Sinh học - Chapter 013: Meiosis and sexual life cycles

Meiosis and Sexual Life CyclesChapter 13Overview: Variations on a ThemeLiving organisms are distinguished by their ability to reproduce their own kindGenetics is the scientific study of heredity and variationHeredity is the transmission of traits from one generation to the nextVariation is demonstrated by the differences in appearance that offspring show from parents and siblings© 2011 Pearson Education, Inc.Concept 13.1: Offspring acquire genes from parents by inheriting chromosomesIn a literal sense, children do not inherit particular physical traits from their parentsIt is genes that are actually inherited© 2011 Pearson Education, Inc.Inheritance of GenesGenes are the units of heredity, and are made up of segments of DNAGenes are passed to the next generation via reproductive cells called gametes (sperm and eggs) Each gene has a specific location called a locus on a certain chromosomeMost DNA is packaged into chromosomes© 2011 Pearson Education, Inc.Comparison of Asexual and Sexual Reproduction In asexual reproduction, a single individual passes genes to its offspring without the fusion of gametesA clone is a group of genetically identical individuals from the same parentIn sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents© 2011 Pearson Education, Inc.Concept 13.2: Fertilization and meiosis alternate in sexual life cyclesA life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism© 2011 Pearson Education, Inc.Sets of Chromosomes in Human CellsHuman somatic cells (any cell other than a gamete) have 23 pairs of chromosomesA karyotype is an ordered display of the pairs of chromosomes from a cell The two chromosomes in each pair are called homologous chromosomes, or homologsChromosomes in a homologous pair are the same length and shape and carry genes controlling the same inherited characters© 2011 Pearson Education, Inc.Figure 13.3bPair of homologousduplicated chromosomesCentromereSisterchromatidsMetaphasechromosome5 mThe sex chromosomes, which determine the sex of the individual, are called X and YHuman females have a homologous pair of X chromosomes (XX)Human males have one X and one Y chromosomeThe remaining 22 pairs of chromosomes are called autosomes© 2011 Pearson Education, Inc.Each pair of homologous chromosomes includes one chromosome from each parentThe 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the fatherA diploid cell (2n) has two sets of chromosomesFor humans, the diploid number is 46 (2n = 46)© 2011 Pearson Education, Inc.In a cell in which DNA synthesis has occurred, each chromosome is replicatedEach replicated chromosome consists of two identical sister chromatids© 2011 Pearson Education, Inc.Figure 13.4Sister chromatidsof one duplicatedchromosomeKeyMaternal set ofchromosomes (n  3)Paternal set ofchromosomes (n  3)Key2n  6CentromereTwo nonsisterchromatids ina homologous pairPair of homologouschromosomes (one from each set)A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n)For humans, the haploid number is 23 (n = 23)Each set of 23 consists of 22 autosomes and a single sex chromosomeIn an unfertilized egg (ovum), the sex chromosome is XIn a sperm cell, the sex chromosome may be either X or Y© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.Fertilization is the union of gametes (the sperm and the egg)The fertilized egg is called a zygote and has one set of chromosomes from each parent The zygote produces somatic cells by mitosis and develops into an adultBehavior of Chromosome Sets in the Human Life Cycle© 2011 Pearson Education, Inc.At sexual maturity, the ovaries and testes produce haploid gametesGametes are the only types of human cells produced by meiosis, rather than mitosisMeiosis results in one set of chromosomes in each gameteFertilization and meiosis alternate in sexual life cycles to maintain chromosome number© 2011 Pearson Education, Inc.Figure 13.5KeyHaploid (n)Diploid (2n)Egg (n)Haploid gametes (n  23)Sperm (n)OvaryTestisMitosis anddevelopmentDiploidzygote(2n  46)Multicellular diploidadults (2n  46)MEIOSISFERTILIZATIONThe Variety of Sexual Life CyclesThe alternation of meiosis and fertilization is common to all organisms that reproduce sexuallyThe three main types of sexual life cycles differ in the timing of meiosis and fertilization © 2011 Pearson Education, Inc.Gametes are the only haploid cells in animalsThey are produces by meiosis and undergo no further cell division before fertilization Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism© 2011 Pearson Education, Inc.Figure 13.6aKeyHaploid (n)Diploid (2n)GametesMEIOSISFERTILIZATIONZygoteMitosisDiploidmulticellularorganism(a) Animalsn2nnn2nPlants and some algae exhibit an alternation of generationsThis life cycle includes both a diploid and haploid multicellular stageThe diploid organism, called the sporophyte, makes haploid spores by meiosis© 2011 Pearson Education, Inc.Each spore grows by mitosis into a haploid organism called a gametophyteA gametophyte makes haploid gametes by mitosisFertilization of gametes results in a