Key Concepts Meiosis nd the Sexul Life Cycle Heredity Trnsmission of trits from one genertion to the next Genetic Vrition Sexul Life Cycle: lterntion etween fertiliztion nd meiosis Diploid vs Hploid Need to understnd the difference etween Mitosis nd Meiosis 1
sexul vs. Sexul reporduction sexul Reproduction CLONES Involves Mitosis: 1 2 identicl progeny Sexul Reproduction Provides Diversity 2 geneticlly distinct cells 1 unique offspring (involves meiosis) Gmetes: reproductive cell Hploid: 1 complete set of chromosomes 1n = 1 set of chromosomes 2n = 2 sets of chromosomes (Diploid) 2 gmetes (1n ech) fuse fertiliztion Zygote (2n) 2
Chromosomes Humns : n = 23 ech with unique in shpe nd size 22 utosomes 1 sex chromosome X or Y Diploid (2n) = 46 chromosomes Somtic cells : 2n Gmetes : 1n 3
Chromosomes Kryotype Ordered visul representtion of the chromosomes in cell Chrcteristics: sed on length, centromere position, nd stining pttern! Norml: 46, X Y norml: 47, X Y, +21 4
Chromosomes Using Kryotype: to Scn for genetic normlities using FISH (Fluorescent in situ hyridiztion) Fluorescent DN proes which hyridize to chromosoml DN 5 -GTCTCCGC-3 3 -TCGGGCTG-5 norml: 47, X Y, +21 5
Chromosomes Somtic Cells : 22 pirs of utosomes (non-sex chromosomes) 2 sex chromosomes (XX or XY) Homologous Chromosomes (Very similr ut not identicl!) B C c Sme order of genes (importnt!!!) Length is the sme Position of centromere is the sme DN is not identicl WHY? Mternl nd pternl contriution Whts the dvntge of hving 2 copies? Increse Diversity, Redundncy promotes Viility 6
Meiosis: Overview process to mke Hploid Gmetes 2n 1n 1 set of ll chromosomes in gmetes Increse diversity S Sister chromtids Meiosis I Homologs line up t Metphse (I) plte Cross-over Meiosis II (very similr to mitosis) Sister chromtids seprte n I Meiosis II 2 identicl progeny 4 cells ll 1n Cross over Telo I 7
Meiosis increses DIVERSITY (1) Independent ssortment of Chromosomes KEY* - In Metphse I ech sister chromtids my line up rndomly mternl or pternl pir my orient fcing different spindle poles Key Mternl set of chromosomes Possiility 1 Pternl set of chromosomes Two eqully prole rrngements of chromosomes t metphse I Possiility 2 2 n = 4 2 23 = > 8 million Metphse II Dughter cells Comintion 1 Comintion 2 Comintion 3 Comintion 4 8
Meiosis increses DIVERSITY (2) Recomintion cross over etween homologous chromosomes. Synpsis (DN exchnge) M P B B B B Sister chromtids Tetrd Chism (physicl site of cross over) Recomintion 4 different Gmetes B B B B 9
Meiosis increses DIVERSITY Detils of Synpsis in Prophse of 1 of Meiosis 1. Homologous chromosomes pir up 2. DN info is trded etween homologs DN intertwines : Chism Cross over events Enzyme cuts DN t Chism nd rejoins them ck together HOMOLOGOUS RECOMBINGTION 3. Recominnts re different from ech other Prt mternl Prt pternl 4. Increses DIVERSITY!!! 5. 4 different Gmetes 10
Stges of Meiosis Meiosis I Prophse I Chromosomes condensed Piring of homologs Cross over/ intertwining t the Chism Nucler envelope is intct WHY? Enzymes cut DN nd rejoin (tkes long time weeks!) When completed: Recominnts mde t the end of Prophse I nucler envelope disssemles Metphse I Homologs line up t the metphse plte nphse I nd Telophse I Homologs seprted Results in 2 dughters: different, with recominnt chromosomes Does not go ck into interphse Does not reform nucler envelope Does not decondense DN Go directly to Meiosis II 11
Stges of Meiosis Meiosis II no S Phse Strt with 2 cells Prophse II nother spindle ssemles t right ngle to previous one The rest is similr to Mitosis Metphse II Sister chromtids line up t the metphse plte nphse II nd Telophse II Sister chromtids seprted Results in 4 cells (gmetes): different, with recominnt chromosomes Gmetes ll 1n! 12
Meiosis INTERPHSE MEIOSIS I: Seprtes homologous chromosomes PROPHSE I METPHSE I NPHSE I Centrosomes (with centriole pirs) Sister chromtids Chismt Spindle Metphse plte Sister chromtids remin ttched Chromtin Tetrd Homologous chromosomes seprte Chromosomes duplicte Homologous chromosomes Tertds line up Pirs of homologous chromosomes split up Figure 13.8 13
Meiosis MEIOSIS II: Seprtes sister chromtids TELOPHSE I ND CYTOKINESIS PROPHSE II METPHSE II NPHSE II TELOPHSE II ND CYTOKINESIS Clevge furrow Sister chromtids seprte Hploid dughter cells forming Two hploid cells form; chromosomes re still doule Figure 13.8 During nother round of cell division, the sister chromtids finlly seprte; four hploid dughter cells result, contining single chromosomes 14
Compre nd Contrst Meiosis nd Mitosis DN Repliction Both strt here Meiosis: no S phse etween I nd II # of Divisions Mitosis: 1 cells identicl Meiosis: 2 cells different Synpsis Not in Mitosis, only in Meiosis I (Prophse I) Line up t plte Mitosis: sister chromtids Meiosis I: homologs Meiosis II: sister chromtids Dughter cells Mitosis: 2 identicl cells Meiosis: 4 unique (hploid 1n) When 2 unique gmetes fuse: Zygote Increses Diversity!! 15