whole process of meiosis

1. MEIOSIS

2. Meiosis
• The form of cell division by which gametes,
with half the number of chromosomes, are
produced.
• Diploid (2n) → haploid (n)
• Meiosis is sexual reproduction.
• Two divisions (meiosis I and meiosis II).

3. Fertilization
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
n=2
3eg
g
sper
m
n=23
2n=46
zygot
e

4. Meiosis
• Sex cells divide to produce gametes
(sperm or egg).
• Gametes have half the # of chromosomes.
• Occurs only in gonads (testes or ovaries).
Male: spermatogenesis
Female: oogenesis
• Meiosis is similar to mitosis with some
chromosomal differences.

5. Spermatogenesis
2n=46
human
sex
cell
diploid
(2n)
n=2
3
n=2
3
meiosis
I
n=2
3
n=2
3
n=2
3
n=2
3
sper
m
haploid
(n)
meiosis
II

6. Meiosis – mouse testes
Parent cell
4 gametes
1st
division
2nd
division

7. Interphase I
• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consist of two
identical sister chromatids attached at their
centromeres.
• Centriole pairs also replicate.

8. Interphase I
• Nucleus and nucleolus visible.
nuclear
membrane
nucleolu
s
cell
membrane
chromati
n

9. Meiosis I (four phases)
• Cell division that reduces the chromosome
number by one-half.
• four phases:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I

10. Prophase I
• Longest and most complex phase (90%).
• Chromosomes condense.
• Synapsis occurs: homologous chromosomes
come together to form a tetrad.
• Tetrad is two chromosomes or four
chromatids (sister and nonsister chromatids).

11. Prophase I - Synapsis
Homologous
chromosomes
sister
chromatids
sister
chromatidsTetrad

12. Homologous Chromosomes
• Pair of chromosomes (maternal and paternal) that are
similar in shape and size.
• Homologous pairs (tetrads) carry genes controlling
the same inherited traits.
• Each locus (position of a gene) is in the same
position on homologues.
• Humans have 23 pairs of homologous chromosomes.
a. 22 pairs of autosomes
b. 01 pair of sex chromosomes

13. Karyotype
• A method of organizing the chromosomes of a
cell in relation to number, size, and type.

14. Homologous Chromosomes
Paterna
l
Materna
l
eye
color
locus
eye
color
locus
hair
color
locus
hair
color
locus

15. Humans have 23 Sets of Homologous Chromosomes
Each Homologous set is made up of 2 Homologues.
Homologue
Homologue

16. Autosomes
(The Autosomes code for most of the offspring’s traits)
In Humans
the
“Autosomes”
are sets 1 - 22

17. Sex Chromosomes
The Sex Chromosomes code for the sex of the offspring.
** If the offspring has two “X” chromosomes it will be a female.
** If the offspring has one “X” chromosome and one “Y”
chromosome it will be a male.
XX chromosome -
female
XY chromosome -
male
In Humans the
“Sex
Chromosomes”
are the 23rd
set

18. Crossing Over
• Crossing over (variation) may occur between
nonsister chromatids at the chiasmata.
• Crossing over: segments of nonsister
chromatids break and reattach to the other
chromatid.
• Chiasmata (chiasma) are the sites of crossing
over.

19. Crossing Over - variation
nonsister
chromatids
chiasmata: site
of crossing over
variatio
Tetrad

20. Another Way Meiosis Makes Lots of
Different Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of
different gamete types produced by independent

21. Prophase I
centriole
s
spindle
fiber
aster
fiber
s

22. Metaphase I
metaphase
plate
OR
metaphase
plate

23. Anaphase I
• Homologous chromosomes separate and
move towards the poles.
• Sister chromatids remain attached at their
centromeres.

24. Anaphase I

25. Telophase I
• Each pole now has haploid set of
chromosomes.
• Cytokinesis occurs and two haploid
daughter cells are formed.

26. Telophase I

27. Meiosis II
• No interphase II
(or very short - no more DNA replication)
• Remember: Meiosis II is similar to mitosis

28. Prophase II
• same as prophase in mitosis

29. Metaphase II
• same as metaphase in mitosis
metaphase
plate
metaphase
plate

30. Anaphase II
• same as anaphase in mitosis
• sister chromatids separate

31. Telophase II
• Same as telophase in mitosis.
• Nuclei form.
• Cytokinesis occurs.
• Remember: four haploid daughter cells
produced.
gametes = sperm or egg

32. Telophase II

33. Meiosis
2n=
4
sex
cell
diploid
(2n)
n=
2
n=
2
meiosis
I
n=
2
n=
2
n=
2
n=
2
sper
m
haploid
(n)
meiosis
II

34. Variation
• Important to population as the raw
material for natural selection.
• Question:
What are the three sexual sources of
genetic variation?

35. Answer:
1. crossing over (prophase I)
2. independent assortment (metaphase I)
3. random fertilization
Remember: variation is good!

36. Fertilization
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
n=2
3eg
g
sper
m
n=23
2n=46
zygot
e