for exams and presentations

1. MEIOSIS
Objective: To understand the process of
Meiosis and to be able to explain how it
differs from mitosis.

2. Meiosis
• cell division by which gametes, with half the number
of chromosomes, are produced.
• Diploid (2n) → haploid(n)
• – Gametes have half the # of chromosomes
• Meiosis is sexual reproduction.
• Two divisions (meiosis I and meiosis II).
• Sex cells produce gametes (sperm or egg).
• Occurs only in gonads (testes or ovaries).
• Male: spermatogenesis
• Female:oogenesis

3. Spermatogenesis
• Formation of functional sperm or spermatozoa from diploid spermatogonia under
control of FSH and Androgens.
• Steps
1.Multiplicative Phase :Diploid germ cells ------->spermatogonia A&B.
spermatogonia A---> acts as a stem cell spermatogonia B----->precursor of
sperms
2 .Growth phase:Spermatogonia increase in size but nourshment from nursing or
sertoli cells. These are called primary spermatocytes.(spermatocytogenisis)
3.maturation phase: Primary spermatocytes ---->Meiosis I--->Secondary
spermatocyes----->meiosis II ------>four haploid spermatids.
4.Differentiation :Non motile rounded spermatids into functional motile
spermatozoa.(Spermiogenesis).

4. Spermatogenesis stages

5. Oogenesis
• Formation of haploid ova/ovum from the diploid egg mother cells of the ovary.
• Occurs in follicles of ovary.
• Only one out of 500 cells in ovary are stimulated by FSH which mature to form ovum.
Steps
1.Multiplicative Phase :Primary germ cells in germinal epithelium ------->several mitosis -------
--->oogonia (2N)
Oogonia--->egg nest ---->one cell of egg nest --->prymary oocyte --->remaining forms
layer around it.
2 .Growth phase: Primary oocyte increase in size by taking food from sorrounding cells.
3.maturation phase: Primary oocyte ---->Meiosis I--->secondary oocyte and first polar body --
-->meiosis II ------>ootid & second polar body.(1st polar body also divide in to two)
Secondary oocyte does not proceed beyond metaphase until sperms enters in it .
second meiotic division is completed after fertilization.
4.Differentiation :Ootid ---->ovum
finally one ovum and 3 polar bodies formed.

7. 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

8. 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

9. 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.

10. Prophase I -Synapsis
Homologous chromosomes
sister chromatids sister chromatids
Tetrad

11. 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.

12. Homologous Chromosomes
Paternal Maternal
eye color
locus
eye color
locus
hair color
locus
hair color
locus

13. 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.

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

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

16. Prophase I
centrioles
spindle fiber
aster
fibers

18. Metaphase I
• Shortest phase
• Tetrads align on the metaphase plate.

19. Metaphase I
metaphase plate
OR
metaphase plate

20. Question:
• How is Meiosis 1 different than
Mitosis?
– You may want to use a double bubble
map to help you with this.
– Talk it out with your table partners.

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

22. Anaphase I

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

24. Telophase I

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

26. Prophase II
• same as prophase in mitosis

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

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

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

30. Telophase II

31. 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

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

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

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

35. Answer:
• 10 chromosomes (haploid)

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

37. 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

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

39. Answer:
• 10 chromosomes

40. Mitosis