1. Biology – Premed Windsor University School of Medicine and Health Sciences DR. UCHE AMAEFUNA.

2. There is more to lectures than the power point slides!
CHAPTER 21 MEIOSIS
There is more to lectures than the power point slides!
Engage your mind

3. Meiosis &
Sexual Reproduction

4. 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!

5. Asexual reproduction Budding Single-celled eukaryotes
Yeast, Paramecium
Amoeba
Simple multicellular eukaryotes
Hydra
What are the disadvantages of asexual reproduction?
What are the advantages?

6.    How about the rest of us? + 46 46 92
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. Human female karyotype
diploid = 2 copies
Human female karyotype 2n
46 chromosomes
23 pairs XX

8. diploid = 2 copies
Human male karyotype 2n
46 chromosomes
23 pairs XY

9. How do we make sperm & eggs?
Must reduce 46 chromosomes  23
must half the number of chromosomes
haploid
zygote 23 46 23 46
meiosis
egg 23 46 23
fertilization
sperm
gametes

10. Meiosis makes sperm & eggs
46 chromosomes to 23 chromosomes
half the number of chromosomes 23 46
meiosis
egg 46 23
diploid
haploid
sperm

11. Paired chromosomes both chromosomes of a pair carry“matching” genes
Homologous chromosomes
both chromosomes of a pair carry“matching” genes
One from mom, one from dad
control same inherited characters (ie. eye color)
homologous = same information
diploid 2n
2n = 4
eye color
(brown?)
eye color
(blue?)
homologous chromosomes
double stranded homologous chromosomes

12. Crossing Over -> variation
Prophase 1
nonsister chromatids
Tetrad
chiasmata: site of crossing over
variation

13. Meiosis 1 overview 1st division of meiosis double stranded
Divide 1
Copy DNA
Line Up 1
4 chromosomes
diploid 2n
prophase 1
metaphase 1
telophase 1
2 chromosomes
haploid 1n

14. 2nd division of meiosis looks like mitosis
Meiosis 2 overview
Bye Bye 2
telophase 2
telophase 1
Line Up 2 4
2nd division of meiosis
looks like mitosis
metaphase 2
2 chromosomes
haploid 1n
gametes

15. Meiosis = reduction division
special cell division in sexually reproducing organisms
reduce number of chromosomes by half
2n  1n
diploid  haploid
makes gametes (sperm, eggs)

16. Meiosis to make gametes
Meiosis & mitosis
Meiosis to make gametes
sperm & egg
Mitosis to make copies of ALL OTHER cells for:
growth
repair
development

17. Sexual reproduction lifecycle
2 copies
diploid 2n
1 copy
haploid 1n
1 copy
haploid 1n
fertilization
meiosis
We’re mixing
things up here!
A good thing?

18. Putting it all together…
meiosis  fertilization  mitosis + development
gametes 46 46 23 23 46 46 46 46 46
meiosis
egg 46 46 23 46 46 23
zygote
mitosis & development
fertilization
mitosis
sperm

19. meiosis keeps chromosome number same from generation to generation
The value of meiosis 1
meiosis keeps chromosome number same from generation to generation
from Mom
Mom
Consider the greater variation with 23 pairs of chromosomes = mixing and matching
Dad
offspring
from Dad

20. The value of meiosis 2 Change over time
We’re
mixing things
up here!
Change over time
meiosis introduces genetic variation
new combinations of traits from generation to generation
Consider the greater variation with 23 pairs of chromosomes = mixing and matching
from Dad
variation
from Mom
offspring
new gametes made by offspring

21. How does this explain why kids are similar to their parents but not the same?
Michael & Kirk Douglas
Baldwin brothers
Martin & Charlie Sheen, Emilio Estevez

22. Meiosis I
Purpose:
    Meiosis I has two main purposes:
It is the reduction division, so it reduces the number of chromosomes in half, making the daughter cells haploid (when the parent cell was diploid).
It is during meiosis I that most of the genetic recombination occurs.

23. Phases:
    Keep in mind that before meiosis begins at all, the DNA undergoes replication, just like it did before mitosis started.  So, when you first see chromosomes in meiosis I, they have sister chromatids, just like in mitosis.  It is just that in meiosis I, we will be talking about tetrads becoming visible, lining up, separating, and decondensing (rather than chromosomes, like in mitosis).  Finally, cytokinesis occurs, too, any time after the tetrads have moved out of the equator (just like in mitosis).

24. Prophase I
    Just like in mitosis, during prophase, DNA condensation occurs, the nuclear envelope and nucleoli disappear, and the spindle starts to form.  The big difference is what is going on with the chromosomes themselves.
    As DNA condensation proceeds and the chromosomes first become visible, they are visible as tetrads.  So, tetrads become visible during prophase.

25. Metaphase I
    Tetrads line up at the equator.  The spindle has completely formed.
    It is during prophase I and metaphase I that genetic recombination is occurring.  Take a look at the genetic recombination page to find out about how that happens here.  Keep in mind that it only happens when there are tetrads, so as soon as anaphase I gets going, genetic recombination is over.

26. Anaphase I
    Tetrads pull apart and chromosomes with two chromatids move toward the poles.

27. Telophase I
    Chromosomes with two chromatids decondense and a nuclear envelope reforms around them.  Each nucleus is now haploid.
    Keep in mind that it is not the number of chromatids per chromosome that determine whether a cell is diploid or haploid, but, it is the number of chromosomes and whether they are paired that determines this.

28. Meiosis II
Purpose:
    At the end of meiosis I, each chromosome still had two chromatids.   That is double the amount of DNA that a cell should have.  So, the entire reason to go through meiosis II is to reduce the amount of DNA back to normal-- basically, to split the chromosomes so that each daughter cell has only one chromatid per chromosome (the normal genetic content).

29. Phases:
    As you read through the phases of meiosis II, you will see that it looks just like mitosis.  It is really similar to mitosis-- so keep that in mind.   The only difference is that the two chromatids per chromosome are not necessarily identical due to genetic recombination occuring in meiosis I.

30. Prophase II
    Chromosomes with two chromatids become visible as they condense (and the nuclear envelope and nucleoli disappear, and the spindle is forming).

31. Metaphase II
    Chromosomes with two chromatids line up at the equator.  The spindle is fully formed.
    Although genetic recombination primarily occurs during meiosis I, the way the chromosomes line up during metaphase II can also help to make unique daughter cells.  I mention this on the genetic recombination page.

32. Anaphase II
    Chromosomes split, so that a chromosome with only one chromatid heads toward each pole.

33. Telophase II
    Chromosomes with only one chromatid decondense and get surrounded by new nuclear envelopes.  The four daughter cells are now all haploid and have the right amount of DNA.  They are ready to develop into sperm or eggs now.