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Meiosis and Sexual Life Cycles

Meiosis and Sexual Life Cycles. Living organisms are distinguished by their ability to reproduce their own kind Heredity Is the transmission of traits from one generation to the next Variation Shows that offspring differ somewhat in appearance from parents and siblings. Inheritance of Genes.

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Meiosis and Sexual Life Cycles

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  1. Meiosis and Sexual Life Cycles • Living organisms are distinguished by their ability to reproduce their own kind • Heredity • Is the transmission of traits from one generation to the next • Variation • Shows that offspring differ somewhat in appearance from parents and siblings

  2. Inheritance of Genes • Genes are segments of DNA, units of heredity • Offspring acquire genes from parents by inheriting chromosomes • Genetics is the scientific study of heredity and hereditary variation

  3. Inheritance of Genes • Each gene in an organism’s DNA has a specific locus on a certain chromosome • We inherit one set of chromosomes from our mother and one set from our father • Two parents give rise to offspring that have unique combinations of genes inherited from the two parents - sexual reproduction

  4. Parent Bud 0.5 mm Figure 13.2 Asexual Reproduction • In asexual reproduction, one parent produces genetically identical offspring by mitosis

  5. Haploid gametes (n = 23) Haploid (n) Ovum (n) Diploid (2n) Sperm Cell (n) FERTILIZATION MEIOSIS Diploid zygote (2n = 46) Ovary Testis Mitosis and development Multicellular diploid adults (2n = 46) Sexual Reproduction - The Human Life Cycle • A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism • Fertilization and meiosis alternate in sexual life cycles • At sexual maturity the ovaries and testes produce haploid gametes by meiosis • Unlike somatic cells, sperm and egg cells are haploid cells, containing only one set of chromosomes • During fertilization, sperm and ovum fuse forming a diploid zygote • The zygote develops into an adult organism

  6. Meiosis • Reduces the chromosome number such that each daughter • Cell has a haploid set of chromosomes • Ensures that the next generation will have: • Diploid number of chromosome • Exchange of genetic information (combination of traits • that differs from that of either parent)

  7. Meiosis • Only diploid cells can divide by meiosis. • Prior to meiosis I, DNA replication occurs. • During meiosis, there will be two nuclear divisions, and the result will be four haploid nuclei. • No replication of DNA occurs between meiosis I and meiosis II.

  8. Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Figure 13.7 Haploid cells with unreplicated chromosomes Meiosis • Meiosis reduces the number of chromosome sets from diploid to haploid • Meiosis takes place in two sets of divisions • Meiosis I reduces the number of chromosomes from diploid to haploid • Meiosis II produces four haploid daughter cells

  9. Meiosis Phases • Meiosis involves the same four phases seen in mitosis • prophase • metaphase • anaphase • telophase • They are repeated during both meiosis I and meiosis II. • The period of time between meiosis I and meiosis II is called interkinesis. • No replication of DNA occurs during interkinesis because the DNA is already duplicated.

  10. Nonsister chromatids Prophase I of meiosis Tetrad Chiasma, site of crossing over Prophase I • Prophase I occupies more than 90% of the time required for meiosis • Chromosomes begin to condense • In synapsis, the 2 members of each homologous pair of chromosomes line up side-by-side, aligned gene by gene, to form a tetrad consisting of 4 chromatids • During synapsis, sometimes there is an exchange of homologous parts between non-sister chromatids. This exchange is called crossing over • Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred

  11. PROPHASE I METAPHASE I ANAPHASE I Sister chromatids remain attached Centromere (with kinetochore) Chiasmata Sister chromatids Metaphase plate Spindle Microtubule attached to kinetochore Homologous chromosomes separate Tetrad Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Tetrads line up Pairs of homologous chromosomes split up Metaphase I • At metaphase I, tetrads 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 tetrad • Microtubules from the other pole are attached to the kinetochore of the other chromosome

