1. What occurs during the phases of meiosis?
Sections to 8.18

2. Homologous Chromosomes
Somatic Cell
Body cells
All cells excepts the sperm and the egg (gametes/sex cells)
Chromosome Number for Humans 46
23 pair
Homologous Chromosomes
2 chromosomes that are the same length, shape, centromere position and code for the same genetic information
1 from each parent

3. 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

4. Autosome vs. Sex Chromosome
Autosomes
Chromosomes that are the same in males and females
Pairs 1 through 22 on a karyotype
Sex Chromosomes
Chromosomes that are different in males and females
Chromosomes that determine sex
Pair 23 on a karyotype
Female – XX
Male XY

5. Diploid vs. Haploid Diploid
Cells that have 2 sets of genetic information
2 sets of homologous chromosomes
Designated by “2n”
In humans 2n= 46
Normal body cells
Haploid
Cells that have 1 set of genetic information
Only one of each homolog
Designated by “n”
In humans n=23
Gametes

6. Gametes vs. Zygote Gametes Sex Cells Sperm or Egg Haploid Zygote
Fertilized egg
Diploid

7. Meiosis Gamete formation
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
The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis – also called Reduction Division (MI = reduction / MII = division)
Each daughter cell has only half as many chromosomes as the parent cell – starts with “2n” cell ends with four “n” cells

9. Prophase I Nuclear membrane breaks down Nucleolus disappears
Chromatin condenses into chromosomes
Synapsis occurs
Homologous chromosomes come together (forming a tetrad)
Crossing over can occur

10. Metaphase I Homologous pairs line up at the metaphase plate
Spindle fibers attach to the kinetechores

11. Anaphase I Homologous chromosomes are pulled apart
Note: centromeres DO NOT split

12. Telophase I and Cytokinesis
Double chromosomes are at the poles
Nuclear membrane reforms
Nucleolus reappears
Cytoplasm Separates
Note: at this point there are 2 genetically unique haploid cells - REDUCTION

13. Prophase II Nuclear membrane breaks down Nucleolus disappears
Chromatin condenses to chromosomes
Note: no synapsis
Note: haploid

14. Metaphase II Chromosomes move to the metaphase plate
Note: single file like mitosis except these cells are haploid

15. Anaphase II Sister chromatids pulled apart
Note: centromere splits just like anaphase of mitosis only with haploid cells

16. Telophase II and Cytokinesis
Chromosomes are at the poles
Nuclear membrane reforms
Cytoplasm divides
Note: produces 4 genetically unique haploid cells

17. Gamete formation in Animals

18. Gamete formation in Plants

19. How does meiosis differ from mitosis?

20. 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
Anaphase
Telophase
Anaphase I
Figure 13.9 A comparison of mitosis and meiosis in diploid cells
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

21. Mitosis vs. Meiosis Fig. 13-9b
SUMMARY
Meiosis
Mitosis
Property
DNA
replication
Number of
divisions
Occurs during interphase before
mitosis begins
One, including prophase, metaphase,
anaphase, and telophase
Synapsis of
homologous
chromosomes
Does not occur
daughter cells
and genetic
composition
Two, each diploid (2n) and genetically
identical to the parent cell
Role in the
animal body
Enables multicellular adult to arise from
zygote; produces cells for growth, repair,
and, in some species, asexual reproduction
Occurs during interphase before meiosis I begins
Two, each including prophase, metaphase, anaphase, and
telophase
Occurs during prophase I along with crossing over
between nonsister chromatids; resulting chiasmata
hold pairs together due to sister chromatid cohesion
Four, each haploid (n), containing half as many chromosomes
as the parent cell; genetically different from the parent
cell and from each other
Produces gametes; reduces number of chromosomes by half
and introduces genetic variability among the gametes
Figure 13.9 A comparison of mitosis and meiosis in diploid cells

22. Independent Assortment of Chromosomes
Homologous pairs of chromosomes orient randomly at metaphase I of meiosis
In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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

24. Crossing Over
Crossing over produces recombinant chromosomes, which combine genes inherited from each parent
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

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

26. Chiasma
The microscopically visible site where crossing over has occurred between chromatids of homologous chromosomes during prophase I