Practical 3 07

1. Practical 3
Meiosis
Gametogenesis

2. Chromosomal sex determination
•Drosophila type
•Male – XY
•Female - XX

3. Sexual reproduction
2n
2n
meiosis
meiosis
n
n
Gametes (haploid)
sperm
oocyte
fertilization 2n
n + n = 2n
zygote
♂
♀
Life cycle:
haploid phase (gametes)
diploid phase (somatic cells)

4. Recommended website
•http://
genetics.gsk.com/chromosomes.htm#chromo

5. Written test
•8 minutes
•Don't forget to put down your name, your
group and the test version.
•In multiple choice questions more than 1
statement could be correct.
•Don't write anything on the question sheet!

6. Meiotic division
1
st
meiotic
division
2
nd
meiotic
division
2n
n n
n n n n

7. First meiotic division
•Heterotypic or reduction
•Prophase
–Leptotene
–Zygotene
–Pachytene
–Diplotene
–Diakinesis
•Metaphase
•Anaphase
•Telophase

8. Leptotene
•Beginning of prophase
•Chromosomes can't be
distinguished.
•Condensation of
chromosomes begins.

9. Zygotene
•Attachment of homologous
chromosomes – the chromosomes
begin to synapse.
•Assemblage of chromosomes is enabled
by synaptonemal complex.

10. Pachytene
•Bivalents or tetrades are present.
–1 bivalent = 2 chromosomes
–1 tetrade = 4 chromatids
–bivalent = tetrade
•Crossing-over between homologous
chromosomes is possible.

11. Crossing-over
•Reciprocal exchange of chromatid segments
between chromosomal homologues

12. Importance of the crossing-over
•New combinations of alleles of genes that are
located on the same chromosome.
•New allele combinations lead to higher variability
of the offspring (important in evolution).
•This increases the potential for genetic variability
between members of the same species.

13. Chromosomes X and Y
•are attached in
pseudoautosomal regions
(PAR) during the 1
st
meiotic
prophase
X
Y
SRY – sex determining region – is
located on chromosome Y in the
neighborhood of PAR.X
Y

14. Task A:
•An infertile man with azoospermia had
been examined in cytogenetic lab. He has
no other striking phenotypic features.
•The cytogenetic analysis revealed
abnormal finding because his cells
contained two X chromosomes while
chromosome Y is missing.
•Explain this surprising result.

15. Crossing-over in PAR
•Nonreciprocal crossing-over
can result in translocation of
the SRY gene from the
chromosome Y to
chromosome X

16. Task A – result
male
female
Normal combination of
gonosomes X;Y translocation
„XX-male“
„XY-female“XX
XY

17. Hermaphroditism – possible
consequence of sex chromosome
abnormalities

18. Prophase of the 1
st
meiotic
division
next periods

19. Pachytene

20. Diplotene
•Condensation of chromosomes
continues
•Homologous chromosomes are divided
•Disassembling of synaptonemal
complex
•At the end of diplotene homologues are
connected only in crossover sites.
These sites are called chiasmata
(singular: chiasma).

21. Diakinesis
•Final period of the 1st meiotic
prophase.
•Chiasmata are transferred to
terminal chromosomal
regions.
•Under specific staining
chromosomes could be well
distinguished under the
microscope.
•Nuclear membrane
disappears.

22. Terminal phases of meiotic division
prophase
metaphase
+ anaphase
2
nd
meiotic
division
(homeotypic)
1
st
meiotic
division
(heterotypic)
Separation of
double-chromatid
homologous
chromosomes

23. Metaphase I
Metaphase I

24. Anaphase I X Anaphase II

26. Gametogenesis
•Spermatogenesis – sperm development
•Oogenesis – oocyte development

27. n n
1st meiotic
division
2nd meiotic
division
n n
primary
spermatocyte
secondary
spermatocyte
sperm
n n
2n
Spermatogenesis

28. n n
1st meiotic
division
2nd meiotic
division
n n
primary
oocyte
secondary
oocyte
mature
oocyte
n
n
2n
polar bodies
polar
body
Oogenesis

29. A schedule of oocyte development
Prophase I
fetus
childhood – dictyotene
mature ovary
fertilization

30. Spermatogenesis X Oogenesis

31. Task B: Risk of chromosomal
nondisjunction defects
•Compare possible risk of chromosomal
abnormalities for two couples that visited
the genetic counselling clinic:
–Couple 1: female (21 years), male (38 years)
–Couple 2: female (36 years), male (32 years)
•Which family has higher risk of numerical
chromosomal aberrations?

