Bài giảng Biology - Chapter 13: Meiosis and Sexual Life Cycles

Chapter 13Meiosis and Sexual Life CyclesOverview: Hereditary Similarity and VariationLiving organismsAre distinguished by their ability to reproduce their own kindHeredityIs the transmission of traits from one generation to the nextVariationShows that offspring differ somewhat in appearance from parents and siblingsFigure 13.1GeneticsIs the scientific study of heredity and hereditary variationConcept 13.1: Offspring acquire genes from parents by inheriting chromosomesInheritance of GenesGenesAre the units of heredityAre segments of DNAEach gene in an organism’s DNAHas a specific locus on a certain chromosomeWe inheritOne set of chromosomes from our mother and one set from our fatherComparison of Asexual and Sexual Reproduction In asexual reproductionOne parent produces genetically identical offspring by mitosisFigure 13.2ParentBud0.5 mm In sexual reproductionTwo parents give rise to offspring that have unique combinations of genes inherited from the two parentsConcept 13.2: Fertilization and meiosis alternate in sexual life cyclesA life cycleIs the generation-to-generation sequence of stages in the reproductive history of an organismSets of Chromosomes in Human CellsIn humansEach somatic cell has 46 chromosomes, made up of two setsOne set of chromosomes comes from each parent5 µmPair of homologouschromosomesCentromereSisterchromatidsFigure 13.3A karyotypeIs an ordered, visual representation of the chromosomes in a cellHomologous chromosomesAre the two chromosomes composing a pairHave the same characteristicsMay also be called autosomesSex chromosomesAre distinct from each other in their characteristicsAre represented as X and Y Determine the sex of the individual, XX being female, XY being maleA diploid cellHas two sets of each of its chromosomesIn a human has 46 chromosomes (2n = 46)In a cell in which DNA synthesis has occurredAll the chromosomes are duplicated and thus each consists of two identical sister chromatidsFigure 13.4KeyMaternal set ofchromosomes (n = 3)Paternal set ofchromosomes (n = 3)2n = 6Two sister chromatidsof one replicatedchromosomeTwo nonsisterchromatids ina homologous pairPair of homologouschromosomes(one from each set)CentromereUnlike somatic cellsGametes, sperm and egg cells are haploid cells, containing only one set of chromosomesBehavior of Chromosome Sets in the Human Life CycleAt sexual maturityThe ovaries and testes produce haploid gametes by meiosisDuring fertilizationThese gametes, sperm and ovum, fuse, forming a diploid zygoteThe zygoteDevelops into an adult organismFigure 13.5KeyHaploid (n)Diploid (2n)Haploid gametes (n = 23)Ovum (n)SpermCell (n)MEIOSISFERTILIZATIONOvaryTestisDiploidzygote(2n = 46)Mitosis anddevelopmentMulticellular diploidadults (2n = 46) The human life cycleThe Variety of Sexual Life CyclesThe three main types of sexual life cyclesDiffer in the timing of meiosis and fertilization In animalsMeiosis occurs during gamete formationGametes are the only haploid cellsGametesFigure 13.6 ADiploidmulticellularorganismKeyMEIOSISFERTILIZATIONnnn2n2nZygoteHaploidDiploidMitosis(a) AnimalsMEIOSISFERTILIZATIONnnnnn2n2nHaploid multicellularorganism (gametophyte)MitosisMitosisSporesGametesMitosisZygoteDiploidmulticellularorganism(sporophyte)(b) Plants and some algaeFigure 13.6 BPlants and some algaeExhibit an alternation of generationsThe life cycle includes both diploid and haploid multicellular stagesMEIOSISFERTILIZATIONnnnnn2nHaploid multicellularorganismMitosisMitosisGametesZygote(c) Most fungi and some protistsFigure 13.6 CIn most fungi and some protistsMeiosis produces haploid cells that give rise to a haploid multicellular adult organismThe haploid adult carries out mitosis, producing cells that will become gametesConcept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploidMeiosisTakes place in two sets of divisions, meiosis I and meiosis IIThe