Meiosis Lecture Presentation PDF

13 Meiosis Lecture Presentation by Cindy S. Malone, PhD, California State University Northridge © 2017 Pearson Education, Ltd. Chapter 13 Ope...

13 Meiosis Lecture Presentation by Cindy S. Malone, PhD, California State University Northridge © 2017 Pearson Education, Ltd. Chapter 13 Opening Roadmap. © 2017 Pearson Education, Ltd. Introduction to Meiosis During sexual reproduction Reproductive cells called gametes unite to form a new individual This process is called fertilization Gametes are called sperm and eggs in animals Meiosis is nuclear division that leads to halving of chromosome number Gametes must contain half the chromosome number At fertilization, full chromosome number is restored © 2017 Pearson Education, Ltd. Chromosomes Come in Distinct Sizes and Shapes Every organism has a characteristic number of chromosomes Sex chromosomes determine the sex of the individual In many animals: Females have two X chromosomes Males have an X and a Y chromosome Autosomes are non-sex chromosomes © 2017 Pearson Education, Ltd. Figure 13.1 4 4 3 2 3 2 Y X X X Male Female © 2017 Pearson Education, Ltd. Chromosomes Come in Distinct Sizes and Shapes Chromosomes of the same type are called homologous chromosomes, or homologs A pair of homologs is called a homologous pair Homologous pairs contain the same genes in the same position along the chromosome However, the two homologs are not identical A gene is a section of DNA that influences one or more hereditary traits Different versions of a specific gene are called alleles Homologs may contain different alleles © 2017 Pearson Education, Ltd. Figure 13.2 Homologous chromosomes Gene for eye color Gene for eye color (allele for red eyes) (allele for purple eyes) Drosophila autosome 2 © 2017 Pearson Education, Ltd. The Concept of Ploidy A karyotype identifies the number and types of chromosomes present in a species Many organisms, including humans, are diploid They have two homologs of each chromosome They have two alleles of each gene Other organisms are haploid They have only one of each type of chromosome They have just one allele of each gene © 2017 Pearson Education, Ltd. The Concept of Ploidy Each species has a haploid number (n) Indicates the number of distinct types of chromosomes present Sex chromosomes count as a single type In humans, n is 23 A cell’s ploidy (n, 2n, 3n, etc.) indicates the number of complete chromosome sets it contains © 2017 Pearson Education, Ltd. The Concept of Ploidy Diploid (2n) cells have A paternal chromosome that came from the father A maternal chromosome that came from the mother Humans are diploid, so 2n = 46 Organisms with three or more versions of each type of chromosome are called polyploid (3n, 4n, etc.) © 2017 Pearson Education, Ltd. Table 13.1 © 2017 Pearson Education, Ltd. An Overview of Meiosis Just before meiosis begins, each chromosome in the diploid (2n) parent cell is replicated When replication is complete, each chromosome has two identical sister chromatids They remain attached along most of their length The two attached sister chromatids are still considered a single replicated chromosome © 2017 Pearson Education, Ltd. Figure 13.3 Unreplicated Unreplicated maternal paternal chromosome chromosome Nuclear envelope Replication Replicated Replicated maternal paternal chromosome chromosome Sister chromatids Homologous pair of replicated chromosomes © 2017 Pearson Education, Ltd. Meiosis Consists of Two Cell Divisions Meiosis consists of two cell divisions 1. M ​ eiosis I—the two homologs of each chromosome pair separate into two daughter cells Each daughter cell has one set of chromosomes A diploid parent produces two haploid daughter cells 2. M ​ eiosis II—the sister chromatids of each chromosome separate into two daughter cells Each of the two daughter cells from meiosis I divides Result is four haploid cells © 2017 Pearson Education, Ltd. Figure 13.4 Parent cell is diploid (2n) and contains MEIOSIS I a homologous pair of replicated chromosomes Homologs separate Daughter cells are haploid (n) and MEIOSIS II Sister contain chromatids just one separate homolog Four daughter cells contain one unreplicated chromosome each (n). In animals, these cells can develop into gametes. © 2017 Pearson Education, Ltd. Meiosis Consists of Two Cell Divisions As in mitosis, chromosome movement is coordinated by microtubules of the spindle apparatus Microtubules attach to kinetochores at the centromere of each chromosome Ends of