1. Meiosis and Sexual Life Cycles
Chapter 13
Meiosis and Sexual Life Cycles

2. 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
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3. Figure 13.1
Figure 13.1 What accounts for family resemblance?

5. Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes
In a literal sense, children do not inherit particular physical traits from their parents
Genes are actually inherited
made up of segments of DNA, packaged into chromosomes
passed to the next generation via reproductive cells called gametes (sperm and eggs)
Each gene has a specific location called a locus on a certain chromosome
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6. Comparison of Asexual and Sexual Reproduction
In asexual reproduction, a single individual passes genes to its offspring without the fusion of gametes
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
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7. Video: Hydra Budding
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8. 0.5 mm Parent Bud (a) Hydra (b) Redwoods Figure 13.2
Figure 13.2 Asexual reproduction in two multicellular organisms.
(a) Hydra
(b) Redwoods

9. 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
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10. 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 of the pairs of chromosomes from a cell
The two chromosomes in each pair are called homologous chromosomes, or homologs
Chromosomes in a homologous pair are the same length and shape and carry genes controlling the same inherited characters
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11. Pair of homologous duplicated chromosomes
Figure 13.3
APPLICATION
TECHNIQUE
5 m
Pair of homologous duplicated chromosomes
Centromere
Figure 13.3 Research Method: Preparing a Karyotype
Sister chromatids
Metaphase chromosome

12. A Dog’s Karyotype

13. The remaining 22 pairs of chromosomes are called autosomes
The sex chromosomes, which determine the sex of the individual, are called X and Y
Human females have a homologous pair of X chromosomes (XX) and males have one X and one Y (XY) chromosome
The remaining 22 pairs of chromosomes are called autosomes
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14. A diploid cell (2n) has two sets of chromosomes
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)
In a cell in which DNA synthesis has occurred, each chromosome is replicated and each replicated chromosome consists of two identical sister chromatids
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15. Chromosome numbers:
All are even numbers – diploid (2n) sets of homologous chromosomes!
All even, as all are diploid, contain pairs of chromosomes.
Ploidy = number of copies of each chromosome. Diploidy

16. Maternal set of chromosomes (n  3)
Figure 13.4
Key
Key
Maternal set of chromosomes (n  3)
2n  6
Paternal set of chromosomes (n  3)
Sister chromatids of one duplicated chromosome
Centromere
Figure 13.4 Describing chromosomes.
Two nonsister chromatids in a homologous pair
Pair of homologous chromosomes (one from each set)

17. For humans, the haploid number is 23 (n = 23)
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
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18. Behavior of Chromosome Sets in the Human Life Cycle
Fertilization is the union of gametes (the sperm and the egg)
The fertilized egg is called a zygote and has one set of chromosomes from each parent
The zygote produces somatic cells by mitosis and develops into an adult
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19. At sexual maturity, the ovaries and testes produce haploid gametes
(REMEMBER- Gametes are the only
types of human cells produced by meiosis, rather than mitosis!)
Meiosis results in one set of chromosomes in each gamete (HAPLOID!)
Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number
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20. Mitosis and development
Figure 13.5
Haploid gametes (n  23)
Key
Haploid (n)
Egg (n)
Diploid (2n)
Sperm (n)
MEIOSIS
FERTILIZATION
Ovary
Testis
Diploid zygote (2n  46)
Figure 13.5 The human life cycle.
Mitosis and development
Multicellular diploid adults (2n  46)

21. sex chromosome combinations possible in the new individual
diploid
germ cells
in female
diploid
germ cells
in male
meiosis, gamete
formation in both
female and male:
eggs
sperm X × Y X × X
fertilization: X X X XX XX Y XY XY
sex chromosome combinations
possible in the new individual
Fig. 11.2, p.170

22. 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
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23. 3 main types of sexual life cycles
Figure 13.6
3 main types of sexual life cycles
Key
Haploid (n)
Haploid multi- cellular organism (gametophyte)
Haploid unicellular or multicellular organism
Diploid (2n) n
Gametes n n
Mitosis n
Mitosis
Mitosis n
Mitosis n n n n n
MEIOSIS
FERTILIZATION
Spores n n
Gametes n
Gametes
MEIOSIS
FERTILIZATION
Zygote
MEIOSIS
FERTILIZATION 2n 2n 2n
Diploid multicellular organism (sporophyte) 2n
Zygote
Diploid multicellular organism 2n
Figure 13.6 Three types of sexual life cycles.
Mitosis
Mitosis
Zygote
(a) Animals
(b) Plants and some algae
(c) Most fungi and some protists

24. Animal sexual life cycle:
Gametes are the only haploid cells in animals
They are produced by meiosis and undergo no further cell division before fertilization
Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism
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25. Diploid multicellular organism Mitosis
Figure 13.6a
Key
Haploid (n)
Diploid (2n) n
Gametes n n
MEIOSIS
FERTILIZATION
Zygote 2n 2n
Figure 13.6 Three types of sexual life cycles.
Diploid multicellular organism
Mitosis
(a) Animals

