1. Chapter 13
Meiosis

2. Fig. 13-1
Figure 13.1 What accounts for family resemblance?

3. I. Inheritance of Genes
Genes = segments of DNA
Genes are passed by gametes (sperm and eggs)
Each gene has a specific location (locus) on a chromosome

4. II. Asexual vs Sexual Reproduction
Asexual reproduction = one parent, genetically identical offspring, mitosis
Sexual reproduction = two parents, offspring have combinations of genes

5. 0.5 mm Parent Bud (a) Hydra (b) Redwoods Fig. 13-2
Figure 13.2 Asexual reproduction in two multicellular organisms
Parent
Bud
(a) Hydra
(b) Redwoods

6. III. Human Chromosomes
Human somatic cells (body cells) have 23 pairs of chromosomes
Karyotype = an ordered display of the pairs of chroms from a cell
Homologous chromosomes = The two chroms in each pair (homologs)
Chroms are same length and carry genes controlling the same traits

7. Fig. 13-3a
APPLICATION
Figure 13.3 Preparing a karyotype

8. TECHNIQUE 5 µm Pair of homologous replicated chromosomes Centromere
Fig. 13-3b
TECHNIQUE
5 µm
Pair of homologous
replicated chromosomes
Centromere
Figure 13.3 Preparing a karyotype
Sister
chromatids
Metaphase
chromosome

9. Sex chromosomes are X and Y
Human females have a pair of X chroms (XX)
Human males have one X and one Y (XY)
Autosomes = 22 pairs of chroms that do not determine sex

10. Each pair of homologous chroms has one chrom from each parent
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 (two identical sister chromatids)

11. Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of
Fig. 13-4
Key
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosome
Figure 13.4 Describing chromosomes
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)

12. Gamete contains a single set of chroms, and is haploid (n) (monoploid)
Humans, haploid number is 23 (n = 23)
Each set of 23 = 22 autosomes + 1 sex chrom
In an unfertilized egg (ovum), the sex chrom is X
In a sperm cell, the sex chrom may be either X or Y

13. IV. Chromosomes in Human Life Cycle
Fertilization = union of gametes
Zygote = fertilized egg and has one set of chroms from each parent (2n)
Produces somatic cells by mitosis and develops into an adult

14. At maturity, ovaries and testes produce haploid (n) gametes
Gametes are only types of human cells produced by meiosis (23 chroms, 1 from each pair)

15. Multicellular diploid adults (2n = 46)
Fig. 13-5
Key
Haploid gametes (n = 23)
Haploid (n)
Egg (n)
Diploid (2n)
Sperm (n)
MEIOSIS
FERTILIZATION
Figure 13.5 The human life cycle
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)

16. V. Stages of Meiosis
First cell division (meiosis I), homologous chroms separate
Meiosis I = two haploid daughter cells with replicated chrom; reductional division
Second cell division (meiosis II), sister chromatids separate
Meiosis II = four haploid daughter cells with unreplicated chroms; equational division

17. Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number
Interphase
Homologous pair of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number

18. Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number
Interphase
Homologous pair of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number
Meiosis I 1
Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes

19. Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number
Interphase
Homologous pair of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number
Meiosis I 1
Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Meiosis II 2
Sister chromatids
separate
Haploid cells with unreplicated chromosomes

20. Figure 13.8 The meiotic division of an animal cell
Prophase I
Metaphase I
Anaphase I
Telophase I and
Cytokinesis
Prophase II
Metaphase II
Anaphase II
Telophase II and
Cytokinesis
Centrosome
(with centriole pair)
Sister chromatids
remain attached
Centromere
(with kinetochore)
Sister
chromatids
Chiasmata
Spindle
Metaphase
plate
Figure 13.8 The meiotic division of an animal cell
Sister chromatids
separate
Haploid daughter cells
forming
Homologous
chromosomes
Homologous
chromosomes
separate
Cleavage
furrow
Fragments
of nuclear
envelope
Microtubule
attached to
kinetochore

21. VI. Meiosis I
Prophase I
Occupies more than 90% of time needed for meiosis
Chromosomes condense
Synapsis = homologous chroms loosely pair up, aligned gene by gene

22. Crossing over = nonsister chromatids exchange DNA segments
Each pair of chromosomes forms a tetrad, a group of four chromatids
Usually has one or more chiasmata, X-shaped regions where crossing over occurred

23. Metaphase I
Tetrads line up at the metaphase plate, one chrom facing each pole
Microtubules attached to the kinetochore of each chromosome of each tetrad

24. Prophase I Metaphase I Centrosome (with centriole pair) Centromere
Fig. 13-8b
Prophase I
Metaphase I
Centrosome
(with centriole pair)
Centromere
(with kinetochore)
Sister
chromatids
Chiasmata
Spindle
Metaphase
plate
Figure 13.8 The meiotic division of an animal cell
Homologous
chromosomes
Fragments
of nuclear
envelope
Microtubule
attached to
kinetochore

25. Anaphase I
Pairs of homologous chroms separate, sister chromatids remain attached

26. Telophase I and Cytokinesis
Each half of the cell has a haploid set of chroms; each chrom still consists of two sister chromatids
Cytokinesis forms two haploid daughter cells
No chrom replication occurs between meiosis I and meiosis II

