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Chapter 13

Chapter 13. Meiosis and Sexual Life Cycles. Hereditary. Overview: Hereditary Similarity and Variation Living organisms Are distinguished by their ability to reproduce their own kind Heredity Is the transmission of traits from one generation to the next

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Chapter 13

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  1. Chapter 13 Meiosis and Sexual Life Cycles

  2. Hereditary • Overview: Hereditary Similarity and Variation • Living organisms • Are distinguished by their ability to reproduce their own kind • Heredity • Is the transmission of traits from one generation to the next • Results from transmission of genes from parents to children • Offspring share similar genes

  3. Figure 13.1 • Variation: inherited differences among individuals of same species • Shows that offspring differ somewhat in appearance from parents and siblings and that is due to the mixing of chromosomes from both parents

  4. Genetics; is the scientific study of heredity and hereditary variation

  5. Inheritance Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes Inheritance is possible because: • DNA is precisely replicated producing copies of genes that can be passed along from parents to offspring. • Sperms and ova carrying each parent’s genes combined in the nucleus of the fertilized egg.

  6. Inheritance of Genes Genes: • Units of hereditary information that are made of DNA and are located on chromosomes. • have specific sequence of nucleotides. • Most genes program cells to synthesize specific proteins • The action of these proteins produce an organism’s inherited traits.

  7. Each gene in an organism’s DNA • Has a specific locus on a certain chromosome • Locus: is a Specific location on a chromosome that contains genes. • We inherit • One set of chromosomes from our mother and one set from our father. • Each species has a characteristic chromosome number, e.g humans have 46.

  8. Parent Bud 0.5 mm Figure 13.2 Comparison of Asexual and Sexual Reproduction • In asexual reproduction • One parent produces genetically identical offspring by mitosis

  9. An individual that reproduces asexually give rise to a clone (identical individuals). • In sexual reproduction • Two parents give rise to offspring that have unique combinations of genes inherited from the two parents ( individuals are genetically different)

  10. A comparison of asexual and sexual reproduction

  11. Fertilization and meiosis • Concept 13.2: Fertilization and meiosis alternate in sexual life cycles • They alternation between the haploid and diploid conditions. • A life cycle • Is the generation-to-generation sequence of stages in the reproductive history of an organism (from conception to the production of its own offspring).

  12. 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 • These chromosomes are different in; • size • position of their centromeres • staining or banding pattern.

  13. Karyotyping • A karyotype • Is an ordered, visual representation of the chromosomes in a cell Pair of homologous chromosomes 5 µm Centromere Sister chromatids Figure 13.3

  14. Homologous chromosomes • Are the two chromosomes composing a pair • Do they have the same characteristics such as size, position of centromere and staining bands. • May also be called autosomes, i.e none sex chromosomes

  15. Sex chromosomes • Are distinct from each other in their characteristics • Are represented as X and Y • Determine the sex of the individual, XX being female, XY being male. • humans have 22 pairs of autosomes and one pair of sex chromosomes.

  16. A diploid cell • Has two sets of each of its chromosomes • In a human cells has 46 chromosomes (2n = 46)

  17. Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Figure 13.4 • In a cell in which DNA synthesis has occurred • All the chromosomes are duplicated and thus each consists of two identical sister chromatids This is a cell with Diploid of 6

  18. Chromosomes in human cells • Unlike somatic cells • Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes For humans a diploid number is 46 (n=46) which is the number of chromosomes in our cell Haploid: condition in which cells contain one set of chromosomes. It is the chromosome number of gametes (n= 23).

  19. Behavior of Chromosome Sets in the Human Life Cycle • At sexual maturity • The ovaries and testes produce haploid gametes by meiosis (n=23). So these gametes are the only cells in the body that are NOT produced by mitosis. • During fertilization • These gametes, sperm and ovum, fuse, forming a diploid zygote • The zygote • Develops into an adult organism with a diploid number of chromosomes in somatic cells

  20. Key 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) Figure 13.5 The human life cycle

  21. The Variety of Sexual Life Cycles • The three main types of sexual life cycles • Differ in the timing of meiosis and fertilization

  22. Key Haploid Diploid n n Gametes n MEIOSIS FERTILIZATION Zygote 2n 2n Diploid multicellular organism Mitosis Figure 13.6 A (a) Animals Sexual life cycle in animals • In animals • Meiosis occurs during gamete formation • Gametes are the only haploid cells • Fertilization produces a diploid zygote

  23. 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 Figure 13.6 B Sexual life cycle in plants and algae • Plants and some algae • Exhibit an alternation of generations • The life cycle includes both diploid and haploid multicellular stages

  24. Reproduction in plants and some algae • The multicellular diploid stage is called sporophyte or spore producing plant. Meiosis in this stage produces haploid cells called: spores. • Haploid spores divide mitotically to generate multicellular haploid stage called gametophyte or gamete producing plant. • Haploid gametophyte produce gametes by mitosis. • Fertilization produces diploid zygote which develops into the next sporophyte generation.

  25. Haploid multicellular organism n Mitosis Mitosis n n n n Gametes MEIOSIS FERTILIZATION 2n Zygote Figure 13.6 C (c) Most fungi and some protists Reproduction in 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

  26. Reproduction in fungi and protists • The only diploid stage is the zygote. • Meiosis occurs immediately after the zygote forms haploid cells. • Resulting haploid cells divide by mitosis to produce a haploid multicellular organism. • Gametes are produced by mitosis from the already haploid organism.

  27. Meiosis • Concept 13.3: Meiosis • Reduces the number of chromosome sets from diploid to haploid • Takes place in two sets of divisions, meiosis I and meiosis II

  28. 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 Homologous chromosomes separate Haploid cells with replicated chromosomes 1 Meiosis II Sister chromatids separate 2 Haploid cells with unreplicated chromosomes The Stages of Meiosis; An overview of meiosis Haploid cells with replicated chromosomes Figure 13.7

  29. Meiosis I • Reduces the number of chromosomes from diploid to haploid • Meiosis II • Produces four haploid daughter cells

  30. MEIOSIS I: Separates homologous chromosomes INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Sister chromatids remain attached Centromere (with kinetochore) Centrosomes (replicate) (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 Figure 13.8

  31. Interphase and meiosis I Interphase I: • chromosomes replicate as in mitosis. • Each duplicated chromosome consists of two sister chromatids. • Centriole pair (in animal cells) also replicate into two pairs. Prophase I: • This is the longer and more complex process than prophase of mitosis. • chromosomes condense. • Synapsis occur, homologous chromosomes come together as pairs by the aid of syanptonemal complex (a protein structure).

  32. Interphase and meiosis I • Since each chromosome consists of two sister chromatids, each homologous pair in synapsis appears as complex of four chromatids: a Tetrad. • In each tetrad, sister chromatids of the same chromosome are attached at their centromeres. None sister chromatids are linked by X-shaped Chiasmata: sites where homologous chromosome strands exchange or crossing over occurs.

  33. Interphase and meiosis I • Centriole pairs move apart and spindle microtubules form. • Nuclear envelope and nucleoli disappear. • Spindle microtubules capture the kinetochores that form on the chromosomes and chromosomes begin moving. • Prophase I takes more than 90% of all meiosis.

  34. MEIOSIS I: Separates homologous chromosomes INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Sister chromatids remain attached Centromere (with kinetochore) Centrosomes (replicate) (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 Figure 13.8

  35. Metaphase I. • Tetrads (chromosomes) are aligned on the metaphase plate. • Chromosomes of homologues point towards opposite poles. • Each homologue is attached to kinetochore microtubules.

  36. Anaphase I: • Homologous chromosomes separate and are moved towards the opposite poles. • Sister chromatids remain attached and move as a unit towards the same pole.

  37. 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 Figure 13.8 • Telophase I, cytokinesis, and meiosis II

  38. Telophase I and Cytokinesis: • The spindle apparatus continue to separate homologuos pairs until the chromosomes reach the poles. • Each pole now has a haploid set of chromosomes (each with two sister chromatids). • Cytokinensis occurs in the same time of telophase I, forming two haploid daughter cells. • In most species, cells immediately prepare for meiosis II. • No DNA replication occurs before meiosis II.

  39. 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 Figure 13.8 • Telophase I, cytokinesis, and meiosis II

  40. Mieosis II: separates sister chromatids Prophase II: • Spindle apparatus forms and chromosomes move towards the metaphase II plate. Metaphase II: • Chromosomes align singly on the metaphase plate. • Kinetochores of sister chromatids point towards opposite poles.

  41. Anaphase II: • Centromeres of sister chromatids separate. • Sister chromatids of each pair move towards opposite poles of the cell. Telophase II and cytokinesis: • Nuclei form at opposite poles of the cell. • Cytokinesis occurs producing four haploid daughter cells.

  42. A Comparison of Mitosis and Meiosis • Meiosis can be distinguished from mitosis by three events in Meiosis (I): • Synapsis and crossing over: Homologous chromosomes physically connect and exchange genetic information • Tetrads on the metaphase plate; At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates • Separation of homologues: At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell • In anaphase II of meiosis, the sister chromatids separate.

  43. A comparison of mitosis and meiosis 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 Homologues separate during anaphase I; sister chromatids remain together Anaphase Telophase Anaphase I Telophase I Sister chromatids separate during anaphase 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 Figure 13.9 Sister chromatids separate during anaphase II

  44. Genetic variation and evolution • Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution • Reshuffling of genetic material in meiosis produces genetic variation • Sexual reproduction contributes to genetic variation by: • Independent assortment • Crossing over during prophase I of meiosis • Random fusion of gametes during fertilization.

  45. Independent Assortment of Chromosomes • Is the random distribution of maternal and paternal homologues to gametes. • according to this, there is a 2n possible combination of maternal and paternal chromosomes. • In humans the possible combinations would be 223 = 8million. • Thus, each human gamete contains on of eight million possible assortment of chromosomes.

  46. 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 • In independent assortment • Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs Figure 13.10

  47. Crossing Over Crossing Over: • The exchange of genetic material between homologues. • Produces recombinant chromosomes that combines genes inherited from two parents. • Occurs when homologous portions of nonsister chromatids trade places during prophase I (X-shaped chiasmata). • In humans, 2-3 crossovers occurs/chromosome

  48. Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes Figure 13.11 Crossing Over • Crossing over • Produces recombinant chromosomes that carry genes derived from two different parents

  49. Random Fertilization • The fusion of gametes • Will produce a zygote with any of about 64 trillion diploid combinations. • In humans, when individual ovum representative of one of 8 million chromosome combinations is fertilized by a sperm cell, which also represents one of 8 million chromosome combinations, the resulting zygote can have one of 64 trillion possible

  50. Mutations • Are the original source of genetic variation • Sexual reproduction • Produces new combinations of variant genes, adding more genetic diversity

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