Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 13 Meiosis and Sexual Life Cycles

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: Hereditary Similarity and Variation Living organisms – Are distinguished by their ability to reproduce their own kind

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Heredity – Is the transmission of traits from 1 generation to the next Variation – Shows that offspring differ somewhat in appearance from parents and siblings Figure 13.1

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetics – scientific study of heredity & hereditary variation

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inheritance of Genes Genes – units of heredity – segments of DNA

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Each gene in an organism’s DNA – Has a specific locus on a certain chromo We inherit – 1 set of chromo’s from mom & 1 from dad

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Comparison of Asexual and Sexual Reproduction asexual reproduction – 1 parent produces genetically identical offspring by mitosis Figure 13.2 Parent Bud 0.5 mm

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings sexual reproduction – 2 parents have offspring w/unique comb’s of genes inherited from both parents

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 13.2: Fertilization & meiosis alternate in sexual life cycles life cycle – generation-to-generation seq of stages in reproductive history of an organism

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sets of Chromosomes in Human Cells humans – somatic cells have 46 chromo’s – 23 from each parent

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 5 µm Pair of homologous chromosomes Centromere Sister chromatids Figure 13.3 karyotype – visual representation of the chromo’s in a cell

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero LE µm Pair of homologous chromosomes Sister chromatids Centromere

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Homologous chromosomes – 2 chromo’s, making a pair – Same length, centromere position, & staining pattern – also called autosomes – 22 pairs in us

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sex chromo’s – Are distinct from each other in their characteristics – X & Y – Determine sex of the indiv, – XX female, – XY male

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings diploid (2n) – 2 sets of each of its chromo’s – human= 46 chromo’s (2n = 46) – Haploid (n) – n=23 in us

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In a cell in which DNA synthesis has occurred – All the chromo’s are duplicated & thus each consists of 2 identical sister chromatids Figure 13.4 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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Unlike somatic cells – Gametes, (sperm & egg cells) are haploid cells, w/ 1 set of chromo’s

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Behavior of Chromosome Sets in the Human Life Cycle At sexual maturity – ovaries & testes make haploid gametes by meiosis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings During fertilization – sperm & ovum, fuse, forming a diploid zygote The zygote – Develops into an adult organism

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 13.5 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) The human life cycle

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Variety of Sexual Life Cycles 3 types of sexual life cycles – Differ in timing of meiosis & fertilization

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In animals – Meiosis occurs during gamete formation – Gametes are the only haploid cells Gametes Figure 13.6 A Diploid multicellular organism Key MEIOSIS FERTILIZATION n n n 2n Zygote Haploid Diploid Mitosis (a) Animals

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings MEIOSISFERTILIZATION n n n n n 2n Haploid multicellular organism (gametophyte) Mitosis Spores Gametes Mitosis Zygote Diploid multicellular organism (sporophyte) (b) Plants and some algae Figure 13.6 B Plants & some algae – Show an alternation of generations – life cycle has both diploid & haploid multicellular stages

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings MEIOSIS FERTILIZATION n n n n n 2n Haploid multicellular organism Mitosis Gametes Zygote (c) Most fungi and some protists Figure 13.6 C In most fungi & some protists – Meiosis makes haploid cells that make a haploid multicellular adult organism – haploid adult carries out mitosis, making cells that will be gametes

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Depending on the type of life cycle, either haploid (n) or diploid (2n) cells can divide by mitosis only diploid cells can undergo meiosis In all 3 life cycles, chromo halving & doubling contribute to genetic variation in offspring

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 13.3: Meiosis reduces the # of chromo sets from diploid to haploid Meiosis – Takes place in 2 sets of divisions, meiosis I & meiosis II

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Stages of Meiosis An overview of meiosis Figure 13.7 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes 1 2 Homologous chromosomes separate Haploid cells with replicated chromosomes Sister chromatids separate Haploid cells with unreplicated chromosomes Meiosis I Meiosis II

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 cell divisions result in 4 daughter cells, rather than the two daughter cells in mitosis Each daughter cell has only ½ as many chromo’s as the parent cell

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Stages of Meiosis 1 st cell division (meiosis I), homologous chromosomes separate Meiosis I results in 2 haploid daughter cells w/ replicated chromosomes In the 2 nd cell division (meiosis II), sister chromatids separate Meiosis II results in 4 haploid daughter cells w/ unreplicated chromosomes

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Meiosis I – Reduces # of chromo’s from diploid to haploid Meiosis II – makes 4 haploid daughter cells

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Centrosomes (with centriole pairs) Sister chromatids Chiasmata Spindle Tetrad Nuclear envelope Chromatin Centromere (with kinetochore) Microtubule attached to kinetochore Tertads line up Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Pairs of homologous chromosomes split up Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example INTERPHASE MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Interphase & meiosis I Figure 13.8

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS MEIOSIS II: Separates sister chromatids Cleavage furrow Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Two haploid cells form; chromosomes are still double Figure 13.8 Telophase I, cytokinesis, and meiosis II

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Comparison of Mitosis and Meiosis Meiosis can be distinguished from mitosis – By 3 events in Meiosis l

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Prophase I Synapsis & crossing over – Homologous chromo’s physically connect & exchange genetic info

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Tetrads on the metaphase plate – In metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Separation of homologues – anaphase I of meiosis- homologous pairs move toward opp poles of the cell – anaphase II of meiosis- sister chromatids separate

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 13.9 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 Tetrad 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 Daughter cells of meiosis II n n nn Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n2n2n2n Daughter cells of mitosis 2n = 6 A comparison of mitosis and meiosis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 13.4: Genetic variation produced in sexual life cycles contributes to evolution Reshuffling of genetic material in meiosis – Produces genetic variation

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Origins of Genetic Variation Among Offspring In species that produce sexually – The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Independent Assortment of Chromosomes Homologous pairs of chromo’s – Orient randomly at metaphase I of meiosis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In independent assortment – Each pair of chromo’s sorts its maternal & paternal homologues into daughter cells independently of the other pairs Figure Key Maternal set of chromosomes Paternal set of chromosomes Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Possibility 2 Metaphase II Daughter cells Combination 1Combination 2Combination 3Combination 4

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Crossing Over Crossing over – Produces recombinant chromosomes that carry genes derived from 2 diff parents Figure Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Random Fertilization The fusion of gametes – Will produce a zygote with any of about 64 trillion diploid combinations

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Evolutionary Significance of Genetic Variation Within Populations Genetic variation – Is the raw material for evolution by natural selection

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutations – Are the original source of genetic variation Sexual reproduction – Produces new combinations of variant genes, adding more genetic diversity