1. Meiosis
Bio 11
Tuesday, October 7, 2008

2. Which one of the following does not occur during mitotic anaphase?  
A) The centromeres of each chromosome divide.  
B) Sister chromatids separate.  
C) The chromatid DNA replicates.  
D) Daughter chromosomes begin to move toward opposite poles of the cell.  
E) All of the choices occur during mitotic anaphase.  

3. Asexual Reproduction is a primary means of propagation for many species

4. Sexual reproduction generates diversity
Variety in offspring improves chances of adaptation to changing environments

5. The offspring of sexual reproduction inherit parental traits
Offspring inherit DNA from each parent
Offspring are not the same
DNA is from the same source

6. Meiosis is the process of cell division that allows gamete formation
LE 8-13
Haploid gametes (n = 23) n
Egg cell
Meiosis is the process of cell division that allows gamete formation
Gametes are haploid (n), somatic cells are diploid (2n) n
Sperm cell
Meiosis
Fertilization
Diploid
zygote
(2n = 46) 2n
Multicellular
diploid adults
(2n = 46)
Mitosis and
development

7. Chromosomes come in pairs
For diploid organims (2n)
One chromosome in a paired set comes from each parent
#’s 1-22 are autosomes
X and Y are sex chromosomes

8. Homologous Chromosomes
LE 8-12
Homologous Chromosomes
Centromere
Sister chromatids

9. Homologous chromosomes
Pairs of chromosomes are homologous
The site for particular genes are called loci (singular: locus)
Identical strands of the same chromosome are sister chromatids
Loci

10. Haploid multicellular organism (gametophyte) Haploid multicellular
LE 13-6
Key
Haploid
Diploid
Haploid multicellular
organism (gametophyte)
Haploid multicellular
organism
Gametes n n
Mitosis n
Mitosis
Mitosis n
Mitosis n n n n n n
Spores n
MEIOSIS
FERTILIZATION n
Gametes
Gametes n
MEIOSIS
FERTILIZATION
MEIOSIS
FERTILIZATION
Zygote 2n 2n 2n 2n
Diploid
multicellular
organism
(sporophyte)
Zygote 2n
Diploid
multicellular
organism
Mitosis
Mitosis
Zygote
Animals
Plants and some algae
Most fungi and some protists

11. Overview of Meiosis
Meiosis consists of 2 major stages, Meiosis I and Meiosis II
Cell copies DNA once, divides twice, creating 4 haploid cells
In meiosis I, homologous pairs of chromosomes are grouped, recombined, and then segregated into two intermediate cells
In meiosis II, those cells are divided

12. The Stages of Meiosis In (meiosis I), homologous chromosomes separate
Meiosis I results in two haploid daughter cells with replicated chromosomes
In the second cell division (meiosis II), sister chromatids separate
Meiosis II results in four haploid daughter cells with unreplicated chromosomes

13. Microtubules attached to kinetochore Centrosomes (with centriole
LE 8-14a
MEIOSIS I
: Homologous chromosome separate
INTERPHASE
PROPHASE I
METAPHASE I
ANAPHASE I
Microtubules
attached to
kinetochore
Centrosomes
(with centriole
pairs)
Metaphase
plate
Sister chromatids
remain attached
Sites of crossing over
Spindle
Nuclear
envelope
Sister
chromatids
Tetrad
Centromere
(with kinetochore)
Homologous
chromosomes separate
Chromatin

14. (Interphase precedes meiosis, of course)
LE 13-8aa
INTERPHASE
MEIOSIS I: Separates homologous chromosomes
(Interphase precedes meiosis, of course)
METAPHASE I
ANAPHASE I
Centrosomes
(with centriole pairs)
Nuclear
envelope
Chromatin
Chromosomes duplicate

15. Division in meiosis I occurs in four phases:
Prophase I
Metaphase I
Anaphase I
Telophase I

16. Prophase I >90% of the time required for meiosis
Chromosomes condense
Synapsis: homologous chromosomes pair up
Crossing over: nonsister chromatids exchange DNA segments
Tetrad: four chromatids
Chiasmata: X-shaped regions where crossing over occurred

