1. Meiosis and Sexual Life Cycles
Chapter 13
A. P. Biology
Mr. Knowles
Liberty Senior High School

2. Mitosis- Review

3. Mutations

4. 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

5. In sexual reproduction:
Two parents give rise to offspring that have unique combinations of genes inherited from the two parents.

6. Leewenhoek’s Early Ideas of Sperm

8. Types of Cells
Somatic (nonreproductive cells) – are diploid. Body cells. Human somatic cells have 46 chromosomes.
Gametes (reproductive cells) – are haploid. Human sperm and ova have 23 chromosomes.

9. Gametes Fuse to Form a Zygoten 2n
Syngamy n

10. During fertilization:
These gametes, sperm and ovum, fuse, forming a diploid zygote.
The zygote
Develops into an adult organism.

11. Fertilization or Syngamy

14. Zygote Undergoes Mitosis

15. Zygote is Diploid (2n)

16. Why is the number of chromosomes in gametes ½ that of somatic cells?
If not, after 10 generations the 46 chromosomes in human cells would increase to over 46 X 210.

17. Meiosis
A process of cell division in which the number of chromosomes in certain cells is halved during gamete formation.
A reduction division.

19. The Human Life Cycle Key Figure 13.5 Haploid gametes (n = 23)
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)

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

21. Meiosis occurs during gamete formation
In animals
Meiosis occurs during gamete formation
Gametes are the only haploid cells
Gametes
Figure 13.6 A
Diploid
multicellular
organism
Key
MEIOSIS
FERTILIZATION n 2n
Zygote
Haploid
Mitosis
(a) Animals

22. (b) Plants and some algae
Exhibit an alternation of generations.
The life cycle includes both diploid and haploid multicellular stages.
MEIOSIS
FERTILIZATION n 2n
Haploid multicellular
organism (gametophyte)
Mitosis
Spores
Gametes
Zygote
Diploid
multicellular
organism
(sporophyte)
(b) Plants and some algae
Figure 13.6 B

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

25. Sexual Life Cycle
Reproduction that alternates between fertilization (a diploid state) and meiosis (a haploid state) – sexual reproduction.
Sexual reproduction – provides DNA from both parents.

26. Some Sexual Life Cycles
Unicellular Protists – individual cells function as gametesundergo meiosis and fuse with others.
Plants- may produce haploid cells undergo mitosis and produce a multicellular haploid organism.
Animals- set aside germ cells undergo meiosis and become haploid gametes.

27. 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
separate
Haploid cells with
replicated chromosomes
Sister chromatids
Haploid cells with unreplicated chromosomes
Meiosis I
Meiosis II
An overview of meiosis

28. Interphase and Meiosis I
Centrosomes
(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
Figure 13.8

29. Telophase I, Cytokinesis, and Meiosis II
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II AND
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

30. Meiosis Two nuclear divisions. Labeled PMAT I and PMAT II.
Prophase I is very different in meiosis than in mitosis.

31. Interphase I

33. Prophase I

34. Synapsis in Prophase I

35. Prophase I
The duplicated homologous chromosomes condense and the ends of chromatids attach to the nuclear envelope.
Homologous pairs align next to each other.
The chromatids of one homologue align in precise register with the chromatids of the other in a process called – synapsis (zipping up a zipper).

36. Prophase I
The DNA of chromatids unwind and base pair with the complementary strand on the the other homologue.
The crossed-over sister chromatids form an X-shaped structure – chiasma.
DNA from one homologue sister chromatid is exchanged with the other – Crossing Over.

37. Crossing Over During Prophase I

38. Prophase I
The chiasmata hold the two pairs of sister chromatids together at the ends – Terminal Chiasmata.
The homologous chromatids have exchanged DNA.

39. Prophase I

40. Metaphase I

41. Metaphase I Terminal chiasmata are still formed.
The nuclear membrane breaks down and spindle fibers are forming.
Only one side of each centromere faces outward toward the growing spindle fiber.
The fibers can attach to kinetochore proteins on only one side of each centromere.

