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CAMPBELL BIOLOGY IN FOCUS 10 Urry • Cain • Wasserman • Minorsky • Jackson • Reece Meiosis and Sexual Life Cycles 鄭先祐(Ayo) 教授 國立臺南大學 生態科學與技術學系 Ayo website: http://myweb.nutn.edu.tw/~hycheng/ What accounts for family resemblance? 2013 Biology 2 Overview: Variations on a Theme • Living organisms are distinguished by their ability to reproduce their own kind • Genetics (遺傳學) is the scientific study of heredity (遺傳) and variation (變異)。 • Heredity (遺傳) is the transmission of traits from one generation to the next • Variation (變異) is demonstrated by the differences in appearance that offspring show from parents and siblings 2013 Biology 3 Meiosis and Sexual Life Cycles 1. Offspring acquire genes from parents by inheriting chromosomes 2. Fertilization and meiosis alternate in sexual life cycles 3. Meiosis reduces the number of chromosome sets from diploid to haploid 4. Genetic variation produced in sexual life cycles contributes to evolution 2013 Biology 4 Concept 10.1: Offspring acquire genes from parents by inheriting chromosomes • In a literal sense, children do not inherit particular physical traits from their parents – It is genes that are actually inherited • Genes(基因) are the units of heredity, and are made up of segments of DNA – Genes are passed to the next generation via reproductive cells called gametes(配子) (sperm and eggs) – Each gene has a specific location called a locus on a certain chromosome (染色體) 2013 Biology 5 Comparison of Asexual and Sexual Reproduction • In asexual reproduction, a single individual passes genes to its offspring without the fusion of gametes – A clone is a group of genetically identical individuals from the same parent • In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents 2013 Biology 6 Figure 13.2 0.5 mm Parent Bud (a) Hydra (b) Redwoods 2013 Biology 7 Asexual reproduction in Hydra (0:24) 2013 Biology 8 Concept 10.2: Fertilization and meiosis alternate in sexual life cycles • A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism Sets of Chromosomes in Human Cells 2013 Biology 9 Sets of Chromosomes in Human Cells • Human somatic cells(體細胞) (any cell other than a gamete) have 23 pairs of chromosomes • A karyotype(染色體組型) is an ordered display of the pairs of chromosomes from a cell • The two chromosomes in each pair are called homologous chromosomes(同源染色體), or homologs – Chromosomes in a homologous pair are the same length and shape and carry genes controlling the same inherited characters 2013 Biology 10 Pair of homologous duplicated chromosomes 5 m Centromere Sister chromatids Metaphase chromosome 2013 Biology 11 • The sex chromosomes, which determine the sex of the individual, are called X and Y – Human females have a homologous pair of X chromosomes (XX) – Human males have one X and one Y chromosome • The remaining 22 pairs of chromosomes are called autosomes • The 46 chromosomes in a human somatic cell are two sets of 23: – one from the mother and one from the father 2013 Biology 12 • A diploid(雙套) cell (2n) has two sets of chromosomes – For humans, the diploid number is 46 (2n = 46) • In a cell in which DNA synthesis has occurred, each chromosome is replicated – Each replicated chromosome consists of two identical sister chromatids • A gamete (sperm or egg) contains a single set of chromosomes, and is haploid(單套) (n) – For humans, the haploid number is 23 (n = 23) – Each set of 23 consists of 22 autosomes and a single sex chromosome 2013 Biology 13 Behavior of Chromosome Sets in the Human Life Cycle • Fertilization(授精) 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 • At sexual maturity, the ovaries and testes produce haploid gametes(配子) – Gametes(配子) are the only types of human cells produced by meiosis, rather than mitosis • Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number 2013 Biology 14 Haploid gametes (n 23) Key Haploid (n) Diploid (2n) Egg (n) Sperm (n) MEIOSIS Ovary FERTILIZATION Testis Diploid zygote (2n 46) Human life cycle (人的生命週期) Mitosis and development Multicellular diploid adults (2n 46) 2013 Biology 15 The Variety of Sexual Life Cycles • The alternation of meiosis and fertilization is common to all organisms that reproduce sexually • The three main types of sexual life cycles differ in the timing of meiosis and fertilization • 01 動物:Gametes are the only haploid cells in animals – They 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 2013 Biology 16 Key Haploid (n) Diploid (2n) Gametes n n n MEIOSIS FERTILIZATION 2n Zygote 2n Diploid multicellular organism Mitosis (a) Animals 2013 Biology 17 • 02植物:Plants and some algae exhibit an alternation of generations (世代交替) – This life cycle includes both a diploid and haploid multicellular stage – The diploid organism, called the sporophyte(孢子體), makes haploid spores(孢 子) by meiosis – Each spore grows by mitosis into a haploid organism called a gametophyte (配子體) – A gametophyte makes haploid gametes by mitosis – Fertilization of gametes results in a diploid sporophyte 2013 Biology 18 Key Haploid (n) Diploid (2n) Haploid multicellular organism (gametophyte) Mitosis n n n Mitosis n n Spores Gametes MEIOSIS Diploid multicellular organism (sporophyte) 2n FERTILIZATION 2n Zygote Mitosis (b) Plants and some algae 2013 Biology 19 • 03 真菌:In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stage – The zygote produces haploid cells by meiosis – Each haploid cell grows by mitosis into a haploid multicellular organism – The haploid adult produces gametes by mitosis 2013 Biology 20 Key Haploid (n) Diploid (2n) Haploid unicellular or multicellular organism Mitosis n n Mitosis n n Gametes MEIOSIS n FERTILIZATION 2n