Meiosis 2

1. Meiosis

2. Pg.
•Title: Meiosis
•Date: 01/27/10
•Obj: Describe how spermatogenesis &
oogenesis occur through meiosis.
•Ess: Explain how sperm & egg contain only 23
chromosomes each through meiosis.

3. Pg.
•Title: Meiosis I & II
•Date: 1/27/10
•In:
•Almost all human cells contain 46
chromosomes. If a sperm has 46
chromosomes & an egg has 46 chromosomes
then how many chromosomes will the “baby”
have?
•Is that a good thing???

4. M
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5. Asexual vs. Sexual Reproduction
•Asexual Reproduction:
Binary Fission & Mitosis
–Produces cells that are
identical copies of
parent cell
•Sexual Reproduction:
Meiosis
–Combines genetic
material from 2 parents
(sperm & egg) so
offspring are genetically
different from parents

6. How many chromosomes in human
cells?
•Humans have 46 (23 pairs) chromosomes
•44 chromosomes (22pairs): autosomal
•2 chromosomes (1 pair): sex chromosomes
(XX, XY)
2 sets of
chromosomes
1 from mom
&
1 from dad

7. How many chromosomes in human
cells?
•All human cells have 46 chromosomes except
for the egg & sperm
•Egg = 23
•Sperm = 23
•All other cells = 46 (2 sets of chromosomes)
–(liver, cardiac, blood cells)
•Diploid (2N) = 2 sets of chromosomes (46)
•Haploid (1N) = 1 set of chromosomes (23)
•N = haploid #
1 set of chromosomes

8. If egg and sperm had same number of chromosomes as other
body cells . . . baby would have too many chromosomes!
No Good!
The Problem!

9. The Solution!
Much Better!
If egg and sperm had half the number of chromosomes than other
body cells . . . baby would have just the right amount!

10. Meiosis
•Makes 4 cells genetically different from parent
cell & from each other
•Production of gametes = sperm & egg
•Contain half the number of chromosomes =
haploid (1N)
•Called the “reduction” division =
–Diploid Haploid
–2N 1N
•Used in sexual reproduction
46 23

11. Meiosis allows sex cells to join to form a complete set of
chromosomes/instructions to make a new organism.
Copyright Pearson Prentice Hall
Haploid
gametes
MEIOSIS
Diploid
zygote
MITOSIS
Diploid
adult
Not in notes

12. You tell me!
•Drosophila (fruit flies)
–Diploid: 2N = 8
–Haploid: 1N = ?
•Lettuce
–Diploid: 2N = 8
–Haploid: 1N = ?
•Goldfish:
–Diploid: 2N = ?
–Haploid: 1N = 48
Not in notes

13. Meiosis is a special kind of cell division
that halves the number of chromosomes
in gametes. This insures that the correct
number of chromosomes are passed on.
Copyright Pearson Prentice Hall
Not in notes

14. The goal is to make
haploid sex cells
Copyright Pearson Prentice Hall
1
st
division
2
nd
division
Not in notes

15. Where does it happen?
•In males-
in the
testes
•In
females-
in the
ovaries

16. Homologous ChromosomesHomologous Chromosomes


Homologous chromosomes
sister chromatids
paternal
sister chromatids
maternal
Tetrad

17. Check out the tetrads!

18. Homologous ChromosomesHomologous Chromosomes
•Pair of chromosomes - maternal
& paternal - similar in shape &
size.
•Homologous pairs – tetrads -
carry genes controlling the same
inherited traits.
•Each locus - position of a gene -
is in the same position on
tetrads.

19. Homologous ChromosomesHomologous Chromosomes

Paternal Maternal
eye color
locus
eye color
locus
hair color
locus
hair color
locus

20. Gametes:
•Sperm or egg
Copyright Pearson Prentice Hall

21. Gametogenesis
•Spermatogenesis
: production of
sperm

22. Gametogenesis
•Oogenesis:
production of egg
Meiosis I Meiosis II

24. Remember, humans have 23 pairs of
chromosomes…………….
•Pairs 1-22 are said to be
Homologous chromosomes (they
go together because they have
same traits but are not identical)
Copyright Pearson Prentice Hall
gene for
eye color
gene for
hair color

25. Copyright Pearson Prentice Hall
Phases of Meiosis
Meiosis I
Prophase IMetaphase IAnaphase ITelophase I
and
Cytokinesis
Interphase I
Meiosis I

26. Copyright Pearson Prentice Hall
Phases of Meiosis
Cells undergo a round of
DNA replication, forming
duplicate chromosomes.
Interphase I

27. Copyright Pearson Prentice Hall
Phases of Meiosis
Each chromosome pairs with its
corresponding homologous
chromosome to form a tetrad.
There are 4 chromatids in a tetrad.
MEIOSIS I
Prophase II

28. Copyright Pearson Prentice Hall
Phases of Meiosis
Crossing-over: Homologous
chromosomes exchange
portions of their chromatids in a
process
Crossing-over produces new
combinations of alleles.

29. Synapsis
and
Crossing
Over in
Action
Copyright Pearson Prentice Hall

30. Copyright Pearson Prentice Hall
Phases of Meiosis
Spindle fibers attach to
the chromosomes.
Tetrads line up in the
middle: metaphase plate.
MEIOSIS I
Metaphase I

31. Copyright Pearson Prentice Hall
Phases of Meiosis
MEIOSIS I
Anaphase I
Spindle fibers pull the
homologous
chromosomes toward
opposite ends of the
cell.
Tetrads split into single
chromosomes.