42. Metaphase I Alignment of each pair on the metaphase plate is random.
Either homologue may be situated toward a given pole.

43. Metaphase I

44. Anaphase I

45. Anaphase I
Spindle fibers contract and break the chiasmata and pull the centromeres toward the poles.
The entire centromere of the homologue proceeds to one pole. (Different than mitosis).
Each pole receives a haploid number of chromosomes, one member of each homologous pair.

46. Anaphase I

47. Anaphase I
Because the orientation of the homologous chromosomes on the metaphase plate is random, genes on separate chromosomes are inherited independently – Independent Assortment.

48. Telophase 1

49. Telophase I Individual chromosomes cluster at the poles.
Each chromosome is made of two chromatids that are not identical because of crossing over that occurred in Prophase I.
Cytokinesis may or may not occur after Telophase I. Nuclei may not reform.

50. Telophase I

51. The Second Meiotic Division
No DNA replication before this division (Different than mitosis).
Undergo a simple mitotic division with the products from Telophase I.
Results are four haploid cells.
Nuclei are reorganized.

53. Prophase II

54. Metaphase II

55. Metaphase II

56. Anaphase II

57. Anaphase II

58. Telophase II

59. Telophase II

64. (before chromosome replication)
Comparison of Mitosis and Meiosis
Figure 13.9
MITOSIS
MEIOSIS
Prophase
Duplicated chromosome
(two sister chromatids)
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
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
Sister chromatids separate during anaphase II
Anaphase
Telophase
Sister chromatids
separate during
anaphase 2n
Daughter cells
of mitosis
2n = 6

65. A Comparison of Mitosis and Meiosis
Meiosis and mitosis can be distinguished from mitosis by three events in Meiosis l:
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

66. Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution
3 Sources of Genetic Variation:
1. Reshuffling of genetic material in meiosis-Crossing Over.
2. Homologous pairs of chromosomes are oriented randomly at metaphase - Independent Assortment.
3. Random Fertilization of Gametes- Will produce a zygote with any of about 64 trillion diploid combinations

67. 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

68. Independent Assortment
Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs
Key
Maternal set of
chromosomes
Paternal set of
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Possibility 2
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4
Figure 13.10

69. Evolutionary Significance of Genetic Variation Within Populations
-Is the raw material for evolution by natural selection
Mutations:
Are the original source of genetic variation
Sexual reproduction:
Produces new combinations of variant genes, adding more genetic diversity

70. The Fate of Haploid Cellsn
Undergo Mitosis Multicellular
Gametes
sperm
ova n n
Haploid Plants
Haploid Fungi

71. Meiotic divisions that result in gametes.
Gametogenesis
Meiotic divisions that result in gametes.

73. Oogenesis Spermatogenesis
Oogonium (female germ cell, 2n)
Spermatogonium (male germ cell, 2n)
Germ cells committed to meiosis
Primary Oocyte (2n)
Primary Spermatocyte (2n)
First Meiotic Division
___Before Ovulation____
Secondary Spermatocyte (n)
Secondary Spermatocyte (n)
Secondary Ooctye (n)
First Polar Body (n)
Second Meiotic Division
___After Fertilization____
Four Spermatids (n)
Second Polar Body (n)
Ovum (n)
Four Spermatozoa (n)

75. Primary Spermatocytes
Seminiferous Tubule
Spermatids
Mature Sperm

76. Rat Sperm

78. Fertilization or Syngamy

79. Zygote is Diploid (2n)

81. Cloning to Save an Endangered Species
Enculeated Cow Egg
Bateng Nucleus
Surrogate Cow
Frozen Bateng Cells
Cloned Bateng

82. Microtubules and Motor Proteins-Spindle Fibers

83. Cloned Animals as of 4-04

84. What animal group hasn’t been cloned…yet?
Primates
Why?

86. Can haploid cells develop into functioning animals or plants?
Bees Do It!

87. How to make a bee colony? Fertilized Egg, (2n) Female Worker, (2n)
Meiosis
Mitosis w/o Cytokinesis
Unfertilized Egg, (1n)
Queen, (2n)
Male Drone, (1n)