1. Overview: Variations on a Theme Living organisms are distinguished by their ability to reproduce their own kind Genetics is the scientific study of heredity and variation Heredity is the transmission of traits from one generation to the next Variation is demonstrated by the differences in appearance that offspring show from parents and siblings Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

2. Inheritance of Genes Genes are segments of DNA that code for the basic units of heredity and are transmitted from one generation to the next In animals and plants, reproductive cells that transmit genes to the next generation are called gametes A locus is the location of a gene on a chromosome Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

3. Comparison of Asexual and Sexual Reproduction In asexual reproduction, one parent produces genetically identical offspring by mitosis A clone is a group of genetically identical individuals from the same parent In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

4. Fig. 13-2 (a) Hydra (b) Redwoods Parent Bud 0.5 mm

5. Concept 13.2: 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, from conception to production of its own offspring Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

6. Sets of Chromosomes in Human Cells Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes A karyotype is an ordered display (picture) of the pairs of chromosomes from a cell The two chromosomes in each pair are called homologous chromosomes, or homologs Both chromosomes of each pair carry genes that control the same inherited characteristics Homologs are similar in length and staining pattern Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

7. Fig. 13-3b TECHNIQUE Pair of homologous replicated chromosomes Centromere Sister chromatids Metaphase chromosome 5 µm

8. The sex chromosomes are called X and Y Human females have a homologous pair of X chromosomes (XX) Human males have one X and one Y chromosome The 22 pairs of chromosomes that do not determine sex are called autosomes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

9. Each pair of homologous chromosomes includes one chromosome from each parent The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father A diploid cell (2n) has two sets of chromosomes For humans, the diploid number is 46 (2n = 46) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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

11. A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n) For humans, the haploid number is 23 (n = 23) Each set of 23 consists of 22 autosomes and a single sex chromosome In an unfertilized egg (ovum), the sex chromosome is X In a sperm cell, the sex chromosome may be either X or Y Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

12. Meiosis and fertilization are the key events in sexually reproducing life cycles During fertilization, a sperm cell combines with an egg cell, fusing one haploid gamete from each parent The result is a fertilized egg called a zygote, which is diploid and represents the first cell of a new human being Behavior of Chromosome Sets in the Human Life Cycle Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

13. Meiosis reduces the numbers of sets of chromosomes from two to one Fertilization restores the diploid number as gametes are combined Both processes alternate in the life cycles of sexually reproducing organisms Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

14. Fig. 13-5 Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Egg (n) Sperm (n) MEIOSISFERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

15. The Variety of Sexual Life Cycles The alternation of meiosis and fertilization is common to all organisms that reproduce sexually The three main types of sexual life cycles differ in the timing of meiosis and fertilization Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

16. In animals, meiosis produces gametes, which undergo no further cell division before fertilization Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

17. Plants and some algae exhibit an alternation of generations The diploid organism, called the sporophyte, makes haploid spores by meiosis Each spore grows by mitosis into a haploid organism called a gametophyte A gametophyte makes haploid gametes by mitosis Fertilization of gametes results in a diploid sporophyte Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

18. In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stage The zygote produces haploid cells by meiosis Each haploid cell grows by mitosis into a haploid multicellular organism The haploid adult produces gametes by mitosis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

19. Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid Like mitosis, meiosis is preceded by the replication of chromosomes Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II The two cell divisions result in four nonidentical daughter cells, rather than the two daughter cells in mitosis Each daughter cell has only half as many chromosomes as the parent cell Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

20. The Stages of Meiosis Interphase Each of the chromosomes makes a copy of itself; DNA is doubled and centrosome divides Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

21. Fig. 13-7-1 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes

22. Fig. 13-7-2 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 1 Haploid cells with replicated chromosomes

23. Division in meiosis I occurs in four phases: –Prophase I –Metaphase I –Anaphase I –Telophase I and cytokinesis

24. Meiosis I Prophase I – Chromosomes condense, resulting in two sister chromatids attached at their centromeres – Synapsis occurs, which is the joining of homologous chromosomes. This newly formed structure is called a tetrad and aligns the homologous chromosomes gene by gene

25. In crossing over, the DNA from one homologue is cut and exchanged with an exact portion of DNA from another homologue – A small part of the DNA from one parent is exchanged with the DNA from the other parent, resulting in increased genetic variation – Where crossing over occurred, criss-crossed regions called chiasmata form, holding the homologues together until anaphase I

26. After crossing over: – Centrioles move away from each other – Nuclear envelope disintegrates – Spindle microtubules attach to kinetochores forming on the chromosomes that begin to move to the metaphase (middle) plate of the cell