32. Copyright Pearson Prentice Hall
Phases of Meiosis
MEIOSIS I
Telophase I and
Cytokinesis
Nuclear membranes form.
Cell separates into two cells.
The two cells produced have
chromosomes and alleles that are
different from each other and from
the diploid cell that entered meiosis
I.

33. Copyright Pearson Prentice Hall
Phases of Meiosis
Meiosis II
The two cells produced by meiosis I now enter a
second meiotic division.
Unlike meiosis I, neither cell goes through
chromosome replication.
Each of the cell’s chromosomes has 2 chromatids.

34. Copyright Pearson Prentice Hall
Phases of Meiosis
Meiosis II
Telophase II
and
Cytokinesis
Prophase II
Metaphase IIAnaphase II
Telophase I and
Cytokinesis I
Meiosis II

35. Copyright Pearson Prentice Hall
Phases of Meiosis
Meiosis I results in two
haploid (N) daughter
cells, each with half the
number of chromosomes
as the original cell.
MEIOSIS II
Prophase II

36. Copyright Pearson Prentice Hall
Phases of Meiosis
The chromosomes line
up in the center of cell.
MEIOSIS II
Metaphase II

37. Copyright Pearson Prentice Hall
Phases of Meiosis
The sister chromatids
separate and move
toward opposite ends of
the cell.
MEIOSIS II
Anaphase II

38. Copyright Pearson Prentice Hall
Phases of Meiosis
Meiosis II results in four
haploid (N) daughter cells.
MEIOSIS II
Telophase II and Cytokinesis

39. Gametes:
•Sperm or egg
Copyright Pearson Prentice Hall

40. Gametogenesis
•Spermatogenesis
: production of
sperm

41. Gametogenesis
•Oogenesis:
production of egg
Meiosis I Meiosis II

43. Copyright Pearson Prentice Hall
Gamete Formation
Gamete Formation
In male animals, meiosis results in four equal-sized
gametes called sperm.

44. Copyright Pearson Prentice Hall
Gamete Formation
In many female animals, only one egg results from
meiosis. The other three cells, called polar bodies,
are usually not involved in reproduction.

45. Interphase IInterphase I
•DNA replicates
forming duplicate
homologous
chromosomes.
(S phase).
•Centriole pairs also
replicate.
•Nucleus &
nucleolus visible.

46. Meiosis IMeiosis I
•Cell division that reduces the
chromosome number by one-half.
•four phases:
a.prophase I
b.metaphase I
c.anaphase I
d.telophase I & cytokinesis
Prophase I
Metaphase I
Anaphase I
Telophase I
&
Cytokinesis

47. Prophase IProphase I
•Longest and most complex
phase (90%).
•Homologous chromosomes come
together to form a tetrad = Synapsis
•Tetrad is two chromosomes or
four chromatids (sister and
nonsister chromatids).

48. Prophase IProphase I - - SynapsisSynapsis


Homologous chromosomes
sister chromatids
paternal
sister chromatids
maternal
Tetrad

49. Prophase IProphase I


centrioles
spindle fiber
spindle
fibers

50. Crossing OverCrossing Over
•Segments of non-sister chromatids break
and reattach to the other chromatid.
•May occur between non-sister
chromatids at the chiasmata.
•Chiasmata (chiasma) - sites of crossing
over.

51. Crossing Over - variation Crossing Over - variation


nonsister chromatids
chiasmata: site
of crossing over
variation
Tetrad

52. Metaphase IMetaphase I
•Shortest phase
•Tetrads align on the
metaphase plate.
•INDEPENDENT
ASSORTMENT OCCURS:
–Orientation of homologous
pair to poles is random.
–Leads to variation

53. Metaphase IMetaphase I


metaphase plate
OR
metaphase plate

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

55. Anaphase IAnaphase I

56. Telophase ITelophase I
•Each pole now has
haploid (1n) set of
chromosomes.
•Cytokinesis occurs
and two haploid
daughter cells are
formed.

57. Telophase ITelophase I

58. Meiosis IIMeiosis II
•No interphase II
•four phases:
a.prophase II
b.metaphase II
c.anaphase II
d.telophase II & cytokinesis
•Similar to mitosis

59. Prophase IIProphase II
•Centrioles separate
•Nuclear envelope
disappears

60. Prophase IIProphase II

61. Metaphase IIMetaphase II
•Sister chromatids
line up at
metaphase plate
•Spindle fibers
attach at
centromere

62. Metaphase IIMetaphase II
metaphase platemetaphase plate

63. Anaphase IIAnaphase II
•sister chromatids separate & move
toward opposite ends of the cell

64. Anaphase II

65. Telophase II
•Nuclei form.
•Cytokinesis occurs.
•Four haploid daughter
cells produced.
–gametes = sperm or
egg

66. Telophase IITelophase II

68. Mitosis vs. Meiosis
•Mitosis •Meiosis
Diploid (2n) Parent CellsDiploid (2n) Parent Cells
Replication of Chromosomes – Replication of Chromosomes –
double the #double the #
End of Mitosis & Meiosis IEnd of Mitosis & Meiosis I
Results in 2 diploid (2n) Results in 2 diploid (2n)
daughter cellsdaughter cells
End of Meiosis II End of Meiosis II
Results in 4 haploid (n) Results in 4 haploid (n)
daughter cells daughter cells

69. Pg. 143 Meiosis Vocab
Word Description Picture
Homologous
chromosomes
Tetrads
Crossing over
Haploid
Diploid
Locus
Spermatogenesis
Oogenesis