Zygote (c) Most fungi and some protists 2013 Biology 21 Concept 10.3: Meiosis reduces the number of chromosome sets from diploid to haploid • Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II – The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis – Each daughter cell has only half as many chromosomes as the parent cell – The result is four haploid daughter cells with unreplicated chromosomes 2013 Biology 22 • Division in meiosis I occurs in four phases – – – – Prophase I Metaphase I Anaphase I Telophase I and cytokinesis • Division in meiosis II also occurs in four phases – – – – Prophase II Metaphase II Anaphase II Telophase II and cytokinesis 2013 Biology 23 Interphase The stages of Meiosis Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Sister chromatids Chromosomes duplicate Diploid cell with duplicated chromosomes 2013 Biology 24 Interphase The stages of Meiosis Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Chromosomes duplicate Sister chromatids Diploid cell with duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with duplicated chromosomes 2013 Biology 25 Interphase The stages of Meiosis Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Sister chromatids Chromosomes duplicate Diploid cell with duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with duplicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unduplicated chromosomes 2013 Biology 26 Prophase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Telophase I and Cytokinesis Anaphase I Metaphase I Sister chromatids remain attached Centromere (with kinetochore) Metaphase plate Fragments Homologous chromosomes of nuclear envelope Homologous chromosomes separate Microtubule attached to kinetochore Cleavage furrow Each pair of homologous chromosomes separates. Chromosomes line up Duplicated homologous chromosomes (red and blue) by homologous pairs. pair and exchange segments; 2n 6 in this example. 2013 Biology Two haploid cells form; each chromosome still consists of two sister chromatids. 27 Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Sister chromatids separate 2013 Biology Haploid daughter cells forming 28 Meiosis (3:35) 2013 Biology 29 A Comparison of Mitosis and Meiosis • Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell • Meiosis 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 2013 Biology 30 • Three events are unique to meiosis, and all three occur in meiosis l 1. Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information 2. At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes 3. At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate 2013 Biology 31 MEIOSIS MITOSIS Parent cell MEIOSIS I Chiasma Prophase I Prophase Duplicated chromosome Chromosome duplication 2n 6 Chromosome duplication Homologous chromosome pair Metaphase Metaphase I Anaphase Telophase Anaphase I Telophase I Daughter cells of meiosis I 2n Haploid n3 MEIOSIS II 2n Daughter cells of mitosis n n n n Daughter cells of meiosis II 2013 Biology 32 • Sister chromatid cohesion allows sister chromatids of a single chromosome to stay together through meiosis I – Protein complexes called cohesins are responsible for this cohesion – In mitosis, cohesins are cleaved at the end of metaphase – In 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) 2013 Biology 33 Concept 10.4: Genetic variation produced in sexual life cycles contributes to evolution • Mutations (changes in an organism’s DNA) are the original source of genetic diversity – Mutations create different versions of genes called alleles • Reshuffling of alleles during sexual reproduction produces genetic variation 2013 Biology 34 Origins of Genetic Variation Among Offspring • The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation • Three mechanisms contribute to genetic variation 1. Independent assortment of chromosomes 2. Crossing over 3. Random fertilization 2013 Biology 35 01. Independent Assortment of Chromosomes • Homologous pairs of chromosomes orient randomly at metaphase I of meiosis • In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs – The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number – For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes 2013 Biology 36 Possibility 2 Possibility 1 Two equally probable arrangements of chromosomes at metaphase I 2013 Biology 37 Possibility 2 Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Metaphase II 2013 Biology 38 Possibility 2 Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 2013 Biology 39 02. Crossing Over • Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent • Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene • In crossing over, homologous portions of two nonsister chromatids trade places 2013 Biology 40 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs 2013 Biology 41 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM 2013 Biology 42 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I 2013 Biology 43 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II 2013 Biology 44 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Daughter cells Recombinant chromosomes 2013 Biology 45 03. Random Fertilization • Random 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 • Crossing over adds even more variation • Each zygote has a unique genetic identity 2013 Biology 46 The Evolutionary Significance of Genetic Variation Within Populations • Natural selection results in the accumulation of genetic variations favored by the environment • Sexual reproduction contributes to the genetic variation in a population, which originates from mutations 2013 Biology 47 問題與討論 • Ayo NUTN website: • http://myweb.nutn.edu.tw/~hycheng/ 2013 Biology 48