1. Meiosis and Sexual Reproduction
Chapter 12

2. Meiosis
A specialized type of cell reproduction specifically designed to create gametes (ova and sperm or spores) for sexual reproduction
Meiosis halves the parental cell’s chromosome number from diploid to haploid
Occurs only in sexually reproducing eukaryotic species

3. Terms and Concepts Asexual reproduction A single parent
No genetic variation
All offspring are genetically identical to each other and the parent
Mechanisms
Mitosis
Prokaryotic fission
Vegetative propagation
Pros and Cons
No energy expended to find a mate (pro)
The phrase “There goes the last female Dodo bird” doesn’t mean extinction (pro)
No genetic variation (con)

5. Fig. 9-1a, p.138

6. Terms and Concepts Sexual Reproduction Two parents required
most of the time—self fertilizing flowers
Genetic variation
Variation is important for survival and adaptation (and ultimately evolution)
Mechanisms
Meiosis, gamete formation, and fertilization
Pros and Cons
Genetic variation (pro)
Must expend energy to attract and find a mate (con)
The phrase “There goes the last female Dodo bird” does mean extinction (con)

7. Fig. 9-1b, p.138

8. Fig. 9-1c, p.138

9. Terms and Concepts Diploid Haploid Homologs Somatic cells Germ cells
Two copies of each type of chromosome
Haploid
One copy of each type of chromosome
Homologs
Pair of chromosomes
Somatic cells
Typical diploid cells of an animal
Germ cells
Cells committed to go through meiosis to produce gametes
Gamete
Reproductive cells (Egg/ova, sperm, spores)
Zygote
The first cell of a new individual (result of fertilization)

10. Meiosis
For sexual reproduction the gametes must have 1/2 the number of chromosomes as the normal cells of an organism
For example human cells have 46 chromosomes
(23 pairs)
If the sperm had 46 and the ovum had 46, then the resulting zygote would have 92 chromosomes!
Over time cells would have more and more chromosomes
The process of meiosis produces gametes with ½ the number of chromosomes

11. Fig. 9-12, p.150

12. Meiosis During meiosis there are two rounds of division Meiosis I
Separates homologs
Meiosis II
Separates sister chromatids

13. cell pairs with its partner,
Each homologue in the
cell pairs with its partner,
then the partners separate.
p.141c

14. Meiosis
The process of meiosis is very similar to mitosis, however there are some key differences that account for producing genetically variable, haploid gametes
The following slides will give an overview of meiosis especially pointing out the key differences from mitosis
However, the actual processes of each phase will not be presented as they are the same as in mitosis
Study figure 12.5

15. Meiosis Meiosis I Prophase I Homologs pair up along their length
The event is called synapsis or pairing of homologous chromosomes
The structure of the two chromosomes is called a tetrad
There are four strands of DNA, two sets of sister chromatids
Crossing over
While the homologs are closely associated they can swap segments of DNA
This creates novel combinations of gene traits in both strands
Figure 12.7

17. Meiosis Meiosis I Metaphase I
Homolog pairs line up at the equator (pushed and pulled by the microtubules of the bipolar spindle, just as in mitosis)
Random Alignment (fig 12.7)
Homologs can be attached to either spindle pole
Each homolog can be packaged into either one of the two new nuclei
Increases the number of potential combinations of maternal and paternal alleles in gametes
There are over 8 million possible combinations of the maternal and paternal chromosomes which adds to genetic variability of meiosis

18. combinations possible1 2 3
combinations possible or or or
Fig. 9-7, p.145

19. Meiosis Meiosis I Anaphase I
One of each duplicated chromosome or homolog is pulled towards a spindle pole by the microtubules
Its homolog moves to the opposite pole
This is the step that creates haploid cells by separating the homologous pairs

20. p.141c

21. Meiosis Meiosis I Telophase I
One of each type of chromosome has arrived at a spindle pole
For most species the cytoplasm will divide
For a few, cytoplasmic division occurs after both rounds of meiosis
Cells often proceed directly into meiosis II without completely finishing telophase I

23. Meiosis
Meiosis II (both cells created by meiosis I will follow through the steps of meiosis II to divide a second time)
Prophase II
The centriole pairs (centrosomes) separate and create a new bipolar spindle
Microtubules latch onto separate chromatids
Chromosomes are
Still duplicated (two chromatids each), but only one of each type of chromosome (haploid)
Generally still condensed following meiosis I
There is no DNA replication between meiosis I and II

24. Meiosis Meiosis II Metaphase II Anaphase II Telophase II
Chromosomes are lined up at the equator
Anaphase II
Sister chromatids separate and are pulled towards opposite spindle poles
Telophase II
New nuclear envelope forms around all four new haploid nuclei
Cytoplasmic division results in four cells each containing a haploid number of unduplicated chromosomes

25. p.141d

26. Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II

27. Meiosis Meiosis introduces genetic variations in traits
Two parents both contribute genes to their offspring
One of each autosomal chromosome and one sex chromosome are passed to offspring
Thus offspring get two of each type of chromosome
Each pair of chromosomes carries the same genes
Genes may not be identical
The differences or traits of the same gene are called alleles

28. Meiosis
Meiosis introduces genetic variations (mixes of different alleles) in traits
Crossing over (figure 12.6)
Prophase I
Random alignment (figure 12.7)
Metaphase I
Remember genetic variation is one of the “pros” of sexual reproduction

29. Meiosis Results of meiosis
1 diploid parent cell → 4 genetically variable haploid daughter cells

30. Questions When homologs swap DNA this is called?
T or F: Homologs have a predictable pattern they use to line up during metaphase I.
What is separated during Anaphase I?
What is separated during Anaphase II?
What processes of meiosis contribute to genetic variation?
How many cells are produced by meiosis?

