Lecture 13: Sexual Life Cycles and Meiosis PDF

Chapter 13 Sexual Life Cycles and Meiosis Lecture Presentations by Nicole Tunbridge and © 2018 Pearson Education Ltd....

Chapter 13 Sexual Life Cycles and Meiosis Lecture Presentations by Nicole Tunbridge and © 2018 Pearson Education Ltd. Kathleen Fitzpatrick Variations on a Theme  Offspring resemble their parents more than they do unrelated individuals  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  Genetics is the scientific study of heredity and variation © 2018 Pearson Education Ltd. Figure 13.1a A sperm fertilizing an egg © 2018 Pearson Education Ltd. Concept 13.1: 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 © 2018 Pearson Education Ltd. Inheritance of Genes  Genes 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 (sperm and eggs)  Most DNA is packaged into chromosomes  Humans have 46 chromosomes in the nuclei of their somatic cells, all cells of the body except gametes and their precursors  A gene’s specific position along a chromosome is called its locus © 2018 Pearson Education Ltd. Comparison of Asexual and Sexual Reproduction  In asexual reproduction, a single individual passes all of its 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  Genetic variations are normal consequences of sexual reproduction.  Genetic variations in asexual reproduction is a consequence of DNA mutations © 2018 Pearson Education Ltd. Figure 13.2 0.5 mm Parent Bud (a) Hydra (b) Redwoods © 2018 Pearson Education Ltd. Video: Hydra Budding © 2018 Pearson Education Ltd. Concept 13.2: 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 © 2018 Pearson Education Ltd.  Sets of Chromosomes in Human Cells  Human somatic cells have 23 pairs of chromosomes  A karyotype is an ordered display of the pairs of chromosomes from a cell. Can be used to detect genetic diseases caused by abnormal number of chromosomes ( Down Syndrome)  The two chromosomes in each pair are called homologous chromosomes, or homologs  Chromosomes in a homologous pair are the same length and shape and carry genes controlling the same inherited characters  If a gene of eye color is located at a particular locus of a chromosome, then the homolog of that chromosome will also have a version of the same gene in the same locus. © 2018 Pearson Education Ltd. Figure 13.3 Application Technique Pair of homologous duplicated chromosomes Centromeres 5 µm Sister chromatids Metaphase chromosome Explain: why it is a metaphase chromosome?? - Each chromosome is duplicated to prepare for anaphase. - Chromosomes can only be seen during metaphase because they are condensed in the cell. © 2018 Pearson Education Ltd.  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 © 2018 Pearson Education Ltd.  Each pair of homologous chromosomes includes one chromosome from each parent  The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father  A diploid cell (2n) has two sets of chromosomes  For humans, the diploid number is 46 (2n = 46) © 2018 Pearson Education Ltd.  In a cell in which DNA synthesis has occurred, each chromosome is replicated  Each replicated chromosome consists of two identical sister chromatids © 2018 Pearson Education Ltd. Figure 13.4 Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Sister chromatids of one duplicated chromosome Centromere Two nonsister Pair of homologous chromatids in chromosomes a homologous pair (one from each set) © 2018 Pearson Education Ltd.  A gamete (sperm or egg) contains a single set of chromosomes and is thus a haploid cell (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 © 2018 Pearson Education Ltd. Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid  Like mitosis, meiosis is preceded by the replication of chromosomes  Meiosis takes place in two consecutive 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 © 2018 Pearson Education Ltd. The Stages of Meiosis  Chromosomes duplicate before meiosis  The resulting sister chromatids are closely associated along their lengths  This is called sister chromatid cohesion  The chromatids are sorted into four haploid daughter cells © 2018 Pearson Education Ltd. Figure 13.7 Interphase Pair of Homologous chromosomes appear to be homologous identical from both parents, however they may chromosomes contain variation of a specific gene at a in diploid specific loci , called allele parent cell Pair of duplicated Chromosomes homologous duplicate chromosomes Sister Diploid cell with chromatids duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with duplicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unduplicated chromosomes © 2018 Pearson Education Ltd. Figure 13.7a Interphase Pair of homologous chromosomes in Interphase: diploid parent cell -G1 phase -S phase -G2 phase Chromosomes Pair of duplicated duplicate homologous chromosomes Sister chromatids Diploid cell with duplicated chromosomes © 2018 Pearson Education Ltd. Figure 13.7b Meiosis I 1 Homologous chromosomes separate Haploid cells with duplicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unduplicated chromosomes © 2018 Pearson Education Ltd.  Division in meiosis I occurs in four phases:  prophase I  metaphase I  anaphase I  telophase I and cytokinesis © 2018 Pearson Education Ltd. Prophase I  In early prophase I, each chromosome pairs with its homolog and crossing over occurs (exchange of genetic material)  X-shaped regions called chiasmata are sites of crossovers  Cross over occurs between homologous chromosomes and not between sister chromatids © 2018 Pearson Education Ltd. Metaphase I  In metaphase I, pairs of homologs 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 pair  Microtubules from the other pole are attached to the kinetochore of the other chromosome © 2018 Pearson Education Ltd. Anaphase I  In anaphase I, pairs of homologous chromosomes separate  One chromosome of each pair moves toward opposite poles, guided by the spindle apparatus  Sister chromatids remain attached at the centromere and move as one unit toward the pole © 2018 Pearson Education Ltd. Telophase I and 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 Cleavage furrow © 2018 Pearson Education Ltd.  