1. Meiosis Reduction Division
Mike Clark, M.D.

2. Meiosis Meiosis is nicknamed reduction division
It is a process where a cell divides (division) but reduces the genetic material to ½ (reduction)
This type of cell division occurs in the gametes (sex cells)
The original parent gamete cells (spermatogonium and oogonium) are diploid (2n) like a somatic cell but the final daughter gamete cells (sperm and egg term ovum) are haploid (1n)

3. Differences between Mitosis and Meiosis
Mitosis occurs in somatic cells – meiosis occurs in gametes
Mitosis has one nuclear division – meiosis has two nuclear divisions
Mitosis produces two new daughter cells – meiosis produces four new daughter cells
The resultant daughter cells in mitosis have 46 pieces of genetic material – the resultant daughter cells in meiosis has 23 pieces of genetic material

4. Figure 27.5 (1 of 2) Mother cell (before chromosome replication)
MITOSIS
MEIOSIS
Replicated
chromosome
Tetrad formed by
synapsis of replicated
homologous
chromosomes
Prophase
Prophase I
Chromosomes
align at the
metaphase plate
Tetrads align at the
metaphase plate
Metaphase
Metaphase I
Sister chromatids
separate during
anaphase
Homologous chromosomes
separate but sister
chromatids remain together
during anaphase I
Daughter
cells of
mitosis
Daughter cells
of meiosis I 2n 2n
No further chromosomal
replication; sister chromatids
separate during
anaphase II
Meiosis II n n n n
Daughter cells of meiosis II
(usually gametes)
Figure 27.5 (1 of 2)

5. Interphase in meiosis occurs prior to the start of meiosis I.
Fig
Interphase
Homologous pair of chromosomes
in diploid parent cell
Interphase in meiosis occurs
prior to the start of meiosis I.
It consists of the same three
Phases as in mitosis – G1,S and
G2.
46 pieces of genetic material
in parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Diploid cell with
replicated
chromosomes
Sister
chromatids
In the S- phase of interphase DNA is duplicated. As noted before the new DNA stays attached to the old (chromatid/chromosome) – thus though we say there are 46 chromosomes – there is actually enough genetic material for 92 chromosomes since one chromosome contains two chromatids. When the chromatids separate they are considered full chromosomes thus there is enough genetic material for 4 haploid (gamete) cells. 92 divided by 4 equals 23 – thus 23 chromosomes in a cell is termed haploid (1n). This is the amount of genetic material that the sperm and egg contain.
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number

6. Fig
Interphase
Homologous pair of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Diploid cell with
replicated
chromosomes
Sister
chromatids
At the end of meiosis I have two daughter cells with 23 doublets of genetic material
(23 chromosomes) but each chromosome has two chromatids – thus enough for 46 singlet chromosomes
Meiosis I
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number 1
Haploid cells with
replicated chromosomes
Homologous
chromosomes
separate

7. Fig
Interphase
Homologous pair of chromosomes
in diploid parent cell
At the completion of meiosis I (after cytokinesis I) - the two cells enter into a phase termed Interkinesis. Interkinesis is similar to Interphase – but it lacks the S-phase – thus DNA is not replicated – it is already enough DNA for 4 haploid cells.
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Meiosis I
At the end of meiosis II – have 4 daughter cells each with ½ the amount of genetic material (haploid).
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number 1
Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Meiosis II 2
Sister chromatids
separate
Haploid cells with unreplicated chromosomes

8. 46 pieces of genetic material in parent cell
Fig
Interphase
Homologous pair of chromosomes
in diploid parent cell
46 pieces of genetic material in parent cell
At the end of meiosis I have two daughter cells with 23 doublets of genetic material
(23 chromosomes) but each chromosome has two chromatids – thus enough for 46 singlet chromosomes
Chromosomes
replicate
Homologous pair of replicated chromosomes
S- phase in interphase
duplicates DNA (but
stays attached chromatid/
chromosome– thus
enough genetic material
for 4 haploid (gamete)
cells
Sister
chromatids
Diploid cell with
replicated
chromosomes
Meiosis I
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number 1
Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Meiosis II
At the end of meiosis II – have 4 daughter cells each with ½ the amount of genetic material (haploid). 2
Sister chromatids
separate
Haploid cells with unreplicated chromosomes

9. Division in meiosis I occurs in four phases:
– Prophase I
– Metaphase I
– Anaphase I
– Telophase I and cytokinesis
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

10. Three events are unique to meiosis, and all three occur in meiosis l:–
Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information
At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes
At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

