1. Meiosis

2. Heredity and Variation
Is the transmission of traits from one generation to the next
Variation
Shows that offspring differ somewhat in appearance and other traits from parents and siblings
Genetics
Is the scientific study of heredity and hereditary variation

3. Breeding in English Shepherds
Mother
Father

4. Breeding in English Shepherds - Offspring

5. Genes – A simple definition
Are the units of heredity
Are segments of DNA
Each gene in an organism’s DNA has a specific locus on a certain chromosome
Sexual organisms inherit one set of chromosomes from the mother and one set from the father

6. The Shallow End of the Gene Pool

7. Asexual Reproduction
For example – budding in Hydra is asexual reproduction that forms clones

8. Asexual Reproduction
Strawberry reproducing by putting out asexual runners – also forming clones

9. Mitosis and development
Haploid gametes (n  23)
Key
Haploid (n)
Egg (n)
Diploid (2n)
Sperm (n)
MEIOSIS
FERTILIZATION
Ovary
Testis
Diploid zygote (2n  46)
Figure 13.5 The human life cycle.
Mitosis and development
Multicellular diploid adults (2n  46)

10. Chromosomes Homologous chromosomes
Are the two chromosomes composing a pair
Have the same characteristics
Autosomes are the non-sex determining chromosomes
Sex chromosomes
Are distinct from each other in their characteristics
Are represented as X and Y in mammals
Determine the sex of the individual, XX being female, XY being male; X and X are homologous in female
A diploid cell
Has two sets of each of its chromosomes
In a human has 46 chromosomes (2n = 46)

12. Pair of homologous duplicated chromosomes
Centromere
Sister chromatids
Figure 13.3 RESEARCH METHOD: Preparing a karyotype
Metaphase chromosome

13. Figure 13.x3 Human female karyotype shown by bright field G-banding of chromosomes

14. Figure 13.x5 Human male karyotype shown by bright field G-banding of chromosomes

15. Hairy Ears – a sex linked trait
Square symbol = male
Round symbol = female
Dark square = hairy ear trait

16. Chromosome number in humans if there was no meiosis prior to reproduction
First Generation - Sperm and Egg with 46 chromosomes each
Second generation 92 chromosomes
Third generation 184 chromosomes
Fourth generation 368 chromosomes
Etc.

18. Sexual life cycles Key Haploid (n)
Haploid multi- cellular organism (gametophyte)
Haploid unicellular or multicellular organism
Diploid (2n) n
Gametes n n
Mitosis n
Mitosis
Mitosis n
Mitosis n n n n n
MEIOSIS
FERTILIZATION
Spores n n
Gametes n
Gametes
MEIOSIS
FERTILIZATION
Zygote
MEIOSIS
FERTILIZATION 2n 2n 2n
Diploid multicellular organism (sporophyte) 2n
Zygote
Diploid multicellular organism 2n
Figure 13.6 Three types of sexual life cycles.
Mitosis
Mitosis
Zygote
(a) Animals
(b) Plants and some algae
(c) Most fungi and some protists

19. Interphase Pair of homologous chromosomes in diploid parent cell
Duplicated pair of homologous chromosomes
Chromosomes duplicate
Sister chromatids
Diploid cell with duplicated chromosomes
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number.

20. Interphase
Pair of homologous chromosomes in diploid parent cell
Duplicated pair of homologous chromosomes
Chromosomes duplicate
Sister chromatids
Diploid cell with duplicated chromosomes
Meiosis I 1
Homologous chromosomes separate
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number.
Haploid cells with duplicated chromosomes

21. Interphase Meiosis I Meiosis II
Pair of homologous chromosomes in diploid parent cell
Duplicated pair of homologous chromosomes
Chromosomes duplicate
Sister chromatids
Diploid cell with duplicated chromosomes
Meiosis I 1
Homologous chromosomes separate
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number.
Haploid cells with duplicated chromosomes
Meiosis II 2
Sister chromatids separate
Haploid cells with unduplicated chromosomes

