2. 2007-2008 Meiosis & Sexual Reproduction

3. Cell division/Asexual reproduction Mitosis ▫produce cells with same information  identical daughter cells ▫exact copies  clones ▫same amount of DNA  same number of chromosomes  same genetic information Aaaargh! I’m seeing double!

4. Asexual reproduction Single-celled eukaryotes ▫yeast (fungi) ▫Protists  Paramecium  Amoeba Simple multicellular eukaryotes ▫Hydra What are the disadvantages of asexual reproduction? What are the advantages? budding

5. How about the rest of us? What if a complex multicellular organism (like us) wants to reproduce? ▫joining of egg + sperm Do we make egg & sperm by mitosis? 46 + 92 eggspermzygote What if we did, then…. Doesn’t work! No!

6. Genetics ▫Is the scientific study of heredity and variation ▫Studies the passing of chromosomes from parents to offspring Meiosis creates gametes, sex cells containing half the species chromosome number Meiosis and Fertilization  maintain a species chromosome number during a sexual life cycle  Increase variation of individuals within a species

7. Heredity ▫Is the transmission of traits from one generation to the next Variation ▫Shows that offspring differ somewhat in appearance from parents and siblings  Two parents give rise to offspring that have unique combinations of genes inherited from the two parents Figure 13.1

8. We inherit ▫One set of chromosomes from our mother and one set from our father Specific location of a gene gametes transmit genes from on generation to the next one chromosome contains hundreds to a few thousand genes each gene has a specific sequence of nucleotides

9. Genes ▫Are the units of heredity ▫Are segments of DNA ▫Contain specific sequence of nucleotides Genes program cells to synthesize specific enzymes and other proteins which create an organism’s inherited traits

10. Sets of Chromosomes in Human Cells In humans ▫Each somatic cell has 46 chromosomes, made up of two sets ▫One set of chromosomes comes from each parent All 46 are visible during mitosis The two chromosomes composing a pair of each type are called homologous chromosomes

11. Human female karyotype 46 chromosomes 23 pairs 22 pairs of autosomes = chromosomes that do not control one’s sex Sex chromosomes

12. Human male karyotype 46 chromosomes 23 pairs homologous chromosomes homologous = “same information” single-stranded

13. Homologous chromosomes ▫Are the two chromosomes composing a pair ▫Have the same characteristics or gene loci  Therefore, every cell has two copies of every gene  Allele: version of a gene  Ex: Hitchhikers thumb, Tongue rolling  All cells have two alleles for every gene – one maternal and one paternal homologous chromosomes sister chromatids single-stranded double-stranded homologous chromosomes

14. 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 At sexual maturity ▫The ovaries and testes produce haploid gametes by meiosis During fertilization ▫These gametes, sperm and ovum, fuse, forming a diploid zygote ▫This starts the human life cycle The zygote (fertilized egg) ▫Develops into an adult organism ▫Generates all of the somatic cells of the organism haploid diploid

15. 23 Meiosis reduces the number of chromosome sets from diploid to haploid ▫Takes place in two sets of divisions, meiosis I and meiosis II ▫Results in 4 daughter cells with half the chromosome number of the parent 23 46 egg sperm 46 meiosis 46 fertilization 23 gametes zygote

16. Overview of Meiosis Meiosis I ▫Reduces the number of chromosomes from one diploid cell to two haploid cells Meiosis II ▫Two haploid daughter cells become four haploid daughter cells http://highered.mcgraw- hill.com/sites/9834092339/student_view0/chapte r11/how_meiosis_works.html 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 DNA replication homologous chromosomes separate Sister chromatids separate

17. Overview of meiosis interphase 1prophase 1metaphase 1anaphase 1 telophase 1 prophase 2 metaphase 2anaphase 2telophase 2 2n = 4 n = 2

18. A Comparison of Mitosis and Meiosis Meiosis and mitosis can be distinguished from mitosis ▫By three events in Meiosis l that are unique to meiosisunique to meiosis http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter11/unique_features_of_meiosis.html

19. Synapsis and crossing over ▫Crossing Over: Homologous chromosomes physically connect and exchange genetic information (sections of DNA)  Synapsis: nonsister chromatids connection preceding crossing over ▫Occurs at the end of Prophase I  Nonsister chromatids remain connected via chiasmata  Chiasmata: location on nonsister chromatids where DNA exchange occurred synapsis

20. Tetrads on the metaphase plate ▫At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates tetrad synapsis

21. Separation of homologues ▫At anaphase I of meiosis, cohesins between nonsister chromatids are cleaved and homologous pairs move toward opposite poles of the cell  Sister chromatids remain attached ▫In anaphase II of meiosis, cohesins between sister chromatids are cleaved and the sister chromatids separate http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter11/the_function_of_cohesin.html

22. 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 http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter11/comparison_of_meiosis_and_mitosis.html

23. Origins of Genetic Variation Among Offspring Three mechanisms that contribute to genetic variation from sexual reproduction: 1.Independent assortment of chromosomes 2.Crossing over 3.Random Fertilization

24. Independent Assortment of Chromosomes Homologous pairs of chromosomes ▫Orient randomly at metaphase I of meiosis ▫Maternal and paternal sister chromatids can be closer to either pole ▫This results in 2 n possibilities  In humans, this means there are 2 23 or 8.4 million possible combinations of maternal and paternal chromosomes metaphase1

25. Independent assortment Figure 13.10 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

26. Homologous pair DNA is not exclusively maternal or paternal though; due to crossing over Crossing over ▫Produces recombinant chromosomes that carry genes derived from two different parents 1.Prophase I: synapsis and crossing over occur, homologs separate slightly 2.Chiasmata and cohesin between nonsister chromatids hold homologs together; they move to the metaphase plate 3.Break down of proteins holding between nonsister chromatid arms together allow homologs with recombinant chromosomes to separate prophase 1

27. Random Fertilization The fusion of gametes ▫Will produce a zygote with any of about 64 trillion diploid combinations ▫This does not even account for variations produced by crossing over

28. Evolutionary Significance of Genetic Variation Within Populations Genetic variation ▫Is the raw material for evolution by natural selection Darwin: a population evolves through the differential reproductive success of its variant members ▫Individuals best suited to their environment reproduce more – leaving offspring behind to continue transmitting beneficial genes Mutations ▫Are the original source of genetic variation