27. Telophase I and Cytokinesis
Fig. 13-8c
Telophase I and
Cytokinesis
Anaphase I
Sister chromatids
remain attached
Figure 13.8 The meiotic division of an animal cell
Homologous
chromosomes
separate
Cleavage
furrow

28. VII. Meiosis II
Prophase II
Spindle apparatus forms
Chroms (composed of two chromatids) move toward the metaphase plate

29. Metaphase II
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
Kinetochores attach to microtubules

30. Prophase II Metaphase II Fig. 13-8e
Figure 13.8 The meiotic division of an animal cell

31. Anaphase II
Sister chromatids separate
Sister chromatids of each chrom now move as two newly individual chroms

32. Telophase II and Cytokinesis
Nuclei form, and the chroms begin decondensing
Cytokinesis makes four daughter cells, each with a haploid set of unreplicated chroms
Daughter cells are genetically distinct from the others and from the parent cell

33. Telephase II and Cytokinesis
Fig. 13-8f
Telephase II and
Cytokinesis
Anaphase II
Figure 13.8 The meiotic division of an animal cell
Sister chromatids
separate
Haploid daughter cells
forming

34. Meiosis - Andersen (9 min)
Mitosis / Meiosis Simulation - Andersen (12 min)

35. VIII. Comparison of Mitosis and Meiosis
Mitosis keeps the number of chromosomes the same, producing cells that are genetically identical to the parent
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
Mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis

36. Replicated chromosome
Fig. 13-9a
MITOSIS
MEIOSIS
MEIOSIS I
Parent cell
Chiasma
Chromosome
replication
Chromosome
replication
Prophase
Prophase I
Homologous
chromosome
pair
2n = 6
Replicated chromosome
Metaphase
Metaphase I
Figure 13.9 A comparison of mitosis and meiosis in diploid cells
Anaphase
Telophase
Anaphase I
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

37. Figure 13.9 A comparison of mitosis and meiosis in diploid cells
Fig. 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
Figure 13.9 A comparison of mitosis and meiosis in diploid cells
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
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

38. Three events are unique to meiosis (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

39. Cell Cycle / Mitosis / Meiosis - Andersen (14 min)

40. IX. Genetic Variation in Offspring
Mvmt of chroms during meiosis and fert gives variation
Mutations (changes in DNA) are original source of genetic diversity
Mutations create different versions of genes (alleles, A or a)
Reshuffling of alleles during sexual reproduction produces genetic variation

41. A. Independent Assortment of Chromosomes
Homologous chroms orient randomly at metaphase I of meiosis
Each pair of chroms sorts maternal and paternal homologues into daughter cells independently of the other pairs
Number of combinations possible is 2n, where n is the haploid number
Humans (n = 23), there are more than 8 million (223) possible combinations

42. Possibility 2 Possibility 1 Two equally probable arrangements of
Fig
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Figure The independent assortment of homologous chromosomes in meiosis

43. Possibility 2 Possibility 1 Two equally probable arrangements of
Fig
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Figure The independent assortment of homologous chromosomes in meiosis
Metaphase II

44. Possibility 1 Possibility 2 Two equally probable arrangements of
Fig
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Figure The independent assortment of homologous chromosomes in meiosis
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4

45. B. Crossing Over
Produces recombinant chromosomes (combine genes inherited from each parent)
Begins in prophase I, as homologous chroms pair up gene by gene
Homologous portions of two nonsister chromatids trade places
Variations come from combining DNA from two parents into a single chromosome

46. Prophase I Nonsister of meiosis chromatids held together
Fig
Prophase I
of meiosis
Nonsister
chromatids
held together
during synapsis
Pair of
homologs
Figure The results of crossing over during meiosis

47. Prophase I Nonsister of meiosis chromatids held together
Fig
Prophase I
of meiosis
Nonsister
chromatids
held together
during synapsis
Pair of
homologs
Chiasma
Centromere
TEM
Figure The results of crossing over during meiosis

48. Prophase I Nonsister of meiosis chromatids held together
Fig
Prophase I
of meiosis
Nonsister
chromatids
held together
during synapsis
Pair of
homologs
Chiasma
Centromere
TEM
Figure The results of crossing over during meiosis
Anaphase I

49. Prophase I Nonsister of meiosis chromatids held together
Fig
Prophase I
of meiosis
Nonsister
chromatids
held together
during synapsis
Pair of
homologs
Chiasma
Centromere
TEM
Figure The results of crossing over during meiosis
Anaphase I
Anaphase II

50. Recombinant chromosomes
Fig
Prophase I
of meiosis
Nonsister
chromatids
held together
during synapsis
Pair of
homologs
Chiasma
Centromere
TEM
Figure The results of crossing over during meiosis
Anaphase I
Anaphase II
Daughter
cells
Recombinant chromosomes

51. C. Random Fertilization
Variation arises because any sperm can fuse with any ovum (unfertilized egg)
Fusion of two gametes (each with 8.4 million possible combinations from ind assort) produces a zygote with about 70 trillion diploid combinations
Crossing over adds even more variation

52. X. Evolution and Genetic Variation
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

53. You should now be able to:
Distinguish between the following terms: somatic cell and gamete; autosome and sex chromosomes; haploid and diploid
Describe the events that characterize each phase of meiosis
Describe three events that occur during meiosis I but not mitosis
Name and explain the three events that contribute to genetic variation in sexually reproducing organisms
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