17. Crossing-Over in Prophase I

18. LE 8-18a
TEM 2,200
Tetrad
Chiasma
Centromere

19. Crossing-over in Prophase I

20. MEIOSIS I: Separates homologous chromosomes
LE 13-8ab
MEIOSIS I: Separates homologous chromosomes
PROPHASE I
METAPHASE I
ANAPHASE I
Centromere
(with kinetochore)
Sister chromatids
remain attached
Sister
chromatids
Chiasmata
Metaphase
plate
Spindle
Tetrad
Microtubule
attached to
kinetochore
Homologous
chromosomes
separate
Homologous chromosomes
(red and blue) pair and
exchange segments; 2n = 6
in this example
Tetrads line up
Pairs of homologous
chromosomes split up

21. Animation: Metaphase I
Tetrads line up at the metaphase plate
Microtubules from one pole are attached
Microtubules from the other pole are attached to the kinetochore of the other chromosome
Animation: Metaphase I

22. Independent Assortment in Metaphase I
Random alignment of maternal/paternal chromosomes at the metaphase plate
Produces genetic variability within populations

23. Independent Assortment in Metaphase I

24. Independent Assortment in Metaphase I

25. Independent Assortment of Chromosomes
Homologous pairs of chromosomes orient randomly at metaphase I of meiosis
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- 1, where n is the haploid number
For humans (n = 23), there are more than 8 million (222) possible combinations of chromosomes

26. MEIOSIS I: Separates homologous chromosomes
LE 13-8ab
MEIOSIS I: Separates homologous chromosomes
PROPHASE I
METAPHASE I
ANAPHASE I
Centromere
(with kinetochore)
Sister chromatids
remain attached
Sister
chromatids
Chiasmata
Metaphase
plate
Spindle
Tetrad
Microtubule
attached to
kinetochore
Homologous
chromosomes
separate
Homologous chromosomes
(red and blue) pair and
exchange segments; 2n = 6
in this example
Tetrads line up
Pairs of homologous
chromosomes split up

27. Anaphase I homologous chromosomes separate
One chromosome moves toward each pole
Sister chromatids remain attached at the centromere

28. Meiosis II Resembles mitosis in its mechanisms
However, only half the chromosomes are present in meiosis II compared with mitosis

29. Haploid daughter Sister chromatids cells forming separate LE 8-14b
MEIOSIS I
: Sister chromatids separate
TELOPHASE I
TELOPHASE I
PROPHASE I
METAPHASE I
ANAPHASE I
AND CYTOKINESIS
AND CYTOKINESIS
Cleavage
furrow
Sister chromatids
separate
Haploid daughter
cells forming

30. MEIOSIS II: Separates sister chromatids
LE 13-8b
MEIOSIS II: Separates sister chromatids
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE 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

31. Animation: Metaphase II
At 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
Animation: Metaphase II

32. Spindle Microtubules bind to the Kinetochore
Prometaphase marks the binding of spindle fibers to kinetochore
Chromosomes will be pulled apart in anaphase

33. MEIOSIS II: Separates sister chromatids
LE 13-8b
MEIOSIS II: Separates sister chromatids
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE 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

34. Animation: 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
Animation: Anaphase II

35. MEIOSIS II: Separates sister chromatids
LE 13-8b
MEIOSIS II: Separates sister chromatids
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE 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

36. Which one of the following statements is false?  
A) A typical body cell is called a somatic cell.  
B) Gametes are haploid cells.  
C) Somatic cells are diploid.  
D) Gametes are made by mitosis.  E) A zygote is a fertilized egg.  

37. 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
Animation: Telophase II and Cytokinesis

38. Nondisjunction in meiosis I Normal meiosis II Normal meiosis II
LE 8-21a
Nondisjunction
in meiosis I
Normal
meiosis II
Normal
meiosis II
Gametes
n + 1
n + 1
n  1
n  1
Number of chromosomes

39. Normal meiosis I Nondisjunction in meiosis I Gametes n + 1 n  1 n n
LE 8-21b
Normal
meiosis I
Nondisjunction
in meiosis I
Gametes
n + 1
n  1 n n
Number of chromosomes

40. LE 8-21c
Egg
cell
n + 1
Sperm
cell
Zygote
2n + 1
n (normal)

41. Downs syndrome is caused by a nondisjunction (trisomy) at chromosome 21

42. Infants with Down syndrome
LE 8-20c 90 80 70 60
Infants with Down syndrome
(per 1,000 births) 50 40 30 20 10 20 25 30 35 40 45 50
Age of mother