microtubules at each kinetochore fray, driving chromosome movement © 2017 Pearson Education, Ltd. Meiosis I is a Reduction Division Meiosis I reduces the chromosome number In most plants and animals, a diploid cell produces four haploid daughter cells In animals, the daughter cells become eggs or sperm by the process of gametogenesis Fertilization results in a diploid cell called a zygote A full complement of chromosomes is restored Each diploid individual receives a haploid chromosome set from its mother and its father © 2017 Pearson Education, Ltd. Figure 13.5 Female Male gamete gamete (egg) (sperm) (n) Fertilization (n) Diploid offspring contains homologous pair of chromosomes Zygote (2n) © 2017 Pearson Education, Ltd. Meiosis I Is a Reduction Division An animal’s life cycle summarizes life from fertilization through offspring production Meiosis in an adult produces haploid gametes that combine during fertilization to form a diploid zygote The zygote develops through mitosis into an adult of the next generation © 2017 Pearson Education, Ltd. Figure 13.6 Diploid (2n) MEIOSIS Haploid (n) Number of chromosomes reduced by half (2n → n) Diploid Haploid adult gametes (n) Sperm Egg (2n) Diploid number of chromosomes restored (n + n → 2n) Zygote (2n) © 2017 Pearson Education, Ltd. The Phases of Meiosis I Meiosis I is a continuous process with five distinct phases: 1. Early prophase I 2. Late prophase I 3. Metaphase I 4. Anaphase I 5. Telophase I © 2017 Pearson Education, Ltd. Early Prophase I In early prophase I Nuclear envelope begins to break down Chromosomes condense The spindle apparatus begins to form The homolog pairs come together in a pairing process called synapsis The structure that results from synapsis is called a bivalent, consisting of two homologs The chromatids of the homologs are called non-sister chromatids © 2017 Pearson Education, Ltd. Late Prophase I In late prophase I The two homologs become attached to microtubules from opposite poles of the spindle apparatus The homologs begin to separate They remain attached at many points, called chiasmata Exchange or crossing over between homologous non-sister chromatids occurs Produces chromosomes with a combination of maternal and paternal alleles © 2017 Pearson Education, Ltd. Metaphase I, Anaphase I, and Telophase I In metaphase I The paired homologs line up at the metaphase plate Alignment of the homologs is random In anaphase I, the paired homologs separate and migrate to opposite ends of the cell In telophase I, the homologs finish migrating to the poles of the cell Then the cell divides in the process of cytokinesis © 2017 Pearson Education, Ltd. Figure 13.7-1 Replicated Bivalent (4 chromatids from chromosomes 2 homologous chromosomes) Nuclear Non-sister Spindle envelope chromatids apparatus Chiasma 2n In this example, Maternal chromosomes n = 3 so 2n = 6 Paternal chromosomes 1. Interphase: 2. Early prophase I 3. Late prophase I 4. Metaphase I 5. Anaphase I Uncondensed chromosomes replicate in parent cell. © 2017 Pearson Education, Ltd. Meiosis I: A Recap Meiosis I results in daughter cells with only one chromosome of each homologous pair They are haploid but still contain replicated chromosomes They have a random assortment of maternal and paternal chromosomes due to 1. Crossing over 2. Random distribution of maternal and paternal homologs to daughter cells © 2017 Pearson Education, Ltd. The Phases of Meiosis II No chromosome replication occurs between meiosis I and meiosis II Like meiosis I, meiosis II is a continuous process, but with four distinct phases: 1. Prophase II 2. Metaphase II 3. Anaphase II 4. Telophase II © 2017 Pearson Education, Ltd. The Phases of Meiosis II Prophase II The spindle apparatus forms One spindle fiber attaches to the centromere of each sister chromatid Metaphase II Replicated chromosomes line up at the metaphase plate © 2017 Pearson Education, Ltd. The Phases of Meiosis II Anaphase II Sister chromatids separate The resulting daughter chromosomes begin moving to opposite sides of the cell Telophase II Chromosomes arrive at opposite sides of the cell A nuclear envelope forms around each haploid set of chromosomes Each cell then undergoes cytokinesis © 2017 Pearson Education, Ltd. Figure 13.7-2 Sister chromatids n n n n n n 6. Telophase I 7. Prophase II 8. Metaphase II 9. Anaphase II 10. Telophase II © 2017 Pearson Education, Ltd. The Phases of Meiosis II Meiosis II results in four haploid daughter cells Each has one of each type of chromosome