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 n=23 egg 2n=46 zygote
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 n=23 n=23 2n=46 sperm haploid (n) meiosis II human
sex cell
diploid (2n)
n=23
meiosis I

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

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. chromatin nuclear membrane
cell membrane
nucleolus

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
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
eye color
locus
hair color
Paternal
Maternal

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

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

17. 21 trisomy – Downs Syndrome
Can you see the extra 21st chromosome?
Is this person male or female?

18. In Humans the “Sex Chromosomes” are the 23rd set
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.
In Humans the “Sex Chromosomes” are the 23rd set
XX chromosome - female
XY chromosome - male

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
Tetrad
nonsister chromatids
chiasmata: site of crossing over
variation

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 assortment.

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 223 = ~8 million combinations 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

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

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

41. Answer: Remember: variation is good! 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 n=23 egg 2n=46 zygote
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/meiosis.ppt