32. Task B – result
•Oogenesis lasts much longer
than spermatogenesis – risk of
chromosomal nondisjunction
defects is age-related.
•The second couple has higher
risk due to 36 years old female.
•In pregnant females older than
35 years prenatal examination
of the fetus is recommended.
35 years … risk 1/365
Age-related risk of the
Down syndrome child
Mother´s age

33. Practical tasks

34. Task 1: Observation of meiosis in
the grasshopper testis
•The slide was prepared from the testis of the
grasshopper. After removing from the individual
the tissue was fixed with methanol + glacial
acetic acid (3 : 1) and stained with the Giemsa-
Romanowski solution. After dehydration the
tissue was mounted in the Canada balsam.
•Find the spermatocytes on the slide using 10x
objective lens.
•Change the objective magnification into 40 or
45x and observe different phases of meiosis. For
their determination use adjacent photos.

35. Task 2: Disjunction of the sex
chromosomes during meiosis
•Redraw the scheme of disjunction of the sex chromosomes during
meiosis of the male to your materials and fill it.
•In the first scheme (A) distinguish the chromosomes according to
their shape and number of chromatids.
•In the second scheme (B) designate the chromosomes using letters
X or Y no matter of the number of chromatids.
XY
A B

36. Task 3: Changes of nuclear DNA
concentration during the cell cycle
•Contrast the two types of cell division
(mitosis X meiosis) by completing the
following graphs below.
•Redraw the picture to your materials and fill
curves demonstrating changes in the DNA
content per one cell:
–during the cell cycle followed with mitosis
–during the cell cycle followed with meiosis.
•(Value „C“ on y-axis represents DNA content per
one cell in G1 phase.)

37. G
1
G
2
S MG
1
C
2C
G
1
G
2
S M
1
M
2
G
1
Cell cycle + mitosis Cell cycle + meiosis
Nuclear
DNA
content
Task 3: Changes of nuclear DNA
concentration during the cell cycle

38. Results

39. Task 3: Observation of meiosis in
the grasshopper testis

40. Task 4: Disjunction of the sex
chromosomes during meiosis
XY
X Y
XX YY
Scheme A Scheme B

41. Task 5: Changes of nuclear DNA
concentration during the cell cycle
G
1
G
2
S MG
1
C
2C
G
1
G
2
S M
1
M
2
G
1
Cell cycle + mitosis Cell cycle + meiosis
Nuclear
DNA

42. Assisted reproduction
•A group of methods that allows reproduction of
infertile couple that can't be therapeuted by
common medical treatment (e.g. using drugs of by
psychological intervention)
•The assisted reproduction could be also used for
prevention of genetic defects in families with
normal fertility but with increased risk of genetically
conditioned disorder for the offspring.

43. Assisted reproduction methods
•Intrauterine artificial insemination
–application of sperm to the uterus of the
female.
•In vitro fertilization and embryo transfer
–fertilization is performed outside the organism
of the mother

44. Artificial insemination
•homologous – the
female is fertilized with
sperm of her husband;
•sperm of anonymous
donor are applied – the
donor and the couple
remain unknown to
each other

45. In vitro fertilization
•Application of gonadotropins (= fertility
drugs) induce maturation of numerous
oocytes.
•The oocytes are removed from ovaries by
biopsy. In a special dish they are mixed
with sperms of the husband.
•After short term cultivation in medium with
nutrients 3 embryos are transferred to the
female's uterus.

46. ICSI
(intracytoplasmatic sperm injection)

47. ICSI protocol

48. Preimplantation diagnostics
•Genetic or other examinations of oocytes or
embryo before transfer to the mother's
•We use
–Polar body
–1 – 2 cells from 8-cell embryo
–More cells from blastocyst

49. Presentation
Ethical issues of assisted
reproduction

50. Next seminar
•In vitro cell cultivation
•Recommended websites:
–http://www.research.umbc.edu/~jwolf/method5.htm
–http://homepages.gac.edu/~cellab/chpts/chpt12/intro12.html
•Be careful when handling cell cultures!
•Test: previous topics (only seminars)

51. See you next week!