One diploid cell with replicated chromosomes Gives rise to four haploid cells with unreplicated chromosomes © 2017 Pearson Education, Ltd. Web Activity: Meiosis © 2017 Pearson Education, Ltd. A Closer Look at Synapsis and Crossing Over Four steps to homolog pairing and crossing over: 1. As chromosomes condense, sister chromatids stay joined along their entire length by cohesions 2. Homologs pair by synapsis and are held together by proteins called the synaptonemal complex 3. Breaks are made in the DNA and cross-over occurs between corresponding segments of non-sister chromatids 4. The synaptonemal complex disassembles and homologs are held together only at chiasmata © 2017 Pearson Education, Ltd. Figure 13.8 Bivalent Crossover between non-sister chromatids Chiasma Chiasma Cohesin Kinetochore proteins microtubule Kinetochores Homologs Synaptonemal Sister chromatids complex 1. Condensation 2. Synapsis (bivalent 3. Crossing over 4. Partial separation formation) and chiasma of homologs formation © 2017 Pearson Education, Ltd. Mitosis versus Meiosis The key difference between the two processes: Homologs pair in meiosis but not in mitosis Because homologs pair in prophase I, they separate during anaphase I, resulting in a reduction division Mitosis produces two diploid daughter cells that are genetically identical to the parent cells Meiosis produces four haploid daughter cells that are genetically distinct from each other and from the parent cell © 2017 Pearson Education, Ltd. Figure 13.9 MITOSIS MEIOSIS 2n Diploid parent cell 2n Diploid parent cell Chromosome replication Chromosome replication Prophase I Prophase 2n 2n Metaphase Metaphase I 2n 2n Anaphase and Telophase Anaphase I and Telophase I 2n n Two diploid daughter Meiosis II cells of mitosis n Four haploid daughter cells of meiosis © 2017 Pearson Education, Ltd. Table 13.2 © 2017 Pearson Education, Ltd. Meiosis Promotes Genetic Variation Meiosis results in four gametes with a chromosome composition different from each other and from the parent cells Independent shuffling of maternal and paternal chromosomes Crossing over during meiosis I Fertilization also introduces variation as haploid sets of chromosomes combine to make a unique offspring © 2017 Pearson Education, Ltd. Meiosis Promotes Genetic Variation The changes in chromosomes produced by meiosis and fertilization are significant Asexual reproduction produces clones that are genetically identical to one another and to the parent Sexual reproduction produces offspring with unique chromosome complements Only sexual reproduction results in a shuffling of the alleles of the parents into the offspring © 2017 Pearson Education, Ltd. The Role of Independent Assortment The random separation of homologous chromosomes during meiosis I can result in A variety of combinations of maternal and paternal chromosomes Called the principle of independent assortment Each daughter cell gets a random assortment of maternal and paternal chromosomes and genes Called genetic recombination Generates a great deal of genetic diversity © 2017 Pearson Education, Ltd. Figure 13.10 (a) Example: An individual has different alleles of two genes implicated in two genetically transmitted diseases. Normal Sickle cell allele allele Cystic Normal fibrosis allele allele Hb-β gene CFTR gene on chromosome 11 on chromosome 7 (b) During meoisis I, bivalents can line up in two different ways before the homologs separate. OR Normal Hb Sickle cell disease Sickle cell disease Normal Hb Cystic fibrosis Normal CFTR Cystic fibrosis Normal CFTR © 2017 Pearson Education, Ltd. The Role of Crossing Over Crossing over produces New combinations of alleles on the same chromosome Combinations that did not exist in each parent Genetic recombination from crossing over and from independent assortment Increases the genetic variability of gametes produced by meiosis Beyond that produced by random assortment of chromosomes © 2017 Pearson Education, Ltd. How Does Fertilization Affect Genetic Variation? Each gamete is genetically unique Even in self-fertilization Where gametes from the same individual combine The offspring will be genetically different from the parent Outcrossing Where gametes from two individuals combine Increases the genetic diversity of the offspring even further © 2017 Pearson Education, Ltd. What Happens When Things Go Wrong in Meiosis? Errors in meiosis are common Over a third of conceptions are spontaneously terminated because of problems in meiosis One infant