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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. Sexual Reproduction • 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

  6. Sex Cells - Gametes • Unlike somatic cells, sperm and egg cells are haploid cells, containing only one set of chromosomes • At sexual maturity the ovaries and testes produce haploid gametes by meiosis

  7. 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)

  8. 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.

  9. Sexual Life Cycles • The three main types of sexual life cycles • Animals • Plants and Algae • Fungi and Protists • Differ in the timing of meiosis and fertilization

  10. Haploid multicellular organism (gametophyte) n Mitosis Mitosis n n n n Spores Gametes MEIOSIS FERTILIZATION Diploid multicellular organism (sporophyte) 2n 2n Zygote Mitosis (b) Plants and some algae Plants and Some Algae • Exhibit an alternation of generations • The life cycle includes both diploid and haploid multicellular stages

  11. Haploid multicellular organism n Mitosis Mitosis n n n n Gametes MEIOSIS FERTILIZATION 2n Zygote (c) Most fungi and some protists Most Fungi and Some Protists • Meiosis produces haploid cells that give rise to a haploid multicellular adult organism • The haploid adult carries out mitosis, producing cells that will become gametes

  12. Key Haploid Diploid n n Gametes n MEIOSIS FERTILIZATION Zygote 2n 2n Diploid multicellular organism Mitosis (a) Animals Animals • Meiosis occurs during gamete formation • Gametes are the only haploid cells

  13. 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

  14. 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.

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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

  22. 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

  23. MEIOSIS I: Separates homologous chromosomes INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Sister chromatids remain attached Centromere (with kinetochore) Centrosomes (with centriole pairs) Chiasmata Metaphase plate Sister chromatids Spindle Nuclear envelope Homologous chromosomes separate Microtubule attached to kinetochore Tetrad Chromatin Pairs of homologous chromosomes split up Chromosomes duplicate Tertads line up Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Interphase and meiosis I

  24. MEIOSIS II: Separates sister chromatids TELOPHASE II AND CYTOKINESIS TELOPHASE I AND CYTOKINESIS METAPHASE II ANAPHASE II PROPHASE II Cleavage furrow Haploid daughter cells forming Sister chromatids separate 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 Telophase I, cytokinesis, and meiosis II

  25. 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

  26. 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

  27. 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

  28. 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

  29. 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 • Reshuffling of different versions of genes during sexual reproduction produces genetic variation

  30. 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

  31. Independent Assortment • Homologous pairs of chromosomes orient randomly at metaphase I of meiosis • In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs • 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

  32. Key Maternal set of chromosomes Possibility 1 Possibility 2 Paternal set of chromosomes Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 Independent Assortment of Chromosomes • Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs

  33. Crossing Over • Crossing over produces recombinant chromosomes, which combine genes inherited from each parent • Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene • In crossing over, homologous portions of two nonsister chromatids trade places • Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome

  34. Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes Crossing Over • Crossing over produces recombinant chromosomes that carry genes derived from two different parents

  35. Random Fertilization • Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) • The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations • Crossing over adds even more variation • Each zygote has a unique genetic identity

  36. Evolutionary Significance of Genetic Variation Within Populations • Natural selection results in accumulation of genetic variations favored by the environment • Sexual reproduction contributes to the genetic variation in a population, which ultimately results from mutations

  37. 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

  38. APPLICATION A karyotype is a display of condensed chromosomes arranged in pairs. Karyotyping can be used to screen for abnormal numbers of chromosomes or defective chromosomes associated with certain congenital disorders, such as Down syndrome. TECHNIQUE Karyotypes are prepared from isolated somatic cells, which are treated with a drug to stimulate mitosis and then grown in culture for several days. A slide of cells arrested in metaphase is stained and then viewed with a microscope equipped with a digital camera. A digital photograph of the chromosomes is entered into a computer, and the chromosomes are electronically rearranged into pairs according to size and shape. Pair of homologous chromosomes 5 µm Centromere RESULTS This karyotype shows the chromosomes from a normal human male. The patterns of stained bands help identify specific chromosomes and parts of chromosomes. Although difficult to discern in the karyotype, each metaphase chromosome consists of two, closely attached sister chromatids (see diagram). Sister chromatids Preparing a Karyotype

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