27. Fig. 13-8b Prophase IMetaphase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore

28. Metaphase I – Homologous pairs of chromosomes are lined up at the metaphase plate – Microtubules from each pole attach to each member of the homologous pairs in preparation for pulling them to opposite sides of the cell

29. Fig. 13-8b Prophase IMetaphase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore

30. 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

31. Fig. 13-8c Anaphase I Telophase I and Cytokinesis Sister chromatids remain attached Homologous chromosomes separate Cleavage furrow

32. Telophase I and Cytokinesis Homologous chromosomes move until they reach opposite poles Each pole contains a haploid set of chromosomes, with each chromosome still consisting of two sister chromatids Cytokinesis occurs during telophase; a cleavage furrow occurs in animal cells and a cell plate in plant cells, both resulting in the formation of two haploid cells Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

33. Fig. 13-8c Anaphase I Telophase I and Cytokinesis Sister chromatids remain attached Homologous chromosomes separate Cleavage furrow

34. Division in meiosis II also occurs in four phases: –Prophase II –Metaphase II –Anaphase II –Telophase II and cytokinesis Meiosis II is very similar to mitosis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

35. Fig. 13-8d Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming

36. Prophase II In prophase II, a spindle apparatus forms In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

37. Metaphase II In metaphase II, the sister chromatids are arranged 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

38. Fig. 13-8e Prophase IIMetaphase II

39. Anaphase II In anaphase II, the sister chromatids separate The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

40. Telophase II and Cytokinesis In telophase II, the chromosomes arrive at opposite poles Nuclei form, and the chromosomes begin decondensing Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

41. 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

42. Fig. 13-8f Anaphase II Telephase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming

43. 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

44. Fig. 13-9 MITOSISMEIOSIS MEIOSIS I Prophase I Chiasma Homologous chromosome pair Chromosome replication Parent cell 2n = 6 Chromosome replication Replicated chromosome Prophase Metaphase Metaphase I Anaphase I Telophase I Haploid n = 3 Daughter cells of meiosis I Anaphase Telophase 2n2n 2n2n Daughter cells of mitosis nn n n MEIOSIS II Daughter cells of meiosis II SUMMARY Meiosis Occurs during interphase before meiosis I begins Two, each including prophase, metaphase, anaphase, and telophase Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Produces gametes; reduces number of chromosomes by half and introduces genetic variability amoung the gametes Mitosis Occurs during interphase before mitosis begins One, including prophase, metaphase, anahase, and telophase Does not occur Two, each diploid (2n) and genetically identical to the parent cell Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Property DNA replication Number of divisions Synapsis of homologous chromosomes Number of daughter cells and genetic composition Role in the animal body

45. Fig. 13-9a MITOSIS MEIOSIS MEIOSIS I Prophase I Chiasma Chromosome replication Homologous chromosome pair Chromosome replication 2n = 6 Parent cell Prophase Replicated chromosome Metaphase Metaphase I Anaphase I Telophase I Haploid n = 3 Daughter cells of meiosis I MEIOSIS II Daughter cells of meiosis II n n n n 2n2n 2n2n Daughter cells of mitosis Anaphase Telophase

46. Fig. 13-9b SUMMARY Meiosis Mitosis Property DNA replication Number of divisions Occurs during interphase before mitosis begins One, including prophase, metaphase, anaphase, and telophase Synapsis of homologous chromosomes Does not occur Number of daughter cells and genetic composition Two, each diploid (2n) and genetically identical to the parent cell Role in the animal body Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Occurs during interphase before meiosis I begins Two, each including prophase, metaphase, anaphase, and telophase Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes

47. 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, it is homologous chromosomes, instead of sister chromatids, that separate Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

48. Sister chromatid cohesion allows sister chromatids of a single chromosome to stay together through meiosis I Protein complexes called cohesins are responsible for this cohesion In mitosis, cohesins are cleaved at the end of metaphase Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

49. Concept 13.4: 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 called alleles Reshuffling of alleles during sexual reproduction produces genetic variation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

50. 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

51. 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

52. 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

53. Fig. 13-12-1 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis

54. Fig. 13-12-2 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere TEM

55. Fig. 13-12-3 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere Anaphase I TEM

56. Fig. 13-12-4 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere Anaphase I Anaphase II TEM

57. Fig. 13-12-5 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere Anaphase I Anaphase II Daughter cells Recombinant chromosomes TEM

58. Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

59. Crossing over adds even more variation Each zygote has a unique genetic identity Animation: Genetic Variation Animation: Genetic Variation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

60. The Evolutionary Significance of Genetic Variation Within Populations Natural selection results in the accumulation of genetic variations favored by the environment Sexual reproduction contributes to the genetic variation in a population, which originates from mutations Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings