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).
II

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

4. Spermatogenesis
n=23
human
sex cell
sperm
n=23
n=23
2n=46
haploid (n)
n=23
diploid (2n) n=23
n=23
meiosis I meiosis II

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

6. Interphase I
• Nucleus and nucleolus visible.
chromatin nuclear
membrane
cell membrane
nucleolus

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

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

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

10. Homologous Chromosomes
• Pair of chromosomes (maternal and paternal) that are
paternal
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

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

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

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

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

15. Prophase I
spindle fiber centrioles
aster
fibers

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

17. Metaphase I
OR
metaphase plate metaphase plate

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

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

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

21. Anaphase I

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

23. Telophase I

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

25. Prophase II
• same as prophase in mitosis

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

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

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

29. Telophase II

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

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

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

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

34. Answer:
• 10 chromosomes (haploid)

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

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

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

38. Answer:
• 10 chromosomes