1. Meiosis & Sexual Reproduction

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

3. + 46 46 92 Okay then….? No! What if we did, then…. Doesn’t work!
Why can’t we just do mitosis to reproduce?
Why produce sperm and egg and even go through the hassle of dating?
If we are going to do it can we make egg & sperm by mitosis?
No!
What if we did, then…. 46 + 46 92
egg
sperm
zygote
Doesn’t work!

4. Types of Reproduction ASEXUAL Produces clones (genetically identical)
Single parent
Little variation in population - only through mutations
Fast and energy efficient
Eg. budding, binary fission
SEXUAL
Meiosis produces gametes (sex cells)
2 parents: male/female
Lots of variation/diversity
Slower and energy consumptive
Eg. humans, trees

5. Chromosomes Somatic (body) cell: 2n = 46 chromosomes
Each pair of homologous chromosomes includes 1 chromosome from each parent
Autosomes: 22 pairs of chromosomes that do not determine sex
Sex chromosomes: X and Y
Females: XX
Males: XY
Gametes (n=23): 22 autosomes + 1 sex chromosome
Egg: 22 + X
Sperm: 22 + X **or** 22 + Y

6. Sexual reproduction creates variability
Sexual reproduction allows us to maintain both genetic similarity & differences.
Jonas Brothers
Baldwin brothers
Martin & Charlie Sheen, Emilio Estevez
any 2 parents will produce a zygote with over 70 trillion (223 x 223) possible diploid combinations

7. The value of sexual reproduction
Sexual reproduction introduces genetic variation
genetic recombination
independent assortment of chromosomes
random alignment of homologous chromosomes in Metaphase 1
crossing over
mixing of alleles across homologous chromosomes
random fertilization
which sperm fertilizes which egg?
Driving evolution
providing variation for natural selection
gametes of offspring do not have same combination of genes as gametes from parents
random assortment in humans produces 223 (8,388,608) different combinations in gametes
metaphase1

8. Human female karyotype
46 chromosomes
23 pairs

9. Human male karyotype
46 chromosomes
23 pairs

10. Homologous chromosomes
Paired chromosomes
both chromosomes of a pair carry “matching” genes
control same inherited characters
homologous = same information
single stranded homologous chromosomes
diploid 2n
2n = 4
double stranded homologous chromosomes

11. Meiosis: production of gametes
chromosome number must be reduced (reduction division)
diploid  haploid
2n  n
humans: 46  23
meiosis reduces chromosome number
makes gametes
fertilization restores chromosome number
haploid  diploid
n  2n
haploid
Warning: meiosis evolved from mitosis, so stages & “machinery” are similar but the processes are radically different. Do not confuse the two!
diploid

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

13. Meiosis 1 1st division of meiosis separates homologous pairs synapsis
single
stranded
Meiosis 1
1st division of meiosis separates homologous pairs
2n = 4
double
stranded
prophase 1
synapsis
2n = 4
double
stranded
metaphase 1
tetrad
reduction
1n = 2
double
stranded
telophase 1

14. Meiosis I (1st division)
Interphase: chromosomes replicated
Prophase I:
Synapsis: homologous chromosomes pair up
Tetrad = 4 sister chromatids
Crossing over at the chiasmata
Metaphase I: Tetrads line up
Anaphase I:
Pairs of homologous chromosomes separate
(Sister chromatids still attached by centromere)
Telophase I & Cytokinesis:
Haploid set of chromosomes in each cell
Each chromosome = 2 sister chromatids
Some species: chromatin & nucleus reforms

15. Trading pieces of DNA Crossing over
during Prophase 1, sister chromatids intertwine
homologous pairs swap pieces of chromosome
DNA breaks & re-attaches
prophase 1
synapsis
tetrad

16. What does this division look like?
Meiosis 2
2nd division of meiosis separates sister chromatids
1n = 2
double
stranded
prophase 2
What does this division look like?
1n = 2
double
stranded
metaphase 2
1n = 2
single
stranded
telophase 2 4

17. Meiosis II (2nd division) = create gametes
Prophase II:
No interphase
No crossing over
Spindle forms
Metaphase II:
Chromosomes line up
Anaphase II:
Sister chromatids separate
Telophase II:
4 haploid cells
Nuclei reappear
Each daughter cell genetically unique

18. Steps of meiosis Meiosis 1 Meiosis 2 interphase 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  1n)
“reduction division”
2nd division of meiosis separates sister chromatids
(1n  1n)
* just like mitosis *

19. 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

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

21. Sperm production Spermatogenesis continuous & prolific process
Epididymis
Testis
germ cell
(diploid)
Coiled
seminiferous
tubules
primary
spermatocyte
(diploid)
MEIOSIS I
secondary
spermatocytes
(haploid)
MEIOSIS II
Vas deferens
spermatids
(haploid)
spermatozoa
Spermatogenesis
continuous & prolific process
each ejaculation = million sperm
Cross-section of
seminiferous tubule

22. Egg production: Oogenesis
Begins in the ovaries of the female fetus before birth
Pauses during first meiotic division
Final development occurs in the ovaries of the adult female
Each month one egg matures as cued by hormones
Completes the first meiotic division and starts the second meiotic division
Last bit of meiosis is finalized at time of fertilization

23. Putting all your egg in one basket!
Oogenesis
MEIOSIS I
MEIOSIS II
first polar body
second
polar body
ovum
(haploid)
secondary
oocyte
primary
(diploid)
germinal cell
primary follicles
mature follicle with
secondary oocyte
ruptured follicle (ovulation)
corpus luteum
developing
follicle
fertilization
fallopian tube
after fertilization

24. Sources of Genetic Variation:
Crossing Over
Exchange genetic material
Recombinant chromosomes

25. Sources of Genetic Variation:
Independent Assortment of Chromosomes
Random orientation of homologous pairs in Metaphase I

26. Sources of Genetic Variation:
Random Fertilization
Any sperm + Any egg
8 million X 8 million = 64 trillion combinations!

27. sperm vs. egg production
Similarities
Both produce haploid cells by meiosis
Both take place in the gonads
both are controlled by hormones
Differences
Spermatogenesis produces 4 sperm each time while oogenesis produces only 1 egg
Formation of mature sperm continually occurs while eggs only mature once a month (on average)
Sperm formation never stops, egg formation ends at menopause
Sperm can be released at anytime while eggs are released only once a month

28. Life cycle: reproductive history of organism, from conception  production of own offspring
Fertilization and meiosis alternate in sexual life cycles
Meiosis: cell division that reduces # of chromosomes (2n  n), creates gametes
Fertilization: combine gametes (sperm + egg)
Fertilized egg = zygote (2n)
Zygote divides by mitosis to make multicellular diploid organism

29. Differences across kingdoms
Not all organisms use haploid & diploid stages in same way
which one is dominant (2n or n) differs
but still alternate between haploid & diploid
must for sexual reproduction

30. Human Life Cycle