Meiosis is the process of cell division that produces gametes, such as sperm and egg cells, with half the number of chromosomes as the original parent cell. It involves two cell divisions - Meiosis I and Meiosis II. In Meiosis I, homologous chromosomes pair up and separate, reducing the chromosome number by half. Meiosis II then separates the sister chromatids, resulting in four haploid daughter cells each containing a single set of chromosomes. This ensures genetic variation between gametes and allows for genetic recombination during fertilization.

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=23
egg
sperm
n=23
2n=46
zygote

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=23
n=23
meiosis I
n=23
n=23
n=23
n=23
sperm
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
nucleolus
cell membrane
chromatin

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 chromatids
Tetrad

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
Paternal Maternal
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. 21 trisomy – Downs
Syndrome
Can you see the
extra 21st
chromosome?
Is this person
male or female?

18. 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

19. Boy or Girl? The Y Chromosome “Decides”
X chromosome
Y chromosome

20. 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.

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

22. 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

23. Sex Chromosomes
XX chromosome - female XY chromosome - male

24. Prophase I
centrioles
spindle fiber
aster
fibers

25. Metaphase I
• Shortest phase
• Tetrads align on the metaphase plate.
• INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
2. Variation
3. Formula: 2n
Example: 2n = 4
then n = 2
thus 22 = 4 combinations

26. Metaphase I
metaphase plate
OR
metaphase plate

27. Question:
• In terms of Independent Assortment -
how many different combinations of
sperm could a human male produce?

28. Answer
• Formula: 2n
• Human chromosomes: 2n = 46
n = 23
• 223 = ~8 million combinations

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

30. Anaphase I

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

32. Telophase I

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

34. Prophase II
• same as prophase in mitosis

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

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

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

38. Telophase II

39. Meiosis
2n=4
sex cell
diploid (2n)
n=2
n=2
meiosis I
n=2
n=2
n=2
n=2
sperm
haploid (n)
meiosis II

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

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

42. Question:
• A cell containing 20 chromosomes (diploid)
at the beginning of meiosis would, at its
completion, produce cells containing how
many chromosomes?

43. Answer:
• 10 chromosomes (haploid)

44. Fertilization
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote

45. Question:
• A cell containing 40 chromatids at the
beginning of meiosis would, at its completion,
produce cells containing how many
chromosomes?

46. Answer:
• 10 chromosomes

47. Sources
• www.ursulinehs.org/powerpoint/meiosi
s.ppt
• www.biology4teachers.com/Cell%20Divi
sion/MEIOSIS.ppt
• http://www.iteachbio.com/Life%20Scien
ce/LifeFunctionsandTheCell/Meiosis.ppt