26. Plant and algae sexual life cycle:
Plants and some algae exhibit an alternation of generations This life cycle includes both a diploid and haploid multicellular stage.
The diploid organism, called the sporophyte, makes haploid spores by meiosis.
Each spore grows by mitosis into a haploid organism called a gametophyte and a gametophyte makes haploid gametes by mitosis! (already haploid so mitosis used to make more IDENTICAL haploid cells!)
Fertilization of gametes results in a diploid sporophyte!
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27. Haploid multi- cellular organism (gametophyte)
Figure 13.6b
Key
Haploid (n)
Diploid (2n)
Haploid multi- cellular organism (gametophyte)
Mitosis n
Mitosis n n n n
Spores
Gametes
MEIOSIS
FERTILIZATION
Figure 13.6 Three types of sexual life cycles. 2n
Moss reproduction video
Diploid multicellular organism (sporophyte) 2n
Zygote
Mitosis
(b) Plants and some algae

28. Most Fungi and some protist sexual life cycle:
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 (Mitosis= make more of the same!)
The haploid adult produces gametes by mitosis
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29. Haploid unicellular or multicellular organism
Figure 13.6c
Key
Haploid (n)
Diploid (2n)
Haploid unicellular or multicellular organism
Mitosis n
Mitosis n n n n
Gametes
Figure 13.6 Three types of sexual life cycles.
MEIOSIS
FERTILIZATION 2n
Zygote
(c) Most fungi and some protists

30. Depending on the type of life cycle:
either haploid or diploid cells can divide by mitosis
However, only diploid cells can undergo meiosis!
In all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspring
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31. 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
results in 4 daughter cells, rather than the 2 daughter cells in mitosis
Each daughter cell has only half as many chromosomes as the parent cell
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32. The Stages of Meiosis
After chromosomes duplicate, two divisions follow
Meiosis I (reductional division): homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomes
Meiosis II (equational division) sister chromatids separate
The result is four haploid daughter cells with unreplicated chromosomes
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33. The single centrosome replicates, forming two centrosomes
Meiosis I is preceded by interphase, when the chromosomes are duplicated to form sister chromatids
The sister chromatids are genetically identical and joined at the centromere
The single centrosome replicates, forming two centrosomes
For the Cell Biology Video Meiosis I in Sperm Formation, go to Animation and Video Files.
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34. For the Cell Biology Video Meiosis I in Sperm Formation, go to Animation and Video Files.
BioFlix: Meiosis
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35. KEY
DIFFERENCE!
Homologous
pairs
separate first

36. CROSSING
OVER!

37. Sister
Chromatids separate
at centromeres

38. Steps of Meiosis I

39. Chromosomes begin to condense
Prophase I
Prophase I typically occupies more than 90% of the time required for meiosis
Chromosomes begin to condense
In synapsis, homologous chromosomes loosely pair up, aligned gene by gene
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40. In crossing over, nonsister chromatids exchange DNA segments
Each pair of chromosomes forms a tetrad, a group of four chromatids
Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred
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41. Metaphase I
In metaphase I, tetrads line up at the metaphase plate (MIDDLE), 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
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42. 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
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43. 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
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44. 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
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45. Steps of Meiosis II

46. In prophase II, a spindle apparatus forms
In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate
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47. Metaphase II
In metaphase II, the sister chromatids are arranged at the metaphase plate (MIDDLE)
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
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48. In anaphase II, the sister chromatids separate
The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles
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49. Telophase II and Cytokinesis
In telophase II, the chromosomes arrive at opposite poles
Nuclei form, and the chromosomes begin decondensing
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50. Telophase II and Cytokinesis
Figure 13.8b
Telophase II and Cytokinesis
Prophase II
Metaphase II
Anaphase II
During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.
Sister chromatids separate
Haploid daughter cells forming
Figure 13.8 Exploring: Meiosis in an Animal Cell

51. 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 different from the others and from the parent cell
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52. 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
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53. Mitosis vs. Meiosis
Just meiosis!
Comparing Mitosis and meiosis

54. Figure 13.9 A comparison of mitosis and meiosis in diploid cells.
Figure 13.9b
SUMMARY
Property
Mitosis
Meiosis
DNA replication
Occurs during interphase before mitosis begins
Occurs during interphase before meiosis I begins
Number of divisions
One, including prophase, metaphase, anaphase, and telophase
Two, each including prophase, metaphase, anaphase, and telophase
Synapsis of homologous chromosomes
Does not occur
Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion
Number of daughter cells and genetic composition
Two, each diploid (2n) and genetically identical to the parent cell
Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other
Figure 13.9 A comparison of mitosis and meiosis in diploid cells.
Role in the animal body
Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction
Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes

55. 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
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56. In mitosis, cohesins are cleaved at the end of metaphase
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
In meiosis, cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) and at the centromeres in anaphase II (separation of sister chromatids)
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57. 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
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58. 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
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59. Independent assortment
Number of combinations: 2n
e.g. 2 chromosomes in haploid
2n = 4; n = 2
2n = 22 = 4 possible combinations

60. Independent assortment

61. In humans… e.g. 23 chromosomes in haploid 2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations!

62. Random fertilization
At least 8 million combinations from Mom, and another 8 million from Dad …
>64 trillion combinations for a diploid zygote!!!

