1. Meiosis
Is a special type of cell division in which certain somatic cells (containing the diploid number of chromosomes) divide into sex cells (gametes, containing the haploid number of chromosomes).
It occurs only in the reproductive organs (the testes of the male and the ovaries of the female) in organisms that reproduce sexually.
Each sex cell (sperm and ova) found is genetically unique.

2. Somatic cells : genetically identical
Sex cells: not genetically identical

3. Meiosis – a somatic (body) cell in the reproductive organs divides into haploid sex cells
Somatic Cell in testes/ovaries
= oogenesis
= spermatogenesis

4. 22 autosomes in an egg plus a sex chromosome which is the x sex chromosome
Autosomes – determines eye color, blood type, height, etc.
Sex chromosome – determines the gender

5. 22 autosomes in a sperm plus a x or y chromosome

6. xx in a zygote = female
xy in a zygote = male

7. How can eggs only have the x sex chromosome?
Because all female cells have 2 xx’s in all their nucleus

10. Sperm – smallest of all human cells
Ovum – largest of all human cells
Egg contributes more to the baby than the sperm does
Egg – most of the nourishment
- sugars, nutrients that is why it needs to be large

11. 22 homologous (matching) pairs of autosomes

12. Chromosome Anomaly
- Down Syndrome = 45 autosomes
- extra chromosome in the egg
- ex. 23 autosomes in an egg
- a small percentage of eggs have an extra chromosome just as a small percentage of sperm have an extra chromosome

13. Per ejaculation – 360 million sperm
Woman releases only 1 egg each month
Woman – about 400 eggs in her lifetime

14. Karyotype – chromosome analysis

15. What about wrong number of sex chromosomes?

16. 22 autosomes plus 2 xx in an egg plus sperm with 22 autosomes and an x sex chromosome – xxx female = mentally retarded

17. 22 autosomes and xx chromosome and 22 autosomes from a sperm with an y chromosome = xxy = person looks male, mental retardation, sterility = pseudohermaphrodites
xxy – klinefelter syndrome
There are no hermaphrodites but there are pseudohermaphrodites = hormonal imbalance

18. 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!

19. Okay then….? + 46 46 92 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!

20. 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

21. Human female karyotype
46 chromosomes
23 pairs

22. Human male karyotype
46 chromosomes
23 pairs

23. 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

24. 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

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

26. Meiosis 1 1st division of meiosis separates homologous pairs synapsis
single
stranded
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

27. 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

28. 4 Meiosis 2 2nd division of meiosis separates sister chromatids
double
stranded
prophase 2
What does this division look like?
1n = 2
double
stranded
metaphase 2
1n = 2
single
stranded
telophase 2 4

29. 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 *

30. 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

31. 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

32. 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

33. Oogenesis MEIOSIS I MEIOSIS II primary follicles germinal cell
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

34. 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