1. Chapter 13: 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. 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

5. 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

6. Human female karyotype
46 chromosomes
23 pairs

7. Human male karyotype
46 chromosomes
23 pairs

8. 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

9. Meiosis: production of gametes
chromosome number must be reduced
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

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

11. 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

12. 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

13. 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

14. 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 *

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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