in every 691 live births in the USA have Down syndrome Caused by an extra copy of chromosome 21 Called trisomy 21 © 2017 Pearson Education, Ltd. Figure 13.11 © 2017 Pearson Education, Ltd. How Do Mistakes Occur? If both homologs or both sister chromatids move to the same pole of the parent cell The products of meiosis will be abnormal This sort of meiotic error is referred to as nondisjunction © 2017 Pearson Education, Ltd. Figure 13.12 n+1 Homologs fail to separate n+1 n–1 2n = 4 n =2 n–1 1. Meiosis I starts 2. Nondisjunction 3. Sister chromatids 4. Aneuploidy results. normally. Bivalents line occurs with one set separate normally in All gametes have too many or up in middle of cell. of homologs. meiosis II. too few chromosomes. © 2017 Pearson Education, Ltd. How Do Mistakes Occur? Nondisjunction results in gametes that Contain an extra chromosome (n + 1) Lack one chromosome (n − 1) Fertilization of an n + 1 gamete leads to trisomy Fertilization of an n − 1 gamete leads to monosomy Cells with too many or too few of a chromosome are called aneuploid © 2017 Pearson Education, Ltd. Why Do Mistakes Occur? Meiotic errors are a result of random errors Maternal age is an important factor in the frequency of trisomy Egg development, or oogenesis, in humans Primary oocytes enter meiosis I during female embryonic development Arrest in prophase I until sexual maturity is reached Don’t complete meiosis until ovulation, years later © 2017 Pearson Education, Ltd. Figure 13.13 1 10 Incidence of Down syndrome per number of births 1 15 1 25 1 40 1 1 1 70 900 1 1 1 1 100 300 200 720 450 Age of mother (years) © 2017 Pearson Education, Ltd. Web Activity: Mistakes in Meiosis © 2017 Pearson Education, Ltd. Why Does Meiosis Exist? Sexual reproduction is relatively uncommon among organisms Most organisms undergo asexual reproduction Asexual reproduction is much more efficient Can produce twice as many offspring in the same amount of time Why wasn’t sexual reproduction eliminated? © 2017 Pearson Education, Ltd. Figure 13.14 Asexual reproduction Sexual reproduction Generation 1 There are only half as many child-producing offspring in the sexual population as in the asexual population Generation 2 Generation 3 © 2017 Pearson Education, Ltd. The Purifying Selection Hypothesis In asexual reproduction, a damaged gene will be inherited by all of that individual’s offspring Sexually reproducing individuals are likely to have offspring that lack deleterious alleles present in the parent Natural selection against deleterious alleles is called purifying selection Over time, purifying selection should steadily reduce the numerical advantage of asexual reproduction © 2017 Pearson Education, Ltd. The Changing-Environment Hypothesis Offspring that are genetically different from their parents Are more likely to survive and produce offspring if the environment changes Offspring that are genetically identical to their parents Are less likely to survive and produce offspring if the environment changes © 2017 Pearson Education, Ltd. The Changing-Environment Hypothesis If a new strain of disease-causing agent evolves All of the asexually produced offspring are likely to be susceptible to that new strain But if the offspring are genetically varied, then it is Likely that some offspring will have combinations of alleles That enable them to fight off the new disease And will produce offspring of their own © 2017 Pearson Education, Ltd. Figure 13.15 Does exposure to evolving pathogens favor outcrossing? In environments where evolving pathogens are present, sexual reproduction by outcrossing will be favored. The presence of evolving pathogens will not favor outcrossing. 1. Start with a pathogen-free population of roundworms with a 20% rate of outcrossing. 2. Divide the population; grow one subgroup in the absence of a pathogen and another subgroup in the presence of an evolving pathogen. Grow without Grow with pathogen pathogen 3. Assess the rate of outcrossing over many generations. The rate of outcrossing will increase in response to exposure by a pathogen. The rate of outcrossing will not be influenced by a pathogen. With pathogen Outcrossing rate Without pathogen Generation Exposure to evolving pathogens favors outcrossing. © 2017 Pearson Education, Ltd. The Changing-Environment Hypothesis Studies support the changing-environment hypothesis Sexual reproduction may be an adaptation that increases the fitness of individuals in certain environments © 2017 Pearson Education, Ltd.