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Transcript
Chapter 13 Meiosis and Sexual Life Cycles AP Biology Overview: Variations on a Theme • Living organisms are distinguished by their ability to reproduce their own kind • Genetics is the scientific study of heredity and variation • Heredity is the transmission of traits from one generation to the next • Variation is demonstrated by the differences in appearance that offspring show from parents and siblings Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes Genes • Children do not inherit particular physical traits from their parents in a literal sense • It is genes that are actually inherited • Genes are units of heredity made up of segments of DNA • Most genes program cells to make specific enzymes and proteins • Cumulative action of these molecules produce an organism’s inherited traits Inheritance of Genes • Genes are passed to the next generation through reproductive cells called gametes • Include sperm and eggs • Male and female gametes unite during fertilization and thus pass on genes of both parents to their offspring • Most of a eukaryotic cell’s DNA is packaged into chromosomes in the nucleus • One chromosome includes 100s-1000s of genes • Each gene has a specific location, called a locus, on a certain chromosome • One set of chromosomes is inherited from each parent Comparison of Asexual and Sexual Reproduction • In asexual reproduction, one parent produces genetically identical offspring by mitosis • Gives rise to clones, or groups of genetically identical individuals from the same parent • Ex) Budding hydra • Fig. 13-2a 0.5 mm In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents • Parent Offspring vary genetically from both siblings and Bud parents (a) Hydra Concept Check 13.1 • 1) How are the traits of parents (such as hair color) transmitted to their offspring? • 2) Explain how asexually reproducing organisms produce offspring that are genetically identical to each other and to their parents. • 3) A horticulturalist breeds orchids, trying to obtain a plant with a unique combination of desirable traits. After many years, she finally succeeds. To produce more plants like this one, should she breed it or clone it? Why? Concept 13.2: Fertilization and meiosis alternate in sexual life cycles Homologous Chromosomes • Human somatic (body) cells have 23 pairs (46 total) of chromosomes • Chromosomes can be distinguished from one another using a microscope due to differences in: • • Size • Position of centromeres • Pattern of colored bands produced by certain stains A karyotype is an ordered display of the pairs of chromosomes from a cell • The two chromosomes in each pair are called homologous chromosomes, or homologs • Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characters • Ex) Genes specifying eye color have equivalent loci on homologous chromosomes Fig. 13-3b TECHNIQUE 5 µm Pair of homologous replicated chromosomes Centromere Sister chromatids Metaphase chromosome Autosomes and Sex Chromosomes • The sex chromosomes are called X and Y • Human females have a homologous pair of X chromosomes (XX) • Human males have one X and one Y chromosome • Only small parts of X and Y are homologous • Most genes on X chromosome do not have counterparts on Y chromosome • Y chromosome also contains genes lacking on X chromosome • The 22 pairs of chromosomes that do not determine sex are called autosomes Diploid Cells • Each pair of homologous chromosomes includes one chromosome from each parent • The 46 chromosomes in a human somatic cell are two sets of 23 chromosomes • One set is inherited from the mother and one from the father • The number of chromosomes in a single set is represented by n • A diploid cell (2n) has two sets of chromosomes • For humans, the diploid number is 46 (2n = 46) Describing Chromosomes • In a cell in which DNA synthesis has occurred, each chromosome is replicated • Fig. 13-4 Each replicated chromosome consists of two identical sister chromatids 2n = 6 Key Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Centromere Pair of homologous chromosomes (one from each set) Haploid Cells • 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 chromosome • In an unfertilized egg (ovum), the sex chromosome is X • In a sperm cell, the sex chromosome may be either X or Y • Each sexually reproducing species has a characteristic haploid and diploid number • Ex) Dogs: n = 39 Behavior of Chromosome Sets in the Human Life Cycle • Fertilization is the union of haploid gametes (the sperm and the egg) • The fertilized egg is called a zygote • Zygote contains one set of chromosomes from each parent and is therefore diploid Fig. 13-5 Key Haploid gametes (n = 23) Haploid (n) Egg (n) Diploid (2n) • Mitosis of the zygote and its descendants produces Sperm (n) all somatic cells during MEIOSIS FERTILIZATION development into an adult Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46) Behavior of Chromosome Sets in the Human Life Cycle • At sexual maturity, the ovaries and