31. From gametes to offspring
Sexual reproduction
Meiosis → four haploid cells
Gamete formation
Sperm formation
Ova/Egg formation
Fertilization
Fusion of the haploid nuclei of the ovum and sperm
Creates a diploid zygote
Adds to variation
Which sperm fertilizes which egg is a matter of chance (see next chapter!)

32. Fig. 9-9, p.147

33. Fig. 9-10a, p.147

34. Fig. 9-12, p.150

35. Sexual Reproduction in Animals
Animals use Gametic Meiosis
Meiosis produces haploid gametes
Gametes do not divide (remain uni-cellular)
Fertilization produces a diploid zygote
Zygote undergoes mitosis to produce a multicellular diploid body (adult animal)

39. Sexual Reproduction in Plants
Plants use Sporic Meiosis (alternation of generations)
Meiosis produces haploid spores
Spores undergo mitosis to produce a multicellular haploid body (gametophyte)
Fertilization produces a diploid zygote
Zygote undergoes mitosis to produce a multicellular diploid body (sporophyte)
Alternates between multi-cellular diploid and multi-cellular haploid bodies

40. multicelled sporophyte
mitosis
Meiosis
Haploid (n) Phase of Cycle
spores
zygote
Diploid (2n)
Phase of Cycle
Fertilization
gametes
mitosis
multicelled gametophyte
mitosis
Stepped Art
Fig, 21.3, p. 326

41. Sexual Reproduction in Plants
Sporophyte (diploid, multicellular)
Produces flowers which contain the germ cells
Germ cells are located in the carpel (ova) and stamen (pollen)

43. Sexual Reproduction in Plants
Production of the ova
Occurs in flowers within the pistil
Made up of 1 or more carpels
Stigma
Style
Ovary

44. Sexual Reproduction in Plants
Production of the ova
Occurs in flowers within the pistil
The ovary contains 1 or more ovules which produce egg sacs
Meiosis of the ovules produces spores
Spores undergo mitosis to become the egg sac (gametophyte)
Meiosis
Spores
Egg Sac
Mitosis

45. Diploid Stage Haploid Stage Meiosis
an ovule
ovary wall
megasporocyte
integument
stalk
ovary (cutaway view)
Diploid Stage
Haploid Stage
Double Fertilization
Meiosis
Fig. 28.6b, p

46. Sexual Reproduction in Plants
Production of pollen
Occurs in flowers in the stamen
Anther
Filament

47. Sexual Reproduction in Plants
Production of pollen
The anther contains germ cells in pollen sacks
Meiosis produces spores
Spores undergo mitosis to become the pollen (gametophyte)

48. pollen sac
anther
(cutaway view)
filament
Germ cell
Diploid Stage
Haploid Stage
Meiosis
Spores
Mitosis
Pollen

50. Sexual Reproduction in Plants
Pollination
The transfer of pollen from an anther to a stigma (not fertilization)
Wind, insects, mammals, etc.
Released pollen lands on the stigma
Pollen grows a tube through the style to the ovary

51. Sexual Reproduction in Plants
Fertilization
Two sperm enter the egg sac from the pollen tube
One fuses with the egg forming the diploid zygote
Develops into the embryo
The other fuses with the central cell to form a triploid endosperm

52. Sexual Reproduction in Plants
Development
A seed develops from each mature fertilized ovule
Fruit develops from the ovary or other tissue
Embryo grows into a new mature sporophyte
Nutritive tissue
Embryo
Seed
Ovule
Ovary

55. Fig. 27-5, p.451

57. Fig. 9-8, p.146

58. Sexual Reproduction in Fungus
Fungus use Zygotic Meiosis
Meiosis of the zygote produces haploid spores
Spores undergo mitosis to produce a multicellular haploid body (mycelium/fungus)
Mycelia of two fungi fuse and develop into a fruiting body (mushroom)
Fertilization produces a diploid zygote
Zygote remains uni-cellular
Zygote is the only diploid cell during the life cycle

59. a g mycelium Diploid Zygote Diploid Stage nuclear fusion meiosis
Haploid Stage
Cells with two nuclei
(n + n) form on gills g
spore (n)
at gill
margin
gill
Mitosis
cap
stalk
mycelium
cytoplasmic fusion
mycelium

60. Summary Terms and concepts Meiosis I and II Sexual reproduction
Pairing of homologs, crossing over, random alignment
4 haploid daughter cells
Sexual reproduction
Meiosis, gamete formation, fertilization
Animal reproduction
Plant reproduction
Fungus reproduction