43. Figure 8.20A

44. Hypotonic Fixative Packed red and solution white blood cells
LE 8-19
Hypotonic
solution
Packed red and
white blood cells
Fixative
White blood
cells
Stain
Centrifuge
Blood
culture
Fluid
Centromere
Sister
chromatids
Pair of homologous
chromosomes
2,600

45. Comparing mitosis and meiosis

47. Property
Mitosis
Meiosis
DNA replication
Divisions
Synapsis and crossing over
Daughter cells, genetic composition
Role in animal body

48. Property
Mitosis
Meiosis
DNA replication
During interphase
Divisions
One
Two
Synapsis and crossing over
Do not occur
In prophase I
Daughter cells, genetic composition
Two diploid, identical to parent cell
Four haploid, different from parent cell and each other
Role in animal body
Produces cells for growth and tissue repair
Produces gametes

49. (before chromosome replication)
LE 13-9
MITOSIS
MEIOSIS
Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Propase
Prophase I
Chromosome
replication
Chromosome
replication
Tetrad formed by
synapsis of homologous
chromosomes
Duplicated chromosome
(two sister chromatids)
2n = 6
Chromosomes
positioned at the
metaphase plate
Tetrads
positioned at the
metaphase plate
Metaphase
Metaphase I
Anaphase
Sister chromatids
separate during
anaphase
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Anaphase I
Telophase
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

50. Review of Meiosis

51. Spermatogenesis vs. Oogenesis

52. Meiosis One spermatocyte produces 4 sperm
One oocyte produces 1 viable egg and 3 non-viable polar bodies

54. Variation from sexual reproduction contributes to evolution
Reshuffling of different versions of genes (alleles) during sexual reproduction produces genetic variation

55. Key Maternal set of chromosomes Possibility 1 Possibility 2
LE 13-10
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

56. LE 13-11
Prophase I
of meiosis
Nonsister
chromatids
Tetrad
Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Recombinant
chromosomes

57. 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

58. 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

59. Explain how junior and sis are not identical.

60. duplicated chromosomes)
LE 8-17a
Coat-color
genes
Eye-color
genes C E
Brown
Black C E C E
Meiosis c e c e c e
White
Pink
Tetrad in parent cell
(homologous pair of
duplicated chromosomes)
Chromosomes of
the four gametes

61. LE 8-18b Coat-color genes Eye-color genes C E (homologous pair of c e
Tetrad
(homologous pair of
chromosomes in synapsis) c e
Breakage of homologous chromatids C E c e
Joining of homologous chromatids C E
Chiasma c e
Separation of homologous
chromosomes at anaphase I C E C e c E c e
Separation of chromatids at
anaphase II and
completion of meiosis C E
Parental type of chromosome C e
Recombinant chromosome c E
Recombinant chromosome c e
Parental type of chromosome
Gametes of four genetic types

62. C E Tetrad c e C E c e Coat-color genes Eye-color genes
LE 8-18ba
Coat-color
genes
Eye-color
genes C E
Tetrad
(homologous pair of
chromosomes in synapsis) c e
Breakage of homologous chromatids C E c e
Joining of homologous chromatids

63. C E c e C E C e c E c e Joining of homologous chromatids Chiasma
LE 8-18bb
Joining of homologous chromatids C E
Chiasma c e
Separation of homologous
chromosomes at anaphase I C E C e c E c e
Separation of chromatids at
anaphase II and
completion of meiosis

64. Separation of chromatids at anaphase II and completion of meiosis
LE 8-18bc
Separation of chromatids at
anaphase II and
completion of meiosis C E
Parental type of chromosome C e
Recombinant chromosome c E
Recombinant chromosome c e
Parental type of chromosome
Gametes of four genetic types

65. Table 8.22

66. Review of mitosis and meiosis

67. LE 8-un149
INTERPHASE (cell growth and chromosome duplication) S
(DNA synthesis) G1 G2
Cytokinesis
Mitosis
(division of
cytoplasm)
(division
of nucleus)
Genetically
identical
“daughter
cells”
MITOTIC PHASE (M)

68. Mitosis and development
LE 8-un149s
Haploid gametes (n = 23) n
Egg cell n
Sperm cell
Meiosis
Fertilization
HUMAN
LIFE
CYCLE
Multicellular
diploid adults
Diploid
zygote 2n
(2n = 46)
(2n = 46)
Mitosis and development

69. Sister chromatids Sister chromatids Homologous pair of chromosomes
LE 8-un1
Sister chromatids
Homologous
pair of
chromosomes
Sister chromatids