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Ch 13 Sexual Life Cycles and Meiosis. Living organisms Have ability to reproduce Heredity Transmission of traits from one generation to the next Variation.

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Presentation on theme: "Ch 13 Sexual Life Cycles and Meiosis. Living organisms Have ability to reproduce Heredity Transmission of traits from one generation to the next Variation."— Presentation transcript:

1 Ch 13 Sexual Life Cycles and Meiosis

2 Living organisms Have ability to reproduce Heredity Transmission of traits from one generation to the next Variation Offspring often differ in appearance from parents and siblings Environmental and genetic influences Genetics Scientific study of heredity and variation Reproduction and the Transmission of Traits

3 Progeny (or offspring) -Inherit chromosomes from parents -DNA in chromosomes encode the genes that specify characteristics For example eye color, blood type, predisposition to disease

4 What are the mechanisms by which chromosomes are passed onto (eukaryotic) offspring? Asexual reproduction Sexual reproduction

5 LE 13-2 Parent 0.5 mm Bud Asexual reproduction Bud forms by mitosis of parent cells Bud genetically identical to parent

6 Sexual Reproduction 1.Meiosis: formation of haploid gametes (eggs, sperm) in parents 2. Fertilization: fusion of gametes from opposite parents--> diploid zygote-->mitotic growth-->embryo-->adult

7 LE 13-5 Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm cell (n) Testis Ovary Mitosis and development Multicellular diploid adults (2n = 46) FERTILIZATIONMEIOSIS Diploid zygote (2n = 46) Human Sexual Reproduction Offspring genetically unique from parents

8 How many chromosomes are inherited by human zygotes? 46 chromosomes or 2n= 46 n= the number of chromosomes in a set 23 from mother, maternal chromosomes 23 from father, paternal “

9 LE 13-3 5 µm Human Karyotype Stained metaphase chromosomes from human somatic cell Why stain metaphase chromosomes? Compact & easier to see com- pared to interphase

10 LE 13-3 5 µm Pair of homologous chromosomes Sister chromatids Centromere

11 Can you tell the difference between a human male and female by the karyotype? Humans have 22 pairs of autosomes (non-sex chromosomes) 1 pair of sex chromosomes Female sex ch.: XX Male sex ch.: XY

12 LE 13-3 5 µm Human Karyotype Male or female?

13 If somatic cells have two sets of chromosomes (diploid), how do gametes end up with only one set (haploid)? Diploid precursors to the egg and sperm undergo meiosis in the testis and ovary.

14 LE 13-7 Homologous pair of chromosomes in diploid parent cell Interphase Homologous pair of replicated chromosomes Chromosomes replicate Meiosis I Diploid cell with replicated chromosomes Sister chromatids Meiosis II Homologous chromosomes separate Sister chromatids separate Haploid cells with replicated chromosomes Haploid cells with unreplicated chromosomes Overview of meiosis aka tetrad Diploid Haploid

15 LE 13-4 Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosomes Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Centromere

16 LE 13-8ab Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Homologous chromosomes (red and blue) pair and exchange segments Homologous recombination (crossing-over) Pairs of homologous chromosomes split up Tetrads line up

17 LE 13-8b Cleavage furrow MEIOSIS II: Separates sister chromatids PROPHASE II METAPHASE IIANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes

18 LE 13-9 Prophase Duplicated chromosome (two sister chromatids) Chromosome replication 2n = 6 Parent cell (before chromosome replication) Chromosome replication MITOSISMEIOSIS Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Tetrads positioned at the metaphase plate Metaphase I Chromosomes positioned at the metaphase plate Metaphase Anaphase Telophase Cytokinesis Homologues separate during anaphase I; sister chromatids remain together Sister chromatids separate during anaphase Daughter cells of meiosis I Haploid n = 3 Anaphase I Telophase I MEIOSIS II Daughter cells of mitosis 2n2n 2n2n n Sister chromatids separate during anaphase II n nn Daughter cells of meiosis II

19 Mitosis produces two genetically identical diploid cells Meiosis produces four genetically distinct haploid cells

20 Mechanisms of Genetic Variation Among Offspring Caused by Sexual Reproduction 1.Crossing over (homologous recombination) (Prophase I) 2. Independent assortment of chromosomes (Metaphase I & II) 3. Random fertilization (post-meiosis)

21 LE 13-11 Prophase I of meiosis Tetrad Nonsister chromatids Chiasma, site of crossing over Recombinant chromosomes Metaphase I Metaphase II Daughter cells Recombination Chiasmata, pl

22 LE 13-10 Key Maternal set of chromosomes Paternal set of chromosomes Possibility 1 Possibility 2 Combination 2 Combination 1 Combination 3 Combination 4 Daughter cells Metaphase II Metaphase I Two equally probable arrangements of chromosomes at Independent Assortment

23 Random Fertilization Any sperm can fuse with any ovum (unfertilized egg) a (human) zygote has about 64 trillion diploid combinations Each zygote has unique genetic identity

24 I think they were talking about us. I hope they had it right! If not maybe we can answer questions.

25 See meiosis animation What is missing?


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