11. Prophase I Metaphase I Anaphase I Centrosome (with centriole pair)
Fig. 13-8a
Telophase I and
Cytokinesis
Prophase I
Metaphase I
Anaphase I
Centrosome
(with centriole pair)
Sister chromatids
remain attached
Centromere
(with kinetochore)
Sister
chromatids
Chiasmata
Spindle
Metaphase
plate
Figure 13.8 The meiotic division of an animal cell
Cleavage
furrow
Homologous
chromosomes
Homologous
chromosomes
separate
Fragments
of nuclear
envelope
Microtubule
attached to
kinetochore

12. Chromosomes begin to condense
Prophase I
Prophase I typically occupies more than 90% of the time required for meiosis
Chromosomes begin to condense
In synapsis, homologous chromosomes loosely pair up, aligned gene by gene
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

13. In crossing over, nonsister chromatids exchange DNA segments
1. Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information
In crossing over, nonsister chromatids exchange DNA segments
Each pair of chromosomes forms a tetrad, a group of four chromatids
Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

14. Crossing Over
Crossing over produces recombinant chromosomes, which combine genes inherited from each parent
Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

15. In crossing over, homologous portions of two nonsister chromatids trade places
Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

16. Prophase I Nonsister of meiosis chromatids held together
Fig
Prophase I
of meiosis
Nonsister
chromatids
held together
during synapsis
Pair of
homologs
Figure The results of crossing over during meiosis

17. Prophase I Nonsister of meiosis chromatids held together
Fig
Prophase I
of meiosis
Nonsister
chromatids
held together
during synapsis
Pair of
homologs
Chiasma
Centromere
TEM
Figure The results of crossing over during meiosis

18. Prophase I Nonsister of meiosis chromatids held together
Fig
Prophase I
of meiosis
Nonsister
chromatids
held together
during synapsis
Pair of
homologs
Chiasma
Centromere
TEM
Anaphase I
Figure The results of crossing over during meiosis

19. Prophase I Nonsister of meiosis chromatids held together
Fig
Prophase I
of meiosis
Nonsister
chromatids
held together
during synapsis
Pair of
homologs
Chiasma
Centromere
TEM
Anaphase I
Figure The results of crossing over during meiosis
Anaphase II

20. Recombinant chromosomes
Fig
Prophase I
of meiosis
Nonsister
chromatids
held together
during synapsis
Pair of
homologs
Chiasma
Centromere
TEM
Anaphase I
Figure The results of crossing over during meiosis
Anaphase II
Daughter
cells
Recombinant chromosomes

21. Without crossing over the newly formed cells would inherit either a full chromosome containing only mom’s or dad’s genes on that chromosome.
Possibility 1
Possibility 2
Metaphase II
Figure The independent assortment of homologous chromosomes in meiosis
By crossing over the situation above would not happen in that each chromosome would have a piece of dad’s genetic material and a piece of mom’s genetic material.

22. Without crossing over the 4 daughter cells below would have no genetic recombination.
Metaphase II
Figure The independent assortment of homologous chromosomes in meiosis
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4

23. 2. At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes Metaphase I
In metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole
Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad
Microtubules from the other pole are attached to the kinetochore of the other chromosome
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

24. Prophase I Metaphase I Centrosome (with centriole pair) Centromere
Fig. 13-8b
Prophase I
Metaphase I
Centrosome
(with centriole pair)
Centromere
(with kinetochore)
Sister
chromatids
Chiasmata
Spindle
Metaphase
plate
Figure 13.8 The meiotic division of an animal cell
Homologous
chromosomes
Fragments
of nuclear
envelope
Microtubule
attached to
kinetochore

25. In anaphase I, pairs of homologous chromosomes separate
3. At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate
Anaphase I
In anaphase I, pairs of homologous chromosomes separate
One chromosome moves toward each pole, guided by the spindle apparatus
Sister chromatids remain attached at the centromere and move as one unit toward the pole
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

26. Telophase I and Cytokinesis
In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids
Cytokinesis usually occurs simultaneously, forming two haploid daughter cells
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

27. In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms
No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

28. Telophase I and Cytokinesis
Fig. 13-8c
Telophase I and
Cytokinesis
Anaphase I
Sister chromatids
remain attached
Figure 13.8 The meiotic division of an animal cell
Homologous
chromosomes
separate
Cleavage
furrow

29. Division in meiosis II also occurs in four phases:
– Prophase II
– Metaphase II
– Anaphase II
– Telophase II and cytokinesis
Meiosis II is very similar to mitosis
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

30. Telophase II and Cytokinesis
Fig. 13-8d
Telophase II and
Cytokinesis
Prophase II
Metaphase II
Anaphase II
Sister chromatids
separate
Haploid daughter cells
forming
Figure 13.8 The meiotic division of an animal cell

31. In prophase II, a spindle apparatus forms
In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

32. Metaphase II
In metaphase II, the sister chromatids are arranged at the metaphase plate
Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical
The kinetochores of sister chromatids attach to microtubules extending from opposite poles
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

