Meiosis is a specialized type of cell division that produces gametes, or sex cells, with half the normal number of chromosomes. It involves two rounds of division to reduce the chromosome number from diploid to haploid. This ensures that fertilization restores the normal chromosome number in offspring. Key aspects of meiosis include homologous chromosome pairing, independent assortment of chromosomes, and crossing over. Errors in meiosis can result in gametes with an abnormal chromosome number and cause conditions like Down syndrome.

1. Meiosis & Sexual
Reproduction

2. Sexual Reproduction
►2 parents
► Exchange of DNA
► Offspring genetically different from parents
► Involves specialized sex cells (gametes)
► Creates diversity

3. Sexual Reproduction in Simple
Organisms
► Conjugation –
 Cytoplasm bridge forms
between 2 cells and an
exchange of DNA takes place
 Occurs between two cells
with different mating types
 Mating types are usually
called plus (+) and minus (-)

4. Sexual Reproduction in Animals
► Each parent produces a GAMETE
 Female gamete – egg
 Male gamete - sperm
► The two gametes combine and produce a zygote
 Called fertilization
► Thezygote then divides by mitosis, grows and
process begins again
 Zygote has the same number of chromosomes as the
parents

5. Somatic cells vs. Gametes
► Somatic cells are body cells
 2 set of chromosomes
 Humans have 46 chromosomes or 23 pairs
► Gametes are sex cells
 1 set of chromosomes
 Humans have 23
chromosomes

6. How are gametes produced?
► Eggs & sperm need to be made and
joined…
► Do we make egg & sperm by mitosis? No!
What if we did, then…
46 + 46 92
egg sperm zygote
Doesn’t work!

7. diploid = 2 copies 2n Human female karyotype
46 chromosomes
23 pairs
XX

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

9. Paired chromosomes
► Homologous chromosomes
 both chromosomes of a pair carry “matching”
genes
►control
same inherited characters
►homologous = same information
diploid
2n eye color eye color
2n = 4 (brown?) (blue?)
homologous double stranded
chromosomes homologous chromosomes

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

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

12. Meiosis
► Specialcell division in sexually reproducing
organisms
► Makes gametes
 Sperm & egg
► Reduction division
 Reduce number of chromosomes
 2n  1n
 diploid  haploid

13. Interphase
► DNA replicates
► Cell prepares for
division

14. Prophase I
► Homologous chromosomes
pair up and form a tetrad
 This is called synapsis
► Crossing over occurs
during synapsis
 Homologous chromosomes
exchange segments of DNA

15. Metaphase I
► Tetrads line up at the
equator.
► Centromeres attach
chromosomes to the
spindle fibers

16. Anaphase I
► Homologous
chromosomes separate
and move to the poles
 Called disjunctions
 Sister chromatids
remain attached

17. Telophase I
► Cytoplasm divides, forming
two new daughter cells
 Cells are haploid

18. Cytokinesis I
► Occurs simultaneously with telophase I
 Forms 2 daughter cells
► Plantcells – cell plate
► Animal cells – cleavage furrow
► There is no further replication of
genetic material prior to meiosis II

19. Prophase II
► Spindle forms
► Sister chromatids
move toward the
equator

20. Metaphase II
► Sister chromatids line
up at the equator
 Similar to mitosis
► Spindle fibers attach at
the centromere

21. Anaphase II
► Sisterchromatids
separate and move to
opposite poles

22. Telophase II
► Nuclei form at poles
and cytokinesis occurs
► After completion of
cytokinesis there are 4
daughter cells
 Each cell is haploid (n)

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

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

25. ► Meiosis Square Dance
Let’s Dance!!

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

27. Putting it all together…
meiosis → fertilization → mitosis + development
gametes
46 23 46
23 46 46
meiosis egg 46 4646
23 46 46
46
46 23 zygote
fertilization mitosis &
mitosis
sperm
development

28. The value of meiosis 1
► Consistency over time
 meiosis keeps chromosome number same
from generation to generation
from Mom
Mom
Dad offspring
from Dad

29. The value of meiosis 2 We’re
mixing things
up here!
► Change over time
 meiosis introduces genetic variation
►gametes of offspring do not have
same genes as gametes from parents
►new combinations of traits
from Dad variation
from Mom
offspring
new gametes made by offspring

30. Fertilization
► Joining of the sperm & egg
► Internal Fertilization
 Gametes fuse inside the body of a female
 Occurs in many land animals
 Offspring have a better chance of survival
 Fewer offspring are produced
► External Fertilization
 Gametes fuse outside the body of a female
 Occurs in many water animals
 Many offspring produced
 Fewer offspring survive

31. Fertilization
► Parthenogenesis
 Development of an unfertilized egg into an adult
animal
 E.g. rotifers, some insects such as bees, wasps,
 E.g. Queen bee can produce fertilized or
unfertilized eggs.
►Fertilized eggs become female workers
►Unfertilized eggs become male drones

32. Independent Assortment
► Chromosomes are separated
and combined independently
of each other during meiosis
 E.g. all your mom’s
chromosomes don’t end up
together in your gametes.
They are randomly mixed.

33. Independent Assortment
► Independent
assortment
produces many different
gametes
 In humans, n=23 so over 8
million different gametes can
be produced…

35. Crossing Over
► Occurs in prophase I, during synapsis
► Homologous chromosomes twist and exchange
pieces of genetic material
► This results in even more different gametes

37. The Key Difference Between Mitosis and Meiosis is
the Way Chromosomes Uniquely Pair and Align in
Meiosis
Mitosis The first (and
distinguishing)
division of meiosis

39. Problems in Meiosis
► Sometimes chromosomes don’t separate
properly
 This is called nondisjunction and result in a cell
with too many or too few chromosomes

40. Problems in Meiosis

41. Problems in Meiosis
Should the gamete with the
chromosome pair be fertilized
then the offspring will not be
‘normal’.
In humans this often occurs
with the 21st pair – producing
a child with Downs Syndrome

42. Trisomy 21– Downs Syndrome
Can you see the extra
21st chromosome?
Is this person male or
female?
•This picture of a person’s
chromosomes is called a
karyotype. The
chromosomes are
organized from largest to
smallest.
•How can karyotypes be
helpful in identifying
diseases?

43. Gametogenesis
► Gametes develop within specialized organs
 Female gametes – egg cells
 Male gametes – sperm cells
 Female gonads – ovaries
 Male gonads - testes

44. Gametogenesis
►Gametogenesis – the formation of
gametes in the gonads
 Two types:
►Oogenesis – formation of eggs by
meiosis
►Spermatogenesis – formation of sperm
by meiosis

45. Spermatogenesis
► Produces 4 haploid sperm

46. Oogenesis
► Produces 1 haploid egg and 3 haploid polar
bodies

47. Hermaphrodites
► Hermaphrodites have both testes and
ovaries
 E.g. earthworm, snails, and hydra