63. Crossing Over
Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent
Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene
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64. 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
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65. Crossing over Chiasmata – sites of crossing over
synapsis- Exchange of genetic material between non-sister chromatids.
Crossing over produces recombinant chromosomes.

66. 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
Crossing over adds even more variation
Each zygote has a unique genetic identity
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67. Animation: Genetic Variation Right-click slide / select “Play”
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68. Meiosis creates genetic variation
During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n)
Meiosis results in genetic variation by shuffling of maternal and paternal chromosomes and crossing over.
During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring.

69. Alterations in chromosome number and individual chromosomes
Many mutations can occur during mitosis or meiosis that will affect the chromosome numbers or alter the information on individual chromosomes.
Mutations can be harmful or beneficial to the organism
Types of mutations include:
Nondisjunction
Deletion
Duplication
Inversion
Translocation

70. Trisomy 21- Cause of Down Syndrome
Nondisjunction animation

71. Various animations showing chromosomes alterations

72. 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
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73. Figure 13.12
Figure A bdelloid rotifer, an animal that reproduces only asexually.
200 m

74. Figure 13.UN01
Prophase I: Each homologous pair undergoes synapsis and crossing over between nonsister chromatids with the subsequent appearance of chiasmata.
Metaphase I: Chromosomes line up as homologous pairs on the metaphase plate.
Figure 13.UN01 Summary figure, Concept 13.3
Anaphase I: Homologs separate from each other; sister chromatids remain joined at the centromere.

75. Figure 13.UN04
Figure 13.UN04 Appendix A: answer to Test Your Understanding, question 8

76. Meiosis and Sexual Life Cycles
Chapter 13
Meiosis and Sexual Life Cycles
Questions prepared by
Louise Paquin McDaniel College

77. Overview Which of the following transmits genes from one generation of a family to another?
DNA
gametes
somatic cells
mitosis
nucleotides
Answer: b
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78. Fertilization Fertilization is to zygote as meiosis is to which of the following?
mitosis
diploid
chromosome
replication
gamete
Answer: e; discuss why

79. Chromosome Number Privet shrubs and humans each have a diploid number of 46 chromosomes per cell. Why are the two species so dissimilar?
Privet chromosomes undergo only mitosis.
Privet chromosomes are shaped differently.
Human chromosomes have genes grouped together differently.
The two species have appreciably different genes.
Privets do not have sex chromosomes.
Answer: d

80. Karyotypes Why is it more practical to prepare karyotypes by viewing somatic diploid cells rather than haploid gametes?
Cells can be made to divide and mitosis can be halted at metaphase; both sets of chromosomes need to be examined.

81. Meiosis Diploid cells may undergo either mitosis or meiosis
Meiosis Diploid cells may undergo either mitosis or meiosis. Can haploid cells? Why or why not?
Haploid cells cannot undergo meiosis because the chromosomes cannot pair.

82. Inquiry Shugoshin is a protein that protects cohesins from cleavage during meiosis I. What would you expect would occur if a premeiotic cell had one normal copy and one abnormal (nonfunctional) copy of the shugoshin gene? Hint: Think of one form on each member of a homologous pair.
Answer: Some gametes will transmit each form.

83. Independent Assortment How and at what stage do chromosomes undergo independent assortment?
meiosis I pairing of homologs
anaphase I separation of homologs
meiosis II separation of homologs
meiosis I metaphase alignment
meiosis I telophase separation
Answer: d

84. Disjunction What allows sister chromatids to separate in which phase of meiosis?
release of cohesin along sister chromatid arms in anaphase I
crossing over of chromatids in prophase I
release of cohesin at centromeres in anaphase I
release of cohesin at centromeres in anaphase II
crossing over of homologues in prophase I
Answer: d

85. Variation What are three ways in which gametes from one individual diploid cell can be different from one another?
Answer: mutation, crossing over, independent assortment

86. Rate and Process Prophase I of meiosis is generally the longest phase of meiosis. Why might this be?
Chromosomes must pair and crossover, besides the cell going through mitosis-like morphological changes.

87. Meiotic Phases In this cell, what phase is represented?
mitotic metaphase
meiosis I anaphase
meiosis I metaphase
meiosis II anaphase
meiosis II metaphase
Answer: c (see Figure 13.9)

88. Recombination and Natural Selection
In which of the following ways are natural selection and recombination due to crossing over in meiosis I related?
a) Recombinants undergo negative selection.
b) Non-recombinants are positively selected in cases of environmental change.
c) Most recombinants have lower fertility than non- recombinants.
d) Some recombinants have positively selected new gene combinations.
e) There is no relationship.
Answer d