1. Meiosis & Sexual Life Cycles
Chapter 13

2. What are disadvantages of asexual reproduction?
Cell Reproduction
Mitosis
produce cells with same information
identical daughter cells
exact copies (clones)
same amount of DNA
same number of chromosomes
asexual reproduction
What are disadvantages of asexual reproduction?
What are advantages?

3. Asexual Reproduction
Single-celled eukaryotes reproduce asexually (mitosis + cytokinesis)
Yeast, Paramecium, Amoeba
Simple multicellular eukaryotes reproduce asexually by budding
Hydra
Budding in Hydra

4. Budding in Yeast
Cell Division in Amoeba

5. Reproduction in multicellular organisms
How is it possible to maintain such genetic continuity in a multicellular organism?
Michael & Kirk Douglas
Baldwin brothers
Martin & Charlie Sheen, Emilio Estevez

6. What has to happen to make gametes?
Sexual Reproduction
But how does a complex multicellular organism reproduce?
joining of gametes
egg (or ovum)
sperm
What has to happen to make gametes?

7. Role of Meiosis in Sexual Cycles
Alternating processes
fertilization & meiosis alternate
meiosis reduces chromosome number
diploid haploid
2n n
fertilization restores chromosome number
haploid diploid
n n

8. Meiosis & fertilization must alternate for sexual reproduction to continue
Alternation of generation dominant multicellular stage (n vs. 2n) differs amongst organisms

9. Multicellular Haploid (n) organism = gametophyte
Multicellular Diploid (2n) organism = sporophyte
Alternation of generations: life history having a multicellular haploid stage which alternates with a multicellular diploid stage
Evolutionary trends: Earlier organisms have dominant gametophyte (n) individual
• gametophyte generation is major stage in life of mosses, visible stage in fungi, and is an independent plant in ferns
However, gametophyte is only an inconspicuous structure within flower or other reproductive organs of "higher" plants

10. Visualizing Chromosomes

11. 23 pairs of chromosomes

12. Human Female Karyotype

13. Human male karyotype

14. What does a karyotype reveal?
Mutation in chromosome number or gross chromosomal abnormalities
Down’s syndrome or Trisomy 21

15. Cri du chat

16. Homologous Chromosomes
Paired chromosomes
both chromosomes of a pair carry genes controlling same inherited characters
homologous = same information
2 copies = diploid = 2n

17. Sexual reproduction: Fertilization
From Mom – female gamete
From Dad – male gamete
1 copy
Haploid 1n
What would happen if meiosis didn’t occur?
-2 copies
-Diploid
-2n
Zygote

18. Making gametes
-2 copies
-diploid
-2n
-1 copy
-haploid
-1n

19. Meiosis = reduction division
special cell division in sexually reproducing organisms
reduce 2n n
diploid haploid (half)
makes gametes (sperm, eggs)
Warning: meiosis evolved from mitosis, so stages & “machinery” are similar but the processes are radically different. Do not confuse!

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

21. 1st division of meiosis separates homologous pairs (2n 1n)
Steps of Meiosis
Meiosis 1
prophase 1
metaphase 1
anaphase 1
telophase 1
Meiosis 2
prophase 2
metaphase 2
anaphase 2
telophase 2
1st division of meiosis separates homologous pairs (2n n)
2nd division of meiosis separates sister chromatids (1n n)
*just like mitosis*

22. Preparing for Meiosis 1st step of meiosis Replication of DNA
Why bother?
meiosis evolved after mitosis
convenient to use “machinery” of mitosis
DNA replicated in S phase of meiosis interphase (just like in mitosis)
2n Single-stranded
2n Double-stranded

23. Preparing for meiosis Synapsis Tetrad
1st division of meiosis separates homologous pairs
Synapsis – pairing of homologous chromosomes – forms a tetrad (4 sister chromatids)
2n single stranded
2n double stranded
Prophse 1
Synapsis
Metaphase 1
2n double stranded
Telophase 1
Tetrad
1n double stranded

24. What does this division look like?
Meiosis 2
2nd division of meiosis separates sister chromatids
What does this division look like?
1n double stranded
Metaphase 2
1n single stranded
cytokinesis

27. Mitosis vs. Meiosis

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

29. New gametes made by offspring
The value of meiosis
Meiosis introduces genetic variation
gametes of offspring do not have same genes as gametes from parents
genetic recombination
random assortment in humans produces 223 (8,388,608) different combinations
From mom
From dad
New gametes made by offspring

30. Random Fertilization
Any 2 parents will produce a zygote with over 70 trillion (223 x 223) diploid combinations

31. And more variation During Prophase 1
homologous pairs swap pieces of chromosome
sister chromatids intertwine
crossing over

32. What are advantages of sexual reproduction? What are disadvantages?
Crossing Over
3 steps
cross over
breakage of DNA
re-fusing of DNA
New combinations of traits
What are advantages of sexual reproduction? What are disadvantages?

33. Genetic variation
Meiosis & crossing over introduce great genetic variation to population - drives evolution
Consider the greater variation with 23 pairs of chromosomes = mixing and matching

34. Sources of genetic variability
Genetic variability in sexual reproduction
independent assortment
homologous chromosomes in Meiosis 1
sister chromatids in Meiosis 2
crossing over
between homologous chromosomes in prophase1
random fertilization
random ovum by a random sperm

35. Spermatogenesis Continuous & prolific process in adult male
Each ejaculation contains 100–650 million sperm
Occurs in seminiferous tubules

36. Structure of sperm

37. What is the advantage of 1 egg released at a time?
Oogenesis
At birth an ovary contains all of the primary oocytes it will ever have
~300 eggs released
Unequal cytokinesis
1 egg
3 polar bodies
polar bodies degenerate
What is the advantage of 1 egg released at a time?