diploid sporophyte© 2011 Pearson Education, Inc.Figure 13.6b2n2nnMEIOSISFERTILIZATIONMitosisMitosisMitosisGametesSporesZygoteHaploid multi-cellular organism(gametophyte)Diploidmulticellularorganism(sporophyte)(b) Plants and some algaennnnHaploid (n)Diploid (2n)KeyIn most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stageThe zygote produces haploid cells by meiosisEach haploid cell grows by mitosis into a haploid multicellular organismThe haploid adult produces gametes by mitosis© 2011 Pearson Education, Inc.Figure 13.6cKeyHaploid (n)Diploid (2n)2nnnnnnMEIOSISFERTILIZATIONMitosisMitosisGametesZygoteHaploid unicellular ormulticellular organism(c) Most fungi and some protistsDepending on the type of life cycle, either haploid or diploid cells can divide by mitosisHowever, only diploid cells can undergo meiosisIn all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspring© 2011 Pearson Education, Inc.Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploidLike mitosis, meiosis is preceded by the replication of chromosomesMeiosis takes place in two sets of cell divisions, called meiosis I and meiosis IIThe two cell divisions result in four daughter cells, rather than the two daughter cells in mitosisEach daughter cell has only half as many chromosomes as the parent cell© 2011 Pearson Education, Inc.The Stages of MeiosisAfter chromosomes duplicate, two divisions followMeiosis I (reductional division): homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomesMeiosis II (equational division) sister chromatids separateThe result is four haploid daughter cells with unreplicated chromosomes© 2011 Pearson Education, Inc.Figure 13.7-3Pair of homologouschromosomes indiploid parent cellDuplicated pairof homologouschromosomesChromosomesduplicateSisterchromatidsDiploid cell withduplicatedchromosomesHomologouschromosomes separateHaploid cells withduplicated chromosomesSister chromatidsseparateHaploid cells with unduplicated chromosomesInterphaseMeiosis IMeiosis II21Meiosis I is preceded by interphase, when the chromosomes are duplicated to form sister chromatidsThe sister chromatids are genetically identical and joined at the centromereThe single centrosome replicates, forming two centrosomes© 2011 Pearson Education, Inc.Division in meiosis I occurs in four phasesProphase IMetaphase IAnaphase ITelophase I and cytokinesis© 2011 Pearson Education, Inc.Figure 13.8aProphase IMetaphase IAnaphase ITelophase I andCytokinesisCentrosome(with centriole pair)SisterchromatidsChiasmataSpindleHomologouschromosomesFragmentsof nuclearenvelopeDuplicated homologouschromosomes (red and blue)pair and exchange segments;2n  6 in this example.Centromere(with kinetochore)MetaphaseplateMicrotubuleattached tokinetochoreChromosomes line upby homologous pairs.Sister chromatidsremain attachedHomologouschromosomesseparateEach pair of homologous chromosomes separates.CleavagefurrowTwo haploid cells form; each chromosomestill consists of two sister chromatids.Prophase IProphase I typically occupies more than 90% of the time required for meiosisChromosomes begin to condenseIn synapsis, homologous chromosomes loosely pair up, aligned gene by gene© 2011 Pearson Education, Inc.In crossing over, nonsister chromatids exchange DNA segmentsEach pair of chromosomes forms a tetrad, a group of four chromatidsEach tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred© 2011 Pearson Education, Inc.Metaphase IIn metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each poleMicrotubules from one pole are attached to the kinetochore of one chromosome of each tetradMicrotubules from the other pole are attached to the kinetochore of the other chromosome© 2011 Pearson Education, Inc.Anaphase IIn anaphase I, pairs of homologous chromosomes separateOne chromosome moves toward each pole, guided by the spindle apparatusSister chromatids remain attached at the centromere and move as one unit toward the pole© 2011 Pearson Education, Inc.Telophase I and CytokinesisIn the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatidsCytokinesis usually occurs simultaneously, forming two haploid daughter cells© 2011 Pearson Education, Inc.In animal cells, a cleavage furrow forms; in plant cells, a cell plate formsNo chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated© 2011 Pearson Education, Inc.Division in meiosis II also occurs in four phasesProphase IIMetaphase IIAnaphase IITelophase II and cytokinesisMeiosis II is very similar to mitosis© 2011 Pearson Education, Inc.Figure 13.8bProphase IIMetaphase IIAnaphase IITelophase II andCytokinesisSister chromatidsseparateHaploid daughtercells formingDuring another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing unduplicated chromosomes.Prophase IIIn prophase II, a spindle apparatus formsIn late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate© 2011 Pearson Education, Inc.Metaphase IIIn metaphase II, the sister chromatids are arranged at the metaphase plateBecause of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identicalThe kinetochores of sister chromatids attach to microtubules extending