23. Telophase I and Cytokinesis Telophase II and Cytokinesis
MEIOSIS I: Separates homologous chromosomes
MEIOSIS I: Separates sister chromatids
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
Sister chromatids
Chiasmata
Centromere (with kinetochore)
Spindle
Metaphase plate
During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.
Cleavage furrow
Homologous chromosomes separate
Fragments of nuclear envelope
Sister chromatids separate
Haploid daughter cells forming
Homologous chromosomes
Microtubule attached to kinetochore
Each pair of homologous chromosomes separates.
Two haploid cells form; each chromosome still consists of two sister chromatids.
Figure 13.8 Exploring: Meiosis in an Animal Cell
Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n  6 in this example.
Chromosomes line up by homologous pairs.

24. Telophase I and Cytokinesis
Prophase I
Metaphase I
Anaphase I
Centrosome (with centriole pair)
Sister chromatids remain attached
Sister chromatids
Chiasmata
Centromere (with kinetochore)
Spindle
Metaphase plate
Cleavage furrow
Homologous chromosomes separate
Homologous chromosomes
Fragments of nuclear envelope
Figure 13.8 Exploring: Meiosis in an Animal Cell
Microtubule attached to kinetochore
Each pair of homologous chromosomes separates.
Two haploid cells form; each chromosome still consists of two sister chromatids.
Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n  6 in this example.
Chromosomes line up by homologous pairs.

25. Telophase II and Cytokinesis
Prophase II
Metaphase II
Anaphase II
During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.
Sister chromatids separate
Haploid daughter cells forming
Figure 13.8 Exploring: Meiosis in an Animal Cell

26. Chromosome duplication Chromosome duplication Duplicated chromosome
MITOSIS
MEIOSIS
Parent cell
Chiasma
MEIOSIS I
Prophase
Prophase I
Chromosome duplication
Chromosome duplication
Duplicated chromosome
Homologous chromosome pair
2n  6
Metaphase
Metaphase I
Anaphase Telophase
Anaphase I
Telophase I
Figure 13.9 A comparison of mitosis and meiosis in diploid cells.
Daughter cells of meiosis I
Haploid n  3 2n 2n
MEIOSIS II
Daughter cells of mitosis n n n n
Daughter cells of meiosis II

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

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

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

30. Independent Assortment
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

31. Mitosis and meiosis have several key differences.
The chromosome number is reduced by half in meiosis, but not in mitosis.
Mitosis produces daughter cells that are genetically identical to the parent and to each other.
Meiosis produces cells that differ from the parent and each other.

32. Three events, unique to meiosis, occur during the first division cycle
Three events, unique to meiosis, occur during the first division cycle During prophase I, homologous chromosomes pair up in a process called synapsis.
A protein zipper, the synaptonemal complex, holds homologous chromosomes together tightly.
Later in prophase I, the joined homologous chromosomes are visible as a tetrad.
At X-shaped regions called chiasmata, sections of nonsister chromatids are exchanged.
Chiasmata is the physical manifestation of crossing over, a form of genetic rearrangement.
Prophase I is longest and most important phase

33. 2. At metaphase I homologous pairs of chromosomes, not individual chromosomes are aligned along the metaphase plate.
In humans, you would see 23 tetrads.
3. At anaphase I, it is homologous chromosomes, not sister chromatids, that separate and are carried to opposite poles of the cell.
Sister chromatids remain attached at the centromere until anaphase II.
The processes during the second meiotic division are virtually identical to those of mitosis.

34. The synaptonemal complex binding together four homologous chromosomes

35. Chiasmata and crossing over

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

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

38. Nonsister chromatids held together during synapsis
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.

39. Nonsister chromatids held together during synapsis
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

40. Nonsister chromatids held together during synapsis
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

42. Human Female vs Male Meiotic Timelines

43. Sexual reproduction leads to genetic variation via:
Independent assortment during meiosis
Crossing over during meiosis
Random mixing of gametes (sperm and egg)