testes produce haploid gametes • Develop from specialized cells called germ cells in the gonads • Gametes are the only types of human cells produced by Fig. 13-5 Key meiosis, rather than mitosis Haploid gametes (n = 23) Haploid (n) Egg (n) Diploid (2n) • Meiosis reduces chromosomes to one set in each gamete Sperm (n) MEIOSIS FERTILIZATION • Fertilization and meiosis alternate in sexual life cycles to Ovary Testis Diploid zygote (2n = 46) maintain chromosome number Mitosis and development Multicellular diploid adults (2n = 46) The Variety of Sexual Life Cycles • The alternation of meiosis and fertilization is common to all organisms that reproduce sexually • Timing of meiosis and fertilization varies, depending on species • These variations can be grouped into 3 main types of life cycles • Life cycle: generation-to generation sequence of stages in the reproductive history of an organism, from conception to reproduction The Variety of Sexual Life Cycles: Diploid Life Cycle • In animals (including humans), meiosis produces gametes • These gametes undergo no further cell division before fertilization • Gametes are the only haploid cells in animals • Fig. 13-6a Gametes fuse to form a diploid zygote Key Haploid (n) Diploid (2n) • This zygote divides by mitosis to n develop into a multicellular (diploid) organism Gametes n n MEIOSIS 2n Diploid multicellular organism (a) Animals FERTILIZATION Zygote 2n Mitosis The Variety of Sexual Life Cycles: Alternation of Generations • Plants and some algae exhibit a second type of life cycle called alternation of generations • This life cycle includes both diploid and haploid stages that are multicellular • Multicellular diploid phase is called the sporophyte Fig. 13-6b • Meiosis in sporophyte produces haploid cells called spores • These spores divide using mitosis, generating a multicellular haploid stage called the gametophyte Key Haploid (n) Diploid (2n) Mitosis n Haploid multicellular organism (gametophyte) • A gametophyte makes haploid gametes using mitosis Fusion of 2 haploid gametes at fertilization results in a diploid zygote that gives rise to the next sporophyte generation n n n Spores • Mitosis n MEIOSIS Gametes FERTILIZATION 2n Diploid multicellular organism (sporophyte) 2n Mitosis (b) Plants and some algae Zygote The Variety of Sexual Life Cycles: The Haploid Life Cycle • In most fungi and some protists, the only diploid stage is the single-celled zygote • Fig. 13-6c Formed by fusion of haploid gametes Key Haploid (n) • • There is no multicellular diploid stage The zygote produces haploid cells by meiosis • Each haploid cell grows by mitosis into Haploid unicellular or multicellular organism Diploid (2n) Mitosis n n n Gametes a haploid multicellular adult organism • The haploid adult produces gametes by mitosis Mitosis n MEIOSIS n FERTILIZATION 2n Zygote (c) Most fungi and some protists The Variety of Sexual Life Cycles: A Comparison • Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis • Only diploid cells can undergo meiosis • Haploid cells cannot be further reduced since they only have 1 set of chromosomes • In all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspring Concept Check 13.2 • 1) How does the karyotype of a human female differ from that of a human male? • 2) How does the alternation of meiosis and fertilization in the life cycles of sexually reproducing organisms maintain the normal chromosome count for each species? • 3) Each sperm of a pea plant contains 7 chromosomes. What are the haploid and diploid numbers for peas? • 4) An certain eukaryote lives as a unicellular organism, but during environmental stress, its cells produce gametes. The gametes fuse, and the resulting zygote undergoes meiosis, generating new single cells. What type of organism could this be? Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid An Introduction to Meiosis • Many steps of meiosis closely resemble corresponding steps in mitosis • Like mitosis, meiosis is preceded by the replication of chromosomes • This replication, however, is followed by TWO consecutive divisions in meiosis • Meiosis I and meiosis II • These 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 Stages of Meiosis • In meiosis, both members of a single homologous pair of chromosomes in a diploid cell are replicated • Fig. 13-7-3 Interphase Homologous pair of chromosomes in diploid parent cell These copies are then sorted into 4 haploid daughter cells • In the first cell division (meiosis I), homologous chromosomes separate • Meiosis I results in two haploid daughter cells with Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes replicated chromosomes Meiosis I • It is called the reductional division 1 Homologous • chromosomes separate In the second cell division (meiosis II), sister chromatids separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids • Meiosis II results in four haploid daughter cells with separate unreplicated chromosomes Haploid cells with unreplicated chromosomes • It is called the equational division