Stages of MeiosisAn overview of meiosisFigure 13.7InterphaseHomologous pairof chromosomesin diploid parent cellChromosomesreplicateHomologous pair of replicated chromosomesSisterchromatidsDiploid cell withreplicatedchromosomes12 Homologous chromosomes separateHaploid cells withreplicated chromosomes Sister chromatids separateHaploid cells with unreplicated chromosomesMeiosis IMeiosis IIMeiosis IReduces the number of chromosomes from diploid to haploidMeiosis IIProduces four haploid daughter cellsCentrosomes(with centriole pairs)SisterchromatidsChiasmataSpindleTetradNuclearenvelopeChromatinCentromere(with kinetochore)Microtubuleattached tokinetochoreTertads line upMetaphaseplateHomologouschromosomesseparateSister chromatidsremain attachedPairs of homologouschromosomes split upChromosomes duplicateHomologous chromosomes(red and blue) pair and exchangesegments; 2n = 6 in this exampleINTERPHASEMEIOSIS I: Separates homologous chromosomesPROPHASE IMETAPHASE IANAPHASE IInterphase and meiosis IFigure 13.8TELOPHASE I ANDCYTOKINESISPROPHASE IIMETAPHASE IIANAPHASE IITELOPHASE II ANDCYTOKINESISMEIOSIS II: Separates sister chromatidsCleavagefurrowSister chromatidsseparateHaploid daughter cellsformingDuring another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomesTwo haploid cellsform; chromosomesare still doubleFigure 13.8Telophase I, cytokinesis, and meiosis IIA Comparison of Mitosis and MeiosisMeiosis and mitosis can be distinguished from mitosisBy three events in Meiosis lSynapsis and crossing overHomologous chromosomes physically connect and exchange genetic informationTetrads on the metaphase plateAt metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase platesSeparation of homologuesAt anaphase I of meiosis, homologous pairs move toward opposite poles of the cellIn anaphase II of meiosis, the sister chromatids separateFigure 13.9MITOSISMEIOSISProphaseDuplicated chromosome(two sister chromatids)ChromosomereplicationChromosomereplicationParent cell(before chromosome replication)Chiasma (site ofcrossing over)MEIOSIS IProphase ITetrad formed bysynapsis of homologouschromosomesMetaphaseChromosomespositioned at themetaphase plateTetradspositioned at themetaphase plateMetaphase IAnaphase ITelophase IHaploidn = 3MEIOSIS IIDaughtercells ofmeiosis IHomologuesseparateduringanaphase I;sisterchromatidsremain togetherDaughter cells of meiosis IInnnnSister chromatids separate during anaphase IIAnaphaseTelophaseSister chromatidsseparate duringanaphase2n2nDaughter cellsof mitosis2n = 6A comparison of mitosis and meiosisConcept 13.4: Genetic variation produced in sexual life cycles contributes to evolutionReshuffling of genetic material in meiosisProduces genetic variationOrigins of Genetic Variation Among OffspringIn species that produce sexuallyThe behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generationIndependent Assortment of ChromosomesHomologous pairs of chromosomesOrient randomly at metaphase I of meiosisIn independent assortmentEach pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairsFigure 13.10KeyMaternal set ofchromosomesPaternal set ofchromosomesPossibility 1Two equally probable arrangements ofchromosomes atmetaphase IPossibility 2Metaphase IIDaughtercellsCombination 1Combination 2Combination 3Combination 4Crossing OverCrossing overProduces recombinant chromosomes that carry genes derived from two different parentsFigure 13.11Prophase Iof meiosisNonsisterchromatidsTetradChiasma,site ofcrossingoverMetaphase IMetaphase IIDaughtercellsRecombinantchromosomesRandom FertilizationThe fusion of gametesWill produce a zygote with any of about 64 trillion diploid combinationsEvolutionary Significance of Genetic Variation Within PopulationsGenetic variationIs the raw material for evolution by natural selectionMutationsAre the original source of genetic variationSexual reproductionProduces new combinations of variant genes, adding more genetic diversity