In animal cells, a cleavage furrow forms  No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated © 2018 Pearson Education Ltd. Figure 13.8a MEIOSIS I: Separates homologous chromosomes Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Centrosome Sister (with Kinetochore chromatids centriole (at centromere) remain Sister pair) attached chroma- Chiasmata tids Spindle Kinetochore micro- microtubules tubules Cleavage Homologous furrow Pair of Fragments chromosomes homo- of nuclear separate envelope Metaphase logous plate chromo- somes Centromere © 2018 Pearson Education Ltd. BioFlix: Meiosis © 2018 Pearson Education Ltd. Video: Meiosis I in Sperm Formation © 2018 Pearson Education Ltd.  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 © 2018 Pearson Education Ltd. 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 © 2018 Pearson Education Ltd. 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 © 2018 Pearson Education Ltd. Anaphase II  In anaphase II, the sister chromatids separate  The sister chromatids of each chromosome now move as two newly Sister chromatids individual chromosomes toward separate opposite poles © 2018 Pearson Education Ltd. Telophase II and Cytokinesis  In telophase II, the chromosomes arrive at opposite poles  Nuclei form, and the chromosomes begin Haploid daughte decondensing cells forming © 2018 Pearson Education Ltd.  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 © 2018 Pearson Education Ltd. Figure 13.8b MEIOSIS II: Separates sister chromatids Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming © 2018 Pearson Education Ltd. BioFlix Animation: Meiosis MEIOSIS II: Separates sister chromatids © 2018 Pearson Education Ltd. A Comparison of Mitosis and Meiosis  Mitosis conserves the number of chromosome sets (2n), producing cells that are genetically identical to the parent cell  Meiosis reduces the number of chromosomes sets from two (diploid, 2n) to one (haploid, n), producing cells that differ genetically from each other and from the parent cell © 2018 Pearson Education Ltd.  Three events are unique to meiosis, and all three occur in meiosis I  Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information  Homologous pairs at the metaphase plate, , rather than individual chromosome (sister chromatids as in mitosis)  Separation of homologs during anaphase I, , rather than separation of sister chromatids in anaphase 1 of mitosis © 2018 Pearson Education Ltd. Figure 13.10a MITOSIS MEIOSIS Parent cell Chiasma MEIOSIS I Prophase Prophase I Chromosome Chromosome Duplicated Pair of duplication duplication chromosome 2n = 6 duplicated homologs Individual Pairs of Metaphase homologous Metaphase I chromosomes line up. chromosomes line up. Anaphase Anaphase I Sister chromatids Homologs Telophase separate. Telophase I separate. Daughter Sister cells of meiosis I chromatids MEIOSIS II 2n 2n separate. Daughter cells n n n n of mitosis Daughter cells of meiosis II © 2018 Pearson Education Ltd. Figure 13.10b © 2018 Pearson Education Ltd.  Sister chromatid cohesion allows sister chromatids to stay together through meiosis I  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) © 2018 Pearson Education Ltd. Concept 13.4: 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 Do we produce the same gamete cells (eggs or sperms) during our whole lifetime??! © 2018 Pearson Education Ltd. 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 © 2018 Pearson Education Ltd. 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 homologs into daughter cells independently of the other pairs © 2018 Pearson Education Ltd.  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 © 2018 Pearson Education Ltd. Figure 13.11_1 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I © 2018 Pearson Education Ltd. Figure 13.11_2 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II © 2018 Pearson Education Ltd. Figure 13.11_3 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 © 2018 Pearson Education Ltd. Crossing Over  Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent  Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome  In humans, an average of one to three crossover events occurs per chromosome © 2018 Pearson Education Ltd. Figure 13.12_1 Prophase I Nonsister chromatids of meiosis held together during synapsis Pair of homologs 1 Synapsis and crossing over Chiasma Centromere TEM © 2018 Pearson Education Ltd. Figure 13.12_2 Prophase I Nonsister chromatids of meiosis held together during synapsis Pair of homologs 1 Synapsis and crossing over Chiasma 2 Movement to the metaphase I plate Centromere TEM Anaphase I © 2018 Pearson Education Ltd. Figure 13.12_3 Prophase I Nonsister chromatids of meiosis held together during synapsis Pair of homologs 1 Synapsis and crossing over Chiasma 2 Movement to the metaphase I plate Centromere TEM 3 Breakdown of Anaphase I proteins holding sister chromatid arms together Anaphase II © 2018 Pearson Education Ltd. Figure 13.12_4 Prophase I Nonsister chromatids of meiosis held together during synapsis Pair of homologs 1 Synapsis and crossing over Chiasma 2 Movement to the metaphase I plate Centromere TEM 3 Breakdown of Anaphase I proteins holding sister chromatid arms together Anaphase II Daughter cells Recombinant chromosomes © 2018 Pearson Education Ltd. Figure 13.12a Chiasma Centromere TEM © 2018 Pearson Education Ltd. Animation: Genetic Variation © 2018 Pearson Education Ltd. 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 © 2018 Pearson Education Ltd.  Crossing over adds even more variation  Each zygote has a unique genetic identity © 2018 Pearson Education Ltd.