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. Meiosis
Sex cells divide to produce gametes (sperm or egg).
Gametes have half the # of chromosomes.
Occurs only in gonads (testes or ovaries).

4. Three sexual life cycles differing in the timing of meiosis and fertilization

5. Moss with Sporophytes
Gametophytes

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

7. Spermatogenesis diploid (2n) Spermato- cyte meiosis II meiosis I sperm
haploid(n)
meiosis II
2n=46
Spermato-
cyte
diploid (2n)
n=23
meiosis I

8. Oogenesis diploid (2n) polar bodies Oocyte meiosis II meiosis I ovum
haploid(n)
meiosis II
2n=46
Oocyte
diploid (2n)
n=23
meiosis I
polar
bodies

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

10. Interphase Nucleus and nucleolus visible. chromatin cell membrane
nuclear membrane
cell membrane
nucleolus

11. 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

12. Prophase I Longest and most complex phase (90%). Chromosomes condense.
Synapsis occurs: homologous chromosomes come together to form a tetrad.

13. 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.
a. 22 pairs of autosomes
b. 01 pair of sex chromosomes

14. Homologous Chromosomes
eye color
locus
hair color
Paternal
Maternal

15. Prophase I
centrioles
spindle fiber
aster
fibers

16. Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad

17. Crossing Over
Crossing over (variation) may occur between nonsister chromatids.
Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.

18. Crossing Over - variation
nonsister chromatids
site of crossing over
Tetrad
variation

20. Metaphase I
equator OR
equator

21. Metaphase I Shortest phase Tetrads align on the equator
INDEPENDENT ASSORTMENT OCCURS: genes for different traits
sort independently of one another
into gametes

22. Anaphase I

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

24. Telophase I

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

27. Meiosis II No interphase (or very short - no more DNA replication)
Remember: Meiosis II is similar to mitosis but the cells are different from each other

28. Prophase II
same as prophase in mitosis

29. Metaphase II
same as metaphase in mitosis
equator

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

31. Telophase II

32. Telophase II Same as telophase in mitosis.
Nuclei form. Cytokinesis occurs.
Remember: four haploid daughter cells produced; 3 polar bodies and an
egg in females, 4 sperm in males
gametes = sperm or egg

35. Fertilization
The fusion of a sperm and egg to form a zygote - a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote

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

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

38. The Results of Alternative Arrangements of Two Homologous Chromosome Pairs on the Metaphase Plate in Meiosis I

39. A Comparison of Mitosis and Meiosis

40. Meiosis Animations:
Comparison of Mitosis and Meiosis:
Independent Assortment:

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

42. Answer:
10 chromosomes (haploid)

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

44. Answer:
10 chromosomes

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

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

47. 1) Three major differences between meiosis and the mitosis are:

48. 2) Compare the number of
chromatids present in human cells
before and after DNA replication
occurs in interphase.
3) Compare and contrast oogenesis
with spermatogenesis.

49. Determine the number of chromo-
somes in a cell whose diploid (2n)
number is 10 after:
a. meiosis I
b. meiosis II
c. mitosis

50. What is the primary difference
between sexual life cycles
in animals when compared to fungi
and plants?

51. Failure of chromosomes to
separate properly during meiosis
may result in a/an ____________
Disorder, such as Down Syndrome.
Explain the evolutionary reason that
Sickle Cell Anemia is more common
among African Americans.

52. If you understand the process of
meiosis then you know that:
It occurs only to make sex cells/gametes.
2) Four different cells are produced.
3) Crossing over (exchange among DNA of
tetrads) occurs in Prophase I to make the
cells genetically different.
4) The amount of DNA is reduced by
half in Meiosis I.