  12. PROPHASE I METAPHASE I ANAPHASE I Sister chromatids remain attached Centromere (with kinetochore) Chiasmata Sister chromatids Metaphase plate Spindle Microtubule attached to kinetochore Homologous chromosomes separate Tetrad Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Tetrads line up Pairs of homologous chromosomes split up Anaphase I • In anaphase I, pairs of homologous chromosomes separate • One chromosome moves toward each pole, guided by the spindle apparatus • Sister chromatids remain attached at the centromere and move as one unit toward the pole

  13. 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 • In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms • No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

  14. TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Haploid daughter cells forming Cleavage furrow Sister chromatids separate Prophase II • Meiosis II is very similar to mitosis • In prophase II, a spindle apparatus forms • In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate

  15. TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Haploid daughter cells forming Cleavage furrow Sister chromatids separate Metaphase II • At metaphase II, the sister chromatids are 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

  16. TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Haploid daughter cells forming Cleavage furrow Sister chromatids separate Anaphase II • At anaphase II, the sister chromatids separate • The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles

  17. TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Haploid daughter cells forming Cleavage furrow Sister chromatids separate Telophase II and Cytokinesis • In telophase II, the chromosomes arrive at opposite poles • Nuclei form, and the chromosomes begin decondensing • 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

  18. A Comparison of Mitosis and Meiosis • Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell • Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell • The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis

  19. A Comparison of Mitosis and Meiosis • Three events are unique to meiosis, and all three occur in meiosis l: • Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information • At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes • At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell. In anaphase II of meiosis, the sister chromatids separate

  20. MITOSIS MEIOSIS Chiasma (site of crossing over) Parent cell (before chromosome replication) MEIOSIS I Prophase I Prophase Chromosome replication Chromosome replication Tetrad formed by synapsis of homologous chromosomes Duplicated chromosome (two sister chromatids) 2n = 6 Tetrads positioned at the metaphase plate Chromosomes positioned at the metaphase plate Metaphase I Metaphase Sister chromatids separate during anaphase Anaphase Telophase Homologues separate during anaphase I; sister chromatids remain together Anaphase I Telophase I Haploid n = 3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II Sister chromatids separate during anaphase II A Comparison Of Mitosis And Meiosis

  21. Comparison • Mitosis • Homologous chromosomes do not pair up • No genetic exchange between homologous chromosomes • One diploid cell produces 2 diploid cells or one haploid cell produces 2 haploid cells • New cells are genetically identical to original cell (except for mutation) • Meiosis • DNA duplication followed by 2 cell divisions • Sysnapsis • Crossing-over • One diploid cell produces 4 haploid cells • Each new cell has a unique combination of genes

  22. Haploid gametes (n = 23) Haploid (n) Ovum (n) Diploid (2n) Sperm Cell (n) FERTILIZATION MEIOSIS Diploid zygote (2n = 46) Ovary Testis Mitosis and development Multicellular diploid adults (2n = 46) Sexual Reproduction - The Human Life Cycle • During fertilization, sperm and ovum fuse forming a diploid zygote • The zygote develops into an adult organism

  23. Spermatocytes to Spermatids • Primary spermatocytes undergo meiosis I, forming two haploid cells called secondary spermatocytes • Secondary spermatocytes undergo meiosis II and their daughter cells are called spermatids • Spermatids are small round cells seen close to the lumen of the tubule • Late in spermatogenesis, spermatids are nonmotile • Spermiogenesis – spermatids lose excess cytoplasm and form a tail, becoming motile sperm

  24. Spermatogenesis Figure 27.8b, c

  25. Oogenesis • Production of female sex cells by meiosis • In the fetal period, oogonia (2n ovarian stem cells) multiply by mitosis and store nutrients • Primordial follicles appear as oogonia are transformed into primary oocytes • Primary oocytes begin meiosis but stall in prophase I • From puberty, each month one activated primary oocyte completes meiosis one to produce two haploid cells • The first polar body • The secondary oocyte • The secondary oocyte arrests in metaphase II and is ovulated • If penetrated by sperm the second oocyte completes meiosis II, yielding: • One large ovum (the functional gamete) • A tiny second polar body

  26. Oogenesis

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