33. Prophase II Metaphase II Fig. 13-8e
Figure 13.8 The meiotic division of an animal cell

34. In anaphase II, the sister chromatids separate
The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

35. Telophase II and Cytokinesis
In telophase II, the chromosomes arrive at opposite poles
Nuclei form, and the chromosomes begin decondensing
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

36. Cytokinesis separates the cytoplasm
At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes
Each daughter cell is genetically distinct from the others and from the parent cell
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

37. Telephase II and Cytokinesis
Fig. 13-8f
Telephase II and
Cytokinesis
Anaphase II
Sister chromatids
separate
Haploid daughter cells
forming
Figure 13.8 The meiotic division of an animal cell

38. Oogenesis Production of female gametes Begins in the fetal period
Oogonia (2n ovarian stem cells) multiply by mitosis and store nutrients
Primary oocytes develop in primordial follicles
Primary oocytes begin meiosis but stall in prophase I and stay there for years – until the woman ovulates
This suspended prophase 1 can late in life lead to Down’s Syndrome in the woman’s offspring

39. Fig
Figure Down syndrome

40. Fig b
Figure Down syndrome

41. Translocated chromosome 22 (Philadelphia chromosome)
Fig
Error – crossing over occurred improperly – the exchange was with non-homologous chromosomes.
Reciprocal
translocation
Normal chromosome 9
Translocated chromosome 9
Translocated chromosome 22
(Philadelphia chromosome)
Normal chromosome 22
Figure Translocation associated with chronic myelogenous leukemia (CML)

42. Oogenesis
Each month after puberty, a few primary oocytes are activated
One is selected each month to resume meiosis I (the one to be ovulated)
Result is two haploid cells
Secondary oocyte
First polar body

43. Oogenesis The secondary oocyte arrests in metaphase II and is ovulated
If penetrated by sperm the second oocyte completes meiosis II, yielding
Ovum (the functional gamete)
Second polar body

44. Figure 27.17 Meiotic events Follicle development in ovary Before birth
Oogonium (stem cell)
Follicle cells
Mitosis
Oocyte
Primary oocyte
Primordial follicle
Growth
Infancy and
childhood
(ovary inactive)
Primary oocyte
(arrested in prophase I;
present at birth)
Primordial follicle
Each month from
puberty to
menopause
Primary follicle
Primary oocyte (still
arrested in prophase I)
Secondary follicle
Spindle
Vesicular (Graafian)
follicle
Meiosis I (completed
by one primary oocyte
each month in response
to LH surge)
Secondary oocyte
(arrested in
metaphase II)
First polar body
Ovulation
Meiosis II of polar
body (may or may
not occur)
Sperm
Ovulated secondary
oocyte
Meiosis II
completed
(only if
sperm
penetration
occurs)
In absence of
fertilization, ruptured
follicle becomes a
corpus luteum and
ultimately degenerates.
Polar bodies
(all polar bodies
degenerate)
Second
polar body
Ovum
Degenating
corpus luteum
Figure 27.17

45. Final Result of Oogenesis (formation of the egg)
Four cells are produced – all 4 with a haploid set of genetic material - but three of the cells are non-functional – termed polar bodies
Only one viable cell is produced - the egg cell (termed the ovum) – this is the cell to be ovulated for the month
The one viable cell (ovum) receives most of the cell cytoplasm
Inasmuch as the placenta will not develop till much later if the egg is fertilized – the developing embryo must live off the food in the ovum’s cytoplasm till the after birth (placenta) is formed

46. Mitosis of Spermatogonia
Begins at puberty
Spermatogonia
Stem cells in contact with the epithelial basal lamina
Each mitotic division  a type A daughter cell and a type B daughter cell

47. Basal lamina
Spermatogonium
(stem cell)
Type A daughter cell
remains at basal lamina
as a stem cell
Mitosis
Type B daughter cell
Growth
Enters meiosis I
and moves to
adluminal
compartment
Primary
spermatocyte
Meiosis I
completed
Secondary
spermatocytes
Meiosis II
Early
spermatids
Late spermatids
Spermatozoa
(b) Events of spermatogenesis,
showing the relative position
of various spermatogenic cells
Figure 27.7b

48. Golgi apparatus Acrosomal vesicle
Approximately 24 days
Golgi
apparatus
Acrosomal
vesicle
Mitochondria
Acrosome
Nucleus 1 2
Centrioles
Spermatid
nucleus
Microtubules
Midpiece
Head
(a) 3
Flagellum
Excess
cytoplasm 4
Tail 5 6 7
(b)
Figure 27.8a, b

49. Final Result of Spermatogenesis
All the four cells (sperm) are viable – thus differing from the female situation