from opposite poles© 2011 Pearson Education, Inc.Anaphase IIIn anaphase II, the sister chromatids separateThe sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles© 2011 Pearson Education, Inc.Telophase II and CytokinesisIn telophase II, the chromosomes arrive at opposite polesNuclei form, and the chromosomes begin decondensing© 2011 Pearson Education, Inc.Cytokinesis separates the cytoplasmAt the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomesEach daughter cell is genetically distinct from the others and from the parent cell© 2011 Pearson Education, Inc.A Comparison of Mitosis and MeiosisMitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cellMeiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell© 2011 Pearson Education, Inc.Figure 13.9aProphaseDuplicatedchromosomeMITOSISChromosomeduplicationParent cell2n  6MetaphaseAnaphaseTelophase2n2nDaughter cellsof mitosisMEIOSISMEIOSIS IMEIOSIS IIProphase IMetaphase IAnaphase ITelophase IHaploidn  3ChiasmaChromosomeduplicationHomologouschromosome pairDaughter cells ofmeiosis IDaughter cells of meiosis IInnnnFigure 13.9bSUMMARYPropertyMitosisMeiosisDNAreplicationNumber ofdivisionsSynapsis ofhomologouschromosomesNumber of daughter cellsand geneticcompositionRole in the animal bodyOccurs during interphase beforemitosis beginsOne, including prophase, metaphase,anaphase, and telophaseDoes not occurTwo, each diploid (2n) and geneticallyidentical to the parent cellEnables multicellular adult to arise fromzygote; produces cells for growth, repair,and, in some species, asexual reproductionOccurs during interphase before meiosis I beginsTwo, each including prophase, metaphase, anaphase,and telophaseOccurs during prophase I along with crossing overbetween nonsister chromatids; resulting chiasmatahold pairs together due to sister chromatid cohesionFour, each haploid (n), containing half as manychromosomes as the parent cell; genetically differentfrom the parent cell and from each otherProduces gametes; reduces number of chromosomesby half and introduces genetic variability among the gametesThree events are unique to meiosis, and all three occur in meiosis lSynapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic informationAt the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomesAt anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate© 2011 Pearson Education, Inc.Sister chromatid cohesion allows sister chromatids of a single chromosome to stay together through meiosis IProtein complexes called cohesins are responsible for this cohesionIn mitosis, cohesins are cleaved at the end of metaphaseIn meiosis, cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) and at the centromeres in anaphase II (separation of sister chromatids)© 2011 Pearson Education, Inc.Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolutionMutations (changes in an organism’s DNA) are the original source of genetic diversityMutations create different versions of genes called allelesReshuffling of alleles during sexual reproduction produces genetic variation© 2011 Pearson Education, Inc.Origins of Genetic Variation Among OffspringThe behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generationThree mechanisms contribute to genetic variationIndependent assortment of chromosomesCrossing overRandom fertilization© 2011 Pearson Education, Inc.Independent Assortment of ChromosomesHomologous pairs of chromosomes orient randomly at metaphase I of meiosisIn independent assortment, each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently of the other pairs© 2011 Pearson Education, Inc.The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid numberFor humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes© 2011 Pearson Education, Inc.Figure 13.10-3Possibility 1Possibility 2Two equally probablearrangements ofchromosomes atmetaphase IMetaphase IIDaughtercellsCombination 1Combination 2Combination 3Combination 4Crossing OverCrossing over produces recombinant chromosomes, which combine DNA inherited from each parentCrossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene© 2011 Pearson Education, Inc.In crossing over, homologous portions of two nonsister chromatids trade placesCrossing over contributes to genetic variation by combining DNA from two parents into a single chromosome© 2011 Pearson Education, Inc.Figure 13.11-5Prophase Iof meiosisNonsister chromatidsheld togetherduring synapsisPair of homologsChiasmaCentromereTEMAnaphase IAnaphase IIDaughtercellsRecombinant chromosomesRandom FertilizationRandom fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations© 2011 Pearson Education, Inc.Crossing over adds even more variationEach zygote has a unique genetic identity© 2011 Pearson Education, Inc.Prophase I: Each homologous pair undergoes synapsisand crossing over between nonsister chromatids withthe subsequent appearance of chiasmata.Metaphase I: Chromosomes line up as homologouspairs on the metaphase plate.Anaphase I: Homologs separate from each other; sisterchromatids remain joined at the centromere.Figure 13.UN01Figure 13.UN03Figure 13.UN04