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Chapter 13 Meiosis Slide 2 Heredity Genes Are the units of heredity Are segments of DNA Each gene in an organisms DNA has a specific locus on a certain chromosome Slide 3 Pairs of Chromosomes A diploid cell Has 2 copies of each of its chromosomestherefore 2 copies of each gene In a human, diploid cells (somatic) have 46 chromosomes (2 n = 46) Slide 4 Pairs of Chromosomes A haploid cell Has only 1 copy of each of its chromosomes therefore 1 copy of each gene In a human, a haploid cell (sex cells: egg & sperm) has 23 chromosomes ( n = 23) Slide 5 Sexual Reproduction In sexual reproduction Two parents give rise to offspring that have unique combinations of genes inherited from the two parents 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 Slide 6 Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46 ) Human Life Cycle Slide 7 Pairs of Chromosomes Homologous chromosomes Are the two chromosomes composing a pair Have the same genes Pair of homologous chromosomes Slide 8 Pairs of Chromosomes Sex chromosomes Are distinct from each other in their characteristics Are represented as X and Y Determine the sex of the individual, XX being female, XY being male Slide 9 Pairs of Chromosomes A karyotype Is an ordered, visual representation of the chromosomes in a cell Slide 10 Chromosomes in Dividing Cell In a cell in which DNA synthesis has occurred All the chromosomes are duplicated and thus each consists of two identical sister chromatids Key Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3) 2n = 6 Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Centromere Slide 11 Meiosis Meiosis is a form of cell division That reduces the number of chromosome sets from diploid to haploid Meiosis Takes place in two sets of divisions, meiosis I and meiosis II Slide 12 Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Chromosomes duplicate Sister chromatids Diploid cell with duplicated chromosomes Homologous chromosomes separate Haploid cells with duplicated chromosomes Sister chromatids separate Haploid cells with unduplicated chromosomes Interphase Meiosis I Meiosis II 21 Overview of Meiosis Slide 13 Stages of Meiosis Meiosis I Reduces the number of chromosomes from diploid to haploid Meiosis II Produces four haploid daughter cells Slide 14 Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Homologous chromosomes Fragments of nuclear envelope Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n 6 in this example. Centromere (with kinetochore) Metaphase plate Microtubule attached to kinetochore Chromosomes line up by homologous pairs. Sister chromatids remain attached Homologous chromosomes separate Each pair of homologous chromosomes separates. Cleavage furrow Two haploid cells form; each chromosome still consists of two sister chromatids. Slide 15 Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Slide 16 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 In crossing over, nonsister chromatids exchange DNA segments Each pair of chromosomes forms a tetrad, a group of four chromatids Each tetrad usually has one or more chiasmata, X- shaped regions where crossing over occurred Slide 17 Metaphase I In Metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad Microtubules from the other pole are attached to the kinetochore of the other chromosome Slide 18 Anaphase I In Anaphase I, pairs of homologous chromosomes separate One chromosome moves toward each pole, guided by the spindle apparatus Sister chromatids remain attached at the centromere and move as one unit toward the pole Slide 19 Telophase I & Cytokinesis In the beginning of Telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids Cytokinesis usually occurs simultaneously, forming two haploid daughter cells In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated Slide 20 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 Slide 21 Metaphase II In Metaphase II, the sister chromatids are arranged at the metaphase plate Because of 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 Slide 22 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 Slide 23 Telophase II & Cytokinesis In Telophase II, the chromosomes arrive at opposite poles Nuclei form, and the chromosomes begin de- condensing 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 Slide 24 Genetic Variation Genetic variation Is the raw material for evolution by natural selection Natural selection results in the accumulation of genetic variations favored by the environment Slide 25 Mechanisms of Variation Meiosis provides various mechanisms for variation by: Crossing over Independent assortment Random fertilization Mutations Slide 26 Crossing Over Crossing over produces recombinant chromosomes Combines DNA from each parent Homologous portions of two nonsister chromatids trade places Crossing over increases genetic variation by combining DNA from two parents into a single chromosome Slide 27 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Daughter cells Recombinant chromosomes Crossing Over Slide 28 Independent Assortment In independent assortment Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs There are 2 n possibilities of arrangements (i.e. 8.4 million in humans) Slide 29 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2Combination 3Combination 4 2n = 4 so 2 n is 2 2 = 4 possible combinations Independent Assortment Slide 30 Random Fertilization Dont forget random fertilization! Adds to genetic variation: any sperm can fuse with any ovum (unfertilized egg) Each gamete has 8.4 million possible chromosome combinations Produces a zygote with any of about 70 trillion diploid combinations Slide 31 MITOSIS MEIOSIS Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrads formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads positioned at the metaphase plate Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Haploid Daughter cells of meiosis II n nn n Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n2n 2n2n Diploid daughter cells of mitosis 2n = 6 Review of Mitosis vs. Meiosis Diploid 2n = 6