Meiosis and Sexual Reproduction in Cell Division

Slide1AP Biology2007-2008 Meiosis & Sexual Reproduction

Slide2AP BiologyCell division / Asexual reproduction  Mitosis  produce cells with same information  identical daughter cells  exact copies  clones  same amount of DNA  same number of chromosomes  same genetic information Aaaargh! I ’ m seeing double!

Slide3AP BiologyAsexual reproduction  Single-celled eukaryotes  yeast (fungi)  Protists  Paramecium  Amoeba  Simple  multicellular eukaryotes  Hydra What are the disadvantages of asexual reproduction? What are the advantages? budding budding

Slide4AP BiologyHow about the rest of us?  What if a complex multicellular organism (like us) wants to reproduce?  joining of egg + sperm  Do we make egg & sperm by mitosis? 46 46 + 92 egg sperm zygote What if we did, then…. Doesn’t work! No!

Slide5AP BiologyHuman female  karyotype 46 chromosomes 23 pairs

Slide6AP BiologyHuman male  karyotype 46 chromosomes 23 pairs

Slide7AP BiologyHomologous chromosomes  Paired chromosomes  both chromosomes of a pair carry “matching” genes  control same inherited characters  homo logous  =  same   information diploid 2n 2n = 4 single stranded homologous chromosomes double stranded homologous chromosomes

Slide8AP BiologyHow do we make sperm & eggs?  Must reduce 46 chromosomes     23  must reduce the number of chromosomes by  half 23 23 46 egg sperm 46 meiosis 46 fertilization 23 23 gametes zygote

Slide9AP BiologyMeiosis: production of gametes  Alternating stages  chromosome number must be reduced  diploid      haploid  2n      n  humans: 46    23  meiosis  reduces chromosome number  makes  gametes  fertilization  restores chromosome number  haploid     diploid  n     2n haploid diploid

Slide10AP BiologySexual reproduction lifecycle  1 copy  haploid  1n  2 copies  diploid  2n  1 copy  haploid  1n meiosis fertilization In the next generation … We ’ re mixing things up here ! A good thing? gametes gametes

Slide11AP BiologyMeiosis  Reduction Division  special cell division for sexual reproduction  reduce 2n     1n  diploid    haploid  “two”    “half”  makes gametes  sperm, eggs Warning :  meiosis evolved from mitosis, so stages & “machinery” are similar but the processes are radically different. Do not confuse the two!

Slide12AP BiologyOverview of meiosis I.P.M.A.T.P.M.A.T interphase 1 prophase 1 metaphase 1 anaphase 1 telophase 1 prophase 2 metaphase 2 anaphase 2 telophase 2 2n = 4 n = 2 n = 2 n = 2

Slide13AP BiologyRepeat after me ! I can’t hear you ! 2nd division  of meiosis separates sister chromatids 1st division  of meiosis separates homologous pairs Double division of meiosis DNA replication Meiosis 1 Meiosis 2

Slide14AP Biology2n =  6 double stranded 2n =  6 single stranded Preparing for meiosis  1st step of meiosis  Duplication of DNA  meiosis evolved after mitosis  convenient to use “machinery” of mitosis  DNA replicated in S phase of  interphase of  MEIOSIS (just like in mitosis) M1 prophase

Slide15AP BiologyRepeat after me ! 2n =  4 single stranded Meiosis 1 2n =  4 double stranded prophase 1  1st division  of meiosis separates   homologous pairs tetrad synapsis 1n  =  2 double stranded telophase 1 2n =  4 double stranded metaphase 1 I can’t hear you ! reduction

Slide16AP BiologyMeiosis 2 1n  =  2 double stranded metaphase 2 1n  =  2 single stranded telophase 2 prophase 2 1n  =  2 double stranded  2nd division  of meiosis separates   sister chromatids What does this division look like? 4

Slide17AP BiologySteps of meiosis  Meiosis 1  interphase  prophase 1  metaphase 1  anaphase 1  telophase 1  Meiosis 2  prophase 2  metaphase 2  anaphase 2  telophase 2 2nd division  of meiosis separates sister chromatids (1n    1n) * just like mitosis * 1st division  of meiosis separates homologous pairs (2n    1n) “reduction division”

Slide18AP BiologyMeiosis 1 & 2

Slide19AP BiologyTrading pieces of DNA  Crossing over  during  Prophase 1 , sister chromatids intertwine  homologous pairs swap pieces of chromosome  DNA breaks & re-attaches tetrad synapsis prophase 1

Slide20AP BiologyCrossing over  3 steps  cross over  breakage of DNA  re-fusing of DNA  New combinations of traits What are the advantages of crossing over  in sexual reproduction?

Slide21AP BiologyMitosis vs. Meiosis

Slide22AP BiologyMitosis vs. Meiosis  Mitosis  1 division  daughter cells genetically  identical to parent cell  produces  2 cells  2n     2n  produces  cells for growth & repair  no crossing over  Meiosis  2 divisions  daughter cells genetically  different from parent  produces  4 cells  2n     1n  produces  gametes  crossing over

Slide23AP Biologymitosis zygote Putting it all together… 23 23 46 egg sperm 46 meiosis 46 23 23 fertilization development meiosis     fertilization    mitosis + development 46 46 46 46 46 46 46 46 gametes

Slide24AP Biology Sexual reproduction introduces genetic variation  genetic recombination  independent assortment  of chromosomes  random alignment of homologous chromosomes in Metaphase 1  crossing over  mixing of alleles across homologous chromosomes  random fertilization  which sperm fertilizes which egg?  Driving evolution  providing variation for natural selection The value of sexual reproduction metaphase1

Slide25AP BiologyVariation from  genetic recombination  Independent assortment of chromosomes  meiosis introduces genetic variation  gametes of offspring do not have same combination of genes as gametes from parents  random assortment in humans produces 2 23  (8,388,608) different combinations in gametes from Dad from Mom offspring new gametes made by offspring

Slide26AP BiologyVariation from  crossing over  Crossing over creates completely new combinations of traits on each chromosome  creates an  infinite variety in gametes

Slide27AP BiologyVariation from random fertilization  Sperm + Egg = ?  any 2 parents will produce a zygote with over 70 trillion (2 23  x 2 23 ) possible diploid combinations

Slide28AP BiologySexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences. Baldwin brothers Jonas Brothers Martin & Charlie Sheen, Emilio Estevez

Slide29AP BiologySperm production Epididymis Testis Coiled seminiferous tubules Vas deferens Cross-section of seminiferous tubule spermatozoa spermatids (haploid) secondary spermatocytes (haploid) primary spermatocyte (diploid) germ cell (diploid) MEIOSIS II MEIOSIS I  Spermatogenesis  continuous & prolific process  each ejaculation = 100-600 million sperm

Slide30AP BiologyEgg production Meiosis 1 completed during egg maturation Meiosis 2 completed triggered by fertilization ovulation  Oogenesis  eggs in ovaries halted before Anaphase 1  Meiosis 1 completed during maturation  Meiosis 2 completed after fertilization  1 egg + 2 polar bodies What is the advantage of this development system? unequal divisions

Slide31AP BiologyOogenesis MEIOSIS I MEIOSIS II first polar body second polar body ovum (haploid) secondary oocyte (haploid) primary oocyte (diploid) germinal cell (diploid) primary follicles mature follicle with secondary oocyte ruptured follicle (ovulation) corpus luteum developing follicle fertilization fallopian tube after fertilization Putting all your egg in one basket !

Slide32AP BiologyDifferences 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

Slide33AP Biology

Slide34AP Biology