Interphase Prior to Meiosis I • Like mitosis, meiosis I is preceded by interphase • During interphase, chromosomes are replicated to form sister chromatids • The sister chromatids are genetically identical and joined at the centromere • The single centrosome replicates, forming two centrosomes Phases of Meiosis I • Division in meiosis I occurs in four phases: – Prophase I – Metaphase I – Anaphase I – Telophase I and cytokinesis Meiosis I: Prophase I Prophase I - typically occupies more than 90% of the time required for meiosis • Chromosomes begin to condense • In synapsis, homologous chromosomes loosely pair up, aligned gene by gene, forming tetrads • Exchange of corresponding segments of DNA (crossing over) occurs during synapsis • • Chiasmata: X-shaped regions where crossing over has occurred and homologs are still associated due to sister chromatid cohesion Fig. 13-8b Synapsis ends in mid-prophase and chromosomes move slightly apart • Centrosome movement, spindle formation, and nuclear envelope breakdown occur as in mitosis • In late prophase, microtubules of spindle fiber attach to the kinetochores of each homolog • Homologous pairs then move toward metaphase plate Prophase I Metaphase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore Meiosis I: Metaphase I Metaphase I • In metaphase I, tetrads line up at the metaphase plate, with one Fig. 13-8b chromosome in each pair facing each pole Prophase I • Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad • Metaphase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Microtubules from the other pole are attached to the kinetochore of the other chromosome Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore Meiosis I: Anaphase I Anaphase I • In anaphase I, pairs of homologous chromosomes separate Fig. 13-8c • Homologs move toward opposite poles, Telophase I and Cytokinesis Anaphase I guided by the spindle apparatus • Sister chromatids remain attached at the Sister chromatids remain attached centromere and move as one unit toward the pole Homologous chromosomes separate Cleavage furrow Meiosis I: Telophase I and Cytokinesis Telophase I and Cytokinesis • In the beginning of telophase I, each half of the cell has a haploid set of chromosomes • Fig. 13-8c Each chromosome still consists of two Telophase I and Cytokinesis Anaphase I sister chromatids • Sister chromatids remain attached Cytokinesis usually occurs simultaneously • Forms two haploid daughter cells • In animal cells, a cleavage furrow forms • • In plant cells, a cell plate forms No replication occurs between meiosis I and II Homologous chromosomes separate Cleavage furrow Phases of Meiosis II • Division in meiosis II also occurs in four phases: – Prophase II – Metaphase II – Anaphase II – Telophase II and cytokinesis • Meiosis II is very similar to mitosis Meiosis II: Prophase II and Metaphase II Prophase II • In prophase II, a spindle apparatus forms • In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate Metaphase II Fig. 13-8e • In metaphase II, the sister chromatids are arranged at the metaphase plate • Due to crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical • The kinetochores of sister chromatids attach to microtubules extending from opposite poles Prophase II Metaphase II Meiosis II: Anaphase II, Telophase II, and Cytokinesis Anaphase II • In anaphase II, the sister chromatids separate • The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles Telophase II and Cytokinesis • In telophase II, the chromosomes arrive at opposite poles • Nuclei form, and the chromosomes begin decondensing • Cytokinesis separates the cytoplasm • At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes • Each daughter cell is genetically distinct from the others and from the parent cell A Comparison of Mitosis and Meiosis • Mitosis conserves the number of chromosome sets • • Produces cells that are genetically identical to the parent cell Meiosis reduces the number Fig. 13-9aof chromosomes sets from two (diploid) to MITOSIS one (haploid) MEIOSIS Parent cell • Produces cells that differ genetically Chromosome replication Prophase Chiasma Chromosome replication Prophase I Homologous chromosome pair 2n = 6 Replicated chromosome MEIOSIS I from each other and from the parent cell • Metaphase Metaphase I Anaphase Telophase Anaphase I Telophase I Haploid n=3 The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis Daughter cells of meiosis I 2n Daughter cells of mitosis 2n MEIOSIS II n n n n Daughter cells of meiosis II Unique Events of Meiosis • Three events unique to meiosis occur in meiosis l: • Synapsis and crossing over in prophase I: • Homologous chromosomes physically connect and exchange genetic information • Homologs on metaphase plate during metaphase I: • There are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes, on the metaphase plate • Separation of homologs at anaphase I: • Homologous chromosomes, rather than sister chromatids, separate Sister Chromatid Cohesion • 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, sister chromatic cohesion is released in 2 steps: • Cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) • Remaining cohesins are cleaved at the centromeres in anaphase II (separation of sister chromatids) Concept Check 13.3 • 1) How are the chromosomes in a cell at metaphase of mitosis similar to and different from the chromosomes in a cell at metaphase at meiosis II? • 2) Given that the synaptonemal complex disappears by the end of prophase, how would the two homologs be associated if crossing over did not occur? What effect might this ultimately have on gamete formation? Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution Mutations and Genetic Variation • 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 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: • Independent assortment of chromosomes • Crossing over • Random fertilization Independent Assortment of Chromosomes • Random orientation of homologous chromosomes at metaphase I contributes to genetic variation • At metaphase I, homologous chromosomes are situated on metaphase plate • Each pair may orient with either its maternal or paternal homolog • Fig. 13-11-3 There is a 50/50 chance that daughter cell of meiosis I will get either homolog • Possibility 2 Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Each homologous pair of chromosomes is also positioned independently of other pairs at metaphase I • Metaphase II Known as independent Daughter cells assortment Combination 1 Combination 2 Combination 3 Combination 4 Independent Assortment of Chromosomes • The number of combinations possible when chromosomes assort independently into gametes is 2n • n is the haploid number • For humans (n = 23), there are more than 8 million (223) Fig. 13-11-3 possible combinations of Possibility 2 Possibility 1 Two equally probable arrangements of chromosomes at metaphase I chromosomes Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 Crossing Over • Crossing over further contributes to genetic variation by producing recombinant chromosomes Fig. 13-12-5 • Combine genes inherited from each parent • • Prophase I of meiosis Pair of homologs During meiosis in humans, an average of 1-3 crossover events occur per chromosome pair Crossing over begins very early in prophase I Nonsister chromatids held together during synapsis Chiasma Centromere TEM Anaphase I • During this time, homologous chromosomes pair up gene by gene Anaphase II • In a single crossover event, corresponding segments of 2 nonsister chromatids (one maternal, one paternal) are exchanged • Daughter cells Recombinant chromosomes Crossing over thus produces chromosomes with new combinations of maternal and paternal alleles Crossing Over • 1) In prophase I, synapsis and crossing over occur Fig. 13-12-5 • • • • Chiasma forms as a result of crossover occurring while sister chromatid cohesion is still present along the arms 2) Chiasmata and attachments between sister chromatids hold homologs together as they move to metaphase I plate 3) Breakdown of proteins holding sister chromatid arms together allows homologs with recombinant chromatids to separate • • Homologs then move slightly apart Occurs during anaphase I 4) During anaphase II, release of sister chromatid cohesion at centromeres allows sister chromatids to separate Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere TEM Anaphase I Anaphase II Daughter cells Recombinant chromosomes Random Fertilization • Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) • In humans, each respective gamete represents 1 of ~8.4 million (223) possible chromosome combinations due to independent assortment • Fusion of two gametes produces a zygote with any of about 70 trillion (223 x 223) diploid combinations The Evolutionary Significance of Genetic Variation Within Populations • Populations evolve through differential reproductive success of its genetically different members • Sexual reproduction contributes to the genetic variation that originally results from mutation in a population • Those individuals best suited to local environment leave the most offspring • Results in an increased frequency of the alleles coding for these favored variations in a population Concept Check 13.4 • 1) What is the original source of all the different alleles of a gene? • 2) The diploid number for fruit flies is 8, while that for grasshoppers is 46. If no crossing over took place, would the genetic variation among offspring from a given pair of parents be greater in fruit flies or grasshoppers? Explain. • 3) Under what circumstances would crossing over during meiosis not contribute to genetic variation among daughter cells? You should now be able to: 1. Distinguish between the following terms: somatic cell and gamete; autosome and sex chromosomes; haploid and diploid 2. Describe the events that characterize each phase of meiosis 3. Describe three events that occur during meiosis I but not mitosis 4. Name and explain the three events that contribute to genetic variation in sexually reproducing organisms