombiat, which combie gees iherited from each paret Crossig over begis very early i prophase I, as homologous pair up gee by gee Daughter cells Combiatio 1 Combiatio 2 Combiatio 3 Combiatio 4 I crossig over, homologous portios of two osister trade places Crossig over cotributes to geetic variatio by combiig DNA from two parets ito a sigle chromosome Prophase I of meiosis Pair of homologs Nosister held together durig syapsis 17

18 Prophase I of meiosis Pair of homologs Nosister held together durig syapsis Prophase I of meiosis Pair of homologs Nosister held together durig syapsis Chiasma Chiasma Cetromere Cetromere TEM TEM Aaphase I Prophase I of meiosis Pair of homologs Nosister held together durig syapsis Prophase I of meiosis Pair of homologs Nosister held together durig syapsis Chiasma Chiasma Cetromere Cetromere TEM TEM Aaphase I Aaphase I Aaphase II Aaphase II Daughter cells Recombiat 18

19 Radom Fertilizatio Radom fertilizatio adds to geetic variatio because ay sperm ca fuse with ay ovum (ufertilized egg) The fusio of two gametes (each with 8.4 millio possible chromosome combiatios from idepedet assortmet) produces a zygote with ay of about 70 trillio diploid combiatios Crossig over adds eve more variatio Each zygote has a uique geetic idetity The Evolutioary Sigificace of Geetic Variatio Withi Populatios Natural selectio results i the accumulatio of geetic variatios favored by the eviromet Sexual reproductio cotributes to the geetic variatio i a populatio, which origiates from mutatios Prophase I: Each homologous pair udergoes syapsis ad crossig over betwee osister. Metaphase I: Chromosomes lie up as homologous pairs o the metaphase plate. Aaphase I: Homologs separate from each other; sister remai joied at the cetromere. 19

20 F H You should ow be able to: 1. Distiguish betwee the followig terms: somatic cell ad gamete; autosome ad sex ; haploid ad diploid 2. Describe the evets that characterize each phase of meiosis 3. Describe three evets that occur durig meiosis I but ot mitosis 4. Name ad explai the three evets that cotribute to geetic variatio i sexually reproducig orgaisms 20