1. Chapter 7
Meiosis

2. Cell division / Asexual reproduction
Mitosis
produce cells with same information
identical daughter cells
exact copies
clones
same number of chromosomes
same genetic information
Aaaargh! I’m seeing double!

3. Asexual reproduction Binary Fission Fission Fragmentation Budding
Prokaryotes
Fission
Amoeba
Fragmentation
Starfish (Echinoderms)
Budding
Yeast, Hydra
What are the disadvantages of asexual reproduction?
What are the advantages?

4. Budding in Yeast Fission in Amoeba

5. Asexual Reproduction Sexual Reproduction
Simple and very primitive
Complex series of nuclear and cellular division
Allows for the production of many offspring in a short period of time
Production of offspring requires more time and a mate of the opposite sex
No genetic diversity
Great genetic diversity
Asexual reproduction does not allow for great genetic diversity
Disadvantage=can’t adapt.
Sexual reproduction allows for great genetic diversity amongst offspring
Genetic diversity is the raw material for evolution (the ability to successfully adapt to a changing environment)

6. Making Gametes (egg & sperm)
What if a complex multicellular organism (like us) wants to reproduce?
joining of egg + sperm
Do we make egg & sperm by mitosis?
No!
What if we did, then….    46 + 46 92
egg
sperm
zygote
Doesn’t work!

7. How do we make sperm & eggs?
Must reduce the chromosome number 46  23
must be half the number of chromosomes
Haploid (n) 23 46
zygote 23 46
egg 23
meiosis 46 23
fertilization
sperm
gametes

8. Meiosis = Reduction Division
special cell division in sexually reproducing organisms
reduces number of chromosomes
2n  1n
diploid  haploid
half
makes gametes
sperm, eggs

9. Double division of meiosis
DNA replication
1st division of meiosis separates homologous pairs
2nd division of meiosis separates sister chromatids

10. 2n = 4
single
stranded
Meiosis 1 Overview
1st division of meiosis separates homologous pairs of chromosomes
2n = 4
double
stranded
prophase1
2n = 4
double
stranded
metaphase1
telophase1
1n = 2
double
stranded

11. Meiosis 2 Overview 2nd division of meiosis separates sister chromatids
double
stranded
Looks like
Mitosis!
prophase2
1n = 2
double
stranded
metaphase2
1n = 2
single
stranded
telophase2

12. Steps of meiosis Meiosis 1
interphase
prophase 1
metaphase 1
anaphase 1
telophase 1
Cytokenesis 1
Meiosis 2
prophase 2
metaphase 2
anaphase 2
telophase 2
Cytokenesis 2
1st division of meiosis separates homologous pairs
(2n  1n)
2nd division of meiosis separates sister chromatids
(1n  1n)
* just like mitosis *

13. Preparing for meiosis 1st step of meiosis Duplication of DNA
Why bother?
meiosis evolved after mitosis
convenient to use “machinery” of mitosis
DNA replicated in S phase of interphase of MEIOSIS (just like in mitosis)
2n = 6
single
stranded
2n = 6
double
stranded
M1 prophase

14. Prophase 1
Homologous chromosomes come together to form a structure called a Tetrad.
There are 4 chromatids in a tetrad.
When the homologous chromosomes attach, this is called Synapsis.
Homologous Chromosomes
A Tetrad formed by Synapsis

15. What are the advantages of sexual reproduction?
Crossing over
Homologous pairs swap
pieces of chromosome
breakage of DNA
re-fusing of DNA
New combinations of traits
synapsis

16. Genetic variation… Crossing over
creates completely new combinations of traits in next generation

17. Metaphase 1
Pairs of homologous chromosomes (tetrads) migrate to the equatorial plane of the cell.

18. Anaphase 1
Homologous chromosomes (not chromatids) separate and move to opposite poles of the cell.

19. Telophase 1 Movement of chromosomes to the poles is completed.
Cytokenesis begins

21. End of Meiosis 1
The cell splits into two haploid cells with duplicated chromosomes.
Chromosomes do not replicate (get copied) between meiosis 1 and meiosis 2.
This means no duplication of chromosomes and no interphase!
Why is this vitally important?

22. **Meiosis 2 is just like Mitosis!!!**
Meiosis 2 in a Nutshell
**Meiosis 2 is just like Mitosis!!!**
Prophase II – New spindle forms
Metaphase II – Duplicated Chromosomes line up in the middle
Anaphase II – Chromosomes are pulled apart & each chromatid becomes a single chromosome and moves to an opposite side of the cell
Telophase II – Nuclear envelope forms, cytokenesis begins

25. Mitosis vs. Meiosis Mitosis Meiosis 1 division
daughter cells genetically identical to parent cell
produces 2 cells
2n  2n
produces cells for growth & repair
no crossing over
Meiosis
2 divisions
daughter cells genetically different from parent
produces 4 cells
2n  1n
produces gametes
crossing over

26. Meiosis in Males and Females
In men = Spermatogenesis produces sperm in testes (gonads)
In women = Oogenesis produces ova in ovaries (gonads)
Both processes = 4 haploid gametes
The only difference between the 2, are the size of the gametes that are produced.

27. Spermatogenesis & Oogenesis
In Spermatogenesis, the cytoplasmic divisions are equal.
In Oogenesis, the cytoplasmic divisions are unequal.
This results in the production of 1 large haploid cell and 3 small haploid cells.
The large cell becomes the ovum, and the 3 small cells become Polar Bodies.
Polar bodies simply house the extra chromosomes until all 3 cells disintegrate.

29. Sources of genetic variability
Genetic variability in sexual reproduction
independent assortment
homologous chromosomes in Meiosis 1
crossing over
between homologous chromosomes in prophase 1
random fertilization
random ovum fertilized by a random sperm
metaphase1

30. Overview of meiosis I.P.M.A.T.P.M.A.T. 2n=4 interphase 1 prophase 1
metaphase 1
anaphase 1
n=2
prophase 2
metaphase 2
anaphase 2
telophase 2
telophase 1

31. What are the DISadvantages of sexual reproduction?
Any Questions??