1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 13 Meiosis and Sexual Life Cycles

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Figure 13.1

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetics; is the scientific study of heredity and hereditary variation

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inheritance of Genes Genes: 1.Units of hereditary information that are made of DNA and are located on chromosomes. 2.have specific sequence of nucleotides. 3.Most genes program cells to synthesize specific proteins 4.The action of these proteins produce an organism’s inherited traits.

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Comparison of Asexual and Sexual Reproduction In asexual reproduction – One parent produces genetically identical offspring by mitosis Figure 13.2 Parent Bud 0.5 mm

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A comparison of asexual and sexual reproduction Asexual reproductionSexual reproduction Single individual is the only parent Single parent passes all its genes to its offspring Offspring are genetically identical to the parents Results in clone Two parents give rise to offspring Each parent passes on half its genes to its offspring Offspring have a combination of genes inherited from both parents Results in greater genetic variation.

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 5 µm Pair of homologous chromosomes Centromere Sister chromatids Figure 13.3 Karyotyping A karyotype – Is an ordered, visual representation of the chromosomes in a cell

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A diploid cell – Has two sets of each of its chromosomes – In a human cells has 46 chromosomes (2n = 46)

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In a cell in which DNA synthesis has occurred – All the chromosomes are duplicated and thus each consists of two identical sister chromatids Figure 13.4 Key Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3) 2n = 6 Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Centromere This is a cell with Diploid of 6

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Variety of Sexual Life Cycles The three main types of sexual life cycles – Differ in the timing of meiosis and fertilization

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sexual life cycle in animals In animals – Meiosis occurs during gamete formation – Gametes are the only haploid cells – Fertilization produces a diploid zygote Gametes Figure 13.6 A Diploid multicellular organism Key MEIOSIS FERTILIZATION n n n 2n Zygote Haploid Diploid Mitosis (a) Animals

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings MEIOSISFERTILIZATION n n n n n 2n Haploid multicellular organism (gametophyte) Mitosis Spores Gametes Mitosis Zygote Diploid multicellular organism (sporophyte) (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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings MEIOSIS FERTILIZATION n n n n n 2n Haploid multicellular organism Mitosis Gametes Zygote (c) Most fungi and some protists Figure 13.6 C 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Stages of Meiosis; An overview of meiosis 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 Haploid cells with replicated chromosomes

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Meiosis I – Reduces the number of chromosomes from diploid to haploid Meiosis II – Produces four haploid daughter cells

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Centrosomes (replicate) (with centriole pairs) Sister chromatids Chiasmata Spindle Tetrad Nuclear envelope Chromatin Centromere (with kinetochore) Microtubule attached to kinetochore Tertads line up Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Pairs of homologous chromosomes split up Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example INTERPHASE MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Interphase and meiosis I Figure 13.8

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Centrosomes (replicate) (with centriole pairs) Sister chromatids Chiasmata Spindle Tetrad Nuclear envelope Chromatin Centromere (with kinetochore) Microtubule attached to kinetochore Tertads line up Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Pairs of homologous chromosomes split up Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example INTERPHASE MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Interphase and meiosis I Figure 13.8

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS MEIOSIS II: Separates sister chromatids Cleavage furrow Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Two haploid cells form; chromosomes are still double Figure 13.8 Telophase I, cytokinesis, and meiosis II

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS MEIOSIS II: Separates sister chromatids Cleavage furrow Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Two haploid cells form; chromosomes are still double Figure 13.8 Telophase I, cytokinesis, and meiosis II

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Comparison of Mitosis and Meiosis Meiosis can be distinguished from mitosis by three events in Meiosis (I): 1.Synapsis and crossing over: Homologous chromosomes physically connect and exchange genetic information 2.Tetrads on the metaphase plate; At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates 3.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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Independent Assortment of Chromosomes Is the random distribution of maternal and paternal homologues to gametes. according to this, there is a 2 n possible combination of maternal and paternal chromosomes. In humans the possible combinations would be 2 23 = 8 million. Thus, each human gamete contains on of eight million possible assortment of chromosomes.

46. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 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

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Crossing Over Crossing over – Produces recombinant chromosomes that carry genes derived from two different parents Figure 13.11 Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes

49. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutations – Are the original source of genetic variation Sexual reproduction – Produces new combinations of variant genes